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Atlas of Peculiar Galaxies
Atlas of Peculiar Galaxies
Atlas of Peculiar Galaxies
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Atlas of Peculiar Galaxies
Alternative namesArp number
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[edit on Wikidata]
The Antennae Galaxies (Arp 244)
Colliding spiral galaxy pair NGC 3808A and NGC 3808B (Arp 87).
NGC 6621/NGC 6622 (Arp 81), a pair of spiral galaxies 100 million years after their colliding.
IC 883 (Arp 193), remnant of two galaxies' merger.
Arp 147, an interacting pair of ring galaxies.

The Atlas of Peculiar Galaxies is a catalog of peculiar galaxies produced by Halton Arp in 1966. A total of 338 galaxies are presented in the atlas, which was originally published in 1966 by the California Institute of Technology. The primary goal of the catalog was to present photographs of examples of the different kinds of peculiar structures found among galaxies.[1]

Background

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Arp realized that the reason why galaxies formed into spiral or elliptical shapes was not well understood. He perceived peculiar galaxies as small "experiments" that astronomers could use to understand the physical processes that distort spiral or elliptical galaxies. With this atlas, astronomers had a sample of peculiar galaxies that they could study in more detail. The atlas does not present a complete overview of every peculiar galaxy in the sky but instead provides examples of the different phenomena as observed in nearby galaxies.

Because little was known at the time of publication about the physical processes that caused the different shapes, the galaxies in the atlas are sorted based on their appearance. Objects 1–101 are individual peculiar spiral galaxies or spiral galaxies that apparently have small companions. Objects 102–145 are elliptical and elliptical-like galaxies. Individual or groups of galaxies with neither elliptical nor spiral shapes are listed as objects 146–268. Objects 269–327 are double galaxies. Finally, objects that simply do not fit into any of the above categories are listed as objects 332–338. Most objects are best known by their other designations, but a few galaxies are best known by their Arp numbers (such as Arp 220).

Today, the physical processes that lead to the peculiarities seen in the Arp atlas are better understood than in the 1960s when Arp's book was published. A large number of the objects have been interpreted as interacting galaxies, including M51 (Arp 85), Arp 220, and the Antennae Galaxies (NGC 4038/NGC 4039, or Arp 244). A few of the galaxies are simply dwarf galaxies that do not have enough mass to produce enough gravity to allow the galaxies to form any cohesive structure. NGC 1569 (Arp 210) is an example of one of the dwarf galaxies in the atlas. A few other galaxies are radio galaxies. These objects contain active galactic nuclei that produce powerful jets of gas called radio jets. The atlas includes the nearby radio galaxies M87 (Arp 152) and Centaurus A (Arp 153).

The peculiar associations present in the catalogue are now interpreted as galaxy mergers or non-interacting line-of-sight overlap, though Arp disputed that idea, claiming that apparent associations were examples of ejections.[2]

Notable Arp galaxies

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The Mice Galaxies (NGC 4676; Arp 242). These two galaxies both have tidal tails that form as a consequence of the galaxies' gravitational interaction. The galaxies are also connected by a tidal bridge, another feature formed by the gravitational interaction.
Merging galaxy pair "The Grasshopper" (alias UGC 4881, FUDGE, Arp 55).
Arp Number Name Magnitude Notes
26 Pinwheel Galaxy (M101) +7.5 spiral galaxy
37 Messier 77 +8.9 radio galaxy
41 NGC 1232 +9.8 spiral galaxy
76 Messier 90 +9.5 spiral galaxy
77 NGC 1097 +9.5 galaxy interacting with its satellite
85 Whirlpool Galaxy (M51) +8.4 galaxy interacting with its satellite
116 Messier 60 +8.8 colliding galaxies
152 Virgo A (M87) +8.6 elliptical galaxy
153 Centaurus A (NGC 5128) +6.6 radio galaxy in a collision?
188 Tadpole Galaxy +14.4 galaxy finishing merging
242 Mice Galaxies +14.7 colliding galaxies
244 Antennae Galaxies +10.3 colliding galaxies
317 Messier 65 +9.2 spiral galaxy
319 NGC 7320 +15 galaxy in colliding group
337 Cigar Galaxy (M82) +8.6 starburst galaxy

Catalog list

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Spiral galaxies

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Low surface brightness

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Ultraviolet image of NGC 2537 (Arp 6)

These are mostly dwarf galaxies or poorly defined spiral galaxies (with the designation Sm) that have low surface brightnesses (i.e. they emit little light per unit area). Low surface brightness galaxies are actually quite common. The exception is NGC 2857 (Arp 1), which is an Sc spiral galaxy (which means that it has a definite structure with loosely wound spiral arms and a faint but well-defined nucleus).[3]

Arp number Common name Notes
1 NGC 2857 Sc spiral galaxy[3]
2 UGC 10310
3 Arp 3
4 Arp 4
5 NGC 3664
6 NGC 2537

Split arms

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This category contains spiral galaxies with arms that split into two separate parts.

Arp number Common name Notes
7 Arp 7
8 NGC 497
9 NGC 2523
10 UGC 1775 Contains an off-center nucleus
11 UGC 717
12 NGC 2608

Detached segments

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M66 (Arp 16)

This category contains spiral galaxies with arms that appear to be segmented. Some spiral arm segments may appear detached because dust lanes in the spiral arms obscure the arms' starlight. Other spiral arms may appear segmented because of the presence of bright star clusters (or discontinuous chains of bright star clusters) in the spiral arms.

Arp number Common name Notes
13 NGC 7448
14 NGC 7314
15 NGC 7393
16 M66
17 UGC 3972
18 NGC 4088

Three-armed

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Usually, most spiral galaxies contain two clearly defined spiral arms, or they contain only fuzzy filamentary spiral structures. Galaxies with three well-defined spiral arms are rare.

Arp number Common name Notes
19 NGC 145
20 UGC 3014
21 Arp 21

One-armed

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NGC 4618 (Arp 23)

One-armed spiral galaxies, including Magellanic spirals, are also rare. In this case, the single spiral arm may actually be formed by a gravitational interaction with another galaxy (as with the Large Magellanic Cloud itself, although it is not a member of the catalog).

Arp number Common name Notes
22 NGC 4027
23 NGC 4618 Interacting with NGC 4625[4]
24 NGC 3445

One heavy arm

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NGC 6946 (Arp 29) spiral galaxy

The spiral arms in these galaxies have an asymmetric appearance. One spiral arm may appear to be considerably brighter than the other. In the photographic plates produced by Arp, the bright arm would look dark or "heavy". While most of these galaxies (such as M101 and NGC 6946) are simply asymmetric spiral galaxies, NGC 6365 is an interacting pair of galaxies where one of the two galaxies is viewed edge-on and just happens to lie where the spiral arm for the other face-on galaxy would be visible.[5]

Arp number Common name Notes
25 NGC 2276
26 M101 Face-on spiral galaxy with five notable companion galaxies[4]
27 NGC 3631
28 NGC 7678
29 NGC 6946
30 NGC 6365 Interacting pair of galaxies, with one galaxy viewed edge-on[5]

Integral sign

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These are galaxies that look like a stretched-out S shape (or like the integral sign used in calculus). Some objects, such as IC 167,[6] are simply ordinary spiral galaxies viewed from an unusual angle. Other objects, such as UGC 10770, are interacting pairs of galaxies with tidal tails that look similar to spiral arms.[7]

Arp number Common name Notes
31 IC 167
32 UGC 10770
33 UGC 8613
34 NGC 4615
35 UGC 212
36 UGC 8548

Low surface brightness companions

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Many of these spiral galaxies are probably interacting with the low surface brightness galaxies in the field of view. In some cases, however, it may be difficult to determine whether the companion is physically near the spiral galaxy or whether the companion is a foreground/background source or a source on the edge of the spiral galaxy.

Arp number Common name Notes
37 M77
38 NGC 6412
39 NGC 1347
40 IC 4271
41 NGC 1232
42 NGC 5829
43 IC 607
44 IC 609
45 UGC 9178 Galaxy triplet[8]
46 UGC 12665
47 Arp 47
48 Arp 48

Small, high surface brightness companions

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Image of NGC 1097 (Arp 77) spiral galaxy

Again, many of these spiral galaxies are probably interacting with companion galaxies, although some of the identified companion galaxies may be foreground/background sources or even bright star clusters within the individual galaxies.

Arp number Common name Notes
49 NGC 5665
50 IC 1520
51 Arp 51
52 Arp 52
53 NGC 3290
54 Arp 54
55 UGC 4881
56 UGC 1432
57 Arp 57
58 UGC 4457
59 NGC 341
60 Arp 60 (PGC 1762846)
61 UGC 3104
62 UGC 6865
63 NGC 2944
64 UGC 9503
65 NGC 90
66 UGC 10396
67 UGC 892
68 NGC 7757
69 NGC 5579
70 UGC 934
71 NGC 6045
72 NGC 5994, NGC 5996
73 IC 1222
74 UGC 1626
75 NGC 702
76 M90
77 NGC 1097
78 NGC 772

Large, high surface brightness companions

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The Whirlpool Galaxy (M51, Arp 85). This object consists of a larger spiral galaxy interacting with an elliptical galaxy.

Galaxies in this category are almost always clearly interacting sources. The most famous of these objects is the Whirlpool Galaxy (M51; Arp 85), which is composed of a spiral galaxy NGC 5194 that is interacting with a smaller elliptical galaxy NGC 5195. The interaction has distorted the shape of both galaxies; the spiral arm pattern has been enhanced in the larger spiral galaxy, and a bridge of stars and gas has formed between the two galaxies. Many of the other galaxies in this category are also connected by bridges.

Arp number Common name Notes
79 NGC 5490C
80 NGC 2633
81 NGC 6621, UGC 11175, NGC 6622
82 NGC 2535, NGC 2536
83 NGC 3799, NGC 3800
84 NGC 5394, NGC 5395
85 Whirlpool Galaxy (M51)
86 NGC 7752, NGC 7753
87 NGC 3808A, NGC 3808B
88 Arp 88
89 NGC 2648
90 NGC 5929, NGC 5930
91 NGC 5953, NGC 5954

Elliptical companions

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Like the spiral galaxies with high surface brightness companions, most of these spiral galaxies are clearly interacting systems. Tidal tails and bridges are visible in many of the images.

Arp number Common name Notes
92 NGC 7603
93 NGC 7284, NGC 7285
94 NGC 3226, NGC 3227
95 IC 4461, IC 4462
96 UGC 3528
97 UGC 7085A
98 UGC 1095
99 NGC 7547, NGC 7549, NGC 7550 Galaxy triplet[9]
100 IC 18, IC 19
101 UGC 10164, UGC 10169

Elliptical and elliptical-like galaxies

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Connected to spiral galaxies

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Different wavelength infrared images of Arp 107 (UGC 5984).

These objects are very similar to the spiral galaxies with elliptical companions. All of the galaxies have features such as tidal tails and tidal bridges that have formed through gravitational interaction.

Arp number Common name Notes
102 Arp 102
103 UGC 10586 Galaxy triplet[10]
104 NGC 5216, NGC 5218
105 NGC 3561
106 NGC 4211
107 UGC 5984
108 Arp 108

Repelling spiral arms

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Based on the description of these objects, it appears that Arp originally thought that the elliptical galaxies were pushing away spiral arms in companion galaxies. However, the tidal spiral arms may actually look distorted because of the interaction. Some of these "repelled" spiral arms are on the opposite side of the spiral galaxy from the elliptical galaxy. Simulations have shown that such features can be formed through gravitational interactions alone; no repelling forces are needed.

Arp number Common name Notes
109 UGC 10053
110 Arp 110
111 NGC 5421 Galaxy group[11]
112 NGC 7805, NGC 7806

Close to and perturbing spiral galaxies

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Galaxy pair NGC 4435 and NGC 4438 (Arp 120)
Messier 60 and NGC 4647 (Arp 116)

This is another category in which the majority of objects are interacting galaxies. As noted in the category name, the spiral galaxies look perturbed. Arp originally described some of the elliptical galaxies as repelling.

Arp number Common name Notes
113 NGC 70 Part of a group of galaxies[12]
114 NGC 2276, NGC 2300
115 UGC 6678 Galaxy triplet[13]
116 Messier 60, NGC 4647
117 IC 982, IC 983
118 NGC 1141, NGC 1142
119 Arp 119
120 NGC 4435, NGC 4438
121 Arp 121
122 NGC 6040
123 NGC 1888, NGC 1889
124 NGC 6361
125 UGC 10491
126 UGC 1449
127 NGC 191 Actually interacting S0 galaxy and spiral galaxy[13]
128 UGC 827
129 UGC 5146
130 IC 5378
131 Arp 131
132 Arp 132

Nearby fragments

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Messier 49, Arp 134
Arp number Common name Notes
133 NGC 541
134 Messier 49
135 NGC 1023
136 NGC 5820

Emanating material

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Arp thought that the elliptical galaxies in this category were ejecting material from their nuclei. Many of the pictures could be interpreted that way. However, these objects are actually a mixture of other phenomena. For example, NGC 2914 (Arp 137) is merely a spiral galaxy with faint spiral arms,[14] and NGC 4015 (Arp 138) is an interacting pair of galaxies where one galaxy is an edge-on spiral galaxy.[15] Some objects, such as NGC 2444 and NGC 2445 (Arp 143), are systems that contain "ring galaxies", which are created when one galaxy (the elliptical galaxies in these examples) passes through the disk of another. This passage causes a gravitational wave in which gas first falls inward and then propagates outward to form the ring structure.[16]

Arp 142: PGC 1237172 (top), NGC 2936, once a standard spiral galaxy (center), and NGC 2937, a smaller elliptical (bottom).[17]
Arp number Common name Notes
137 NGC 2914 Spiral galaxy with faint spiral arms[14]
138 NGC 4015 Interacting pair of galaxies[15]
139 Arp 139 Interacting pair of galaxies[18]
140 NGC 274, NGC 275 Interacting pair of galaxies[19]
141 UGC 3730 Ring galaxy system[20]
142 NGC 2936, NGC 2937, PGC 1237172 Galaxy triplet[21]
143 NGC 2444, NGC 2445 Ring galaxy system[22]
144 NGC 7828, NGC 7829 Ring galaxy system[23]
145 UGC 1840 Ring galaxy system[24]

Amorphous galaxies

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Galaxies in this category are referred to by Arp as galaxies that are neither spiral nor elliptical in shape. Although he does not use the term "amorphous" to describe these galaxies, it is the best description of these galaxies.

Many of these galaxies are either interacting galaxies or galaxies that are the remnants of the merger of two smaller galaxies. The interaction process will produce various tidal features, such as tidal tails and tidal bridges, that may last well after the progenitor galaxies' disks and nuclei have merged. Although the tidal tails are described as several different visual phenomena ("counter-tails", "filaments", "loops"), they are all manifestations of the same phenomena.

Associated rings

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Interacting galaxy pair Arp 148 (Mayall's Object)

As noted above, these ring galaxies may have formed when a companion galaxy passed through the ring galaxy. The interaction would produce a wave effect that would first draw matter into the center and then cause it to propagate outward in a ring.[16]

Arp number Common name Notes
146 Arp 146
147 IC 298
148 Arp 148

Jets

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Giant elliptical galaxy Messier 87 with its relativistic jet (Arp 152)

These are galaxies that appear to be ejecting material outwards from their nuclei. The "jets" themselves look similar to water spraying out of a hose. In the case of IC 803 (Arp 149) and NGC 7609 (Arp 150), the jets are simply part of the amorphous structure produced by the interacting galaxies. In Arp 151[citation needed] and Messier 87 (Arp 152), however, the jets are ionized gas that has been ejected from the environment around supermassive black holes in the galaxies' active galactic nuclei.[25][26] These jets, sometimes called relativistic jets or radio jets, are powerful sources of synchrotron radiation, especially at radio wavelengths.

Arp number Common name Notes
149 IC 803 Interacting galaxies[27]
150 NGC 7609 Interacting galaxies[28]
151 Arp 151 Seyfert galaxy (contains an active galactic nucleus)[25]
152 Messier 87 Seyfert galaxy (contains an active galactic nucleus)[26]

Interior absorption

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Merging galaxy pair named NGC 520 (Arp 157).
Giant elliptical galaxy NGC 1316.

Galaxies in this category feature dark dust lanes that obscure part of the disk of the galaxy. All of these galaxies are the products of two galaxies merging. NGC 520 (Arp 157) is one of the best examples of an intermediate-stage merger, where the two progenitor galaxies' disks have coalesced together but the nuclei have not. Centaurus A (Arp 153) and NGC 1316 (Arp 154) are both effectively elliptical galaxies with unusual dust lanes; their kinematics and structure indicate that they have undergone merging events recently. NGC 4747 (Arp 159) may be nothing more than an edge-on spiral galaxy with a significantly dark dust lanes.[29]

Arp number Common name Notes
153 Centaurus A Notable radio galaxy; contains an active galactic nucleus)[30]
154 NGC 1316 Notable radio galaxy; contains an active galactic nucleus)[30]
155 NGC 3656
156 UGC 5184
157 NGC 520 Notable intermediate-stage merger
158 NGC 523
159 NGC 4747 Spiral galaxy with dark dust lanes[29]
160 NGC 4194 Also known as the Medusa Galaxy

Diffuse filaments

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The filaments in these objects may represent tidal tails from galaxy interactions. Many of the galaxies are the remnants of the mergers of two spiral galaxies to form a single elliptical galaxy. However, NGC 3414 (Arp 162) appears to be merely an unusual S0 galaxy with a very small disk relative to its bulge size.[4] NGC 4670 (Arp 163) is a blue compact dwarf galaxy with extremely strong star formation activity;[31] it is clearly too small to be the merger remnant of two spiral galaxies like the other merger remnants in this sample, although it may have been involved in a much smaller interaction.

Arp number Common name Notes
161 UGC 6665
162 NGC 3414 S0 galaxy[4]
163 NGC 4670 Blue compact dwarf galaxy[31]
164 NGC 455
165 NGC 2418
166 NGC 750, NGC 751 Interacting galaxies

Diffuse counter-tails

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All of these objects are galaxies involved in gravitational interactions. These counter-tails are tidal features caused by the gravitational interactions between two galaxies, just like similar features described in the Arp catalog. Messier 32 (Arp 168), a dwarf galaxy interacting with the Andromeda Galaxy,[4] is included in this category (although the "diffuse counter-tail" is very difficult to see in Arp's photograph).

Arp number Common name Notes
167 NGC 2672, NGC 2673
168 Messier 32 Dwarf galaxy interacting with Andromeda Galaxy[4]
169 NGC 7236, NGC 7237, NGC 7237C Galaxy triplet[32]
170 NGC 7578
171 NGC 5718, IC 1042
172 IC 1178, IC 1181

Narrow counter-tails

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NGC 3068 (Arp 174)

This is another category containing galaxies with tidal tails produced by gravitational interactions. These tidal tails are narrower and better defined than the tidal tails in objects 167–172.

Arp number Common name Notes
173 UGC 9561
174 NGC 3068
175 IC 3481, IC 3481A, IC 3483 Galaxy triplet[33]
176 NGC 4933 Galaxy triplet[34]
177 Arp 177
178 NGC 5613, NGC 5614, NGC 5615 Galaxy triplet[35]

Narrow filaments

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NGC 1614 (Arp 186).
The Tadpole Galaxy (UGC 10214; Arp 188). The "narrow filament", which appears to be tidal feature caused by a gravitational interaction, can be seen extending across this image.

This category contains a mixture of different types of objects. Like the galaxies with diffuse filaments or galaxies with counter-tails, some of the galaxies in this category have been involved in interactions, and the filaments are tidal features created by those interactions. Other sources, however, are simply individual spiral galaxies with faint spiral arms that are described as "filaments" by Arp.

Arp number Common name Notes
179 Arp 179
180 Arp 180 Interacting galaxy pair[36]
181 NGC 3212, NGC 3215 Interacting galaxy pair[37]
182 NGC 7674, NGC 7674A Interacting galaxy pair[38]
183 UGC 8560 Spiral galaxy[39]
184 NGC 1961 Spiral galaxy[40]
185 NGC 6217 Spiral galaxy[41]
186 NGC 1614 Spiral galaxy involved in recent interaction[42]
187 Arp 187
188 Tadpole Galaxy Galaxy involved in recent interaction
189 NGC 4651 Tidal star streams[43]
190 UGC 2320 Interacting galaxy pair[44]
191 UGC 6175 Interacting galaxy pair[45]
192 NGC 3303 Interacting galaxy pair[46]
193 IC 883 Merger remnant

Material ejected from nuclei

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Arp 194. The third galaxy at the bottom of the image is actually a further object, not part of the system.
Arp 195, an interacting galaxy triplet located in the constellation Lynx, about 747 million light-years from Earth

The ejecta in many of these objects appear to be tidal features created by gravitational interactions. In some cases (such as for NGC 5544 and NGC 5545 in Arp 199), the "ejecta" are clearly a spiral galaxy viewed edge-on that happens to line up with another galaxy's nucleus.

Almost all of the objects in this category are interacting or have recently undergone interactions. NGC 3712 (Arp 203) is an exception; it is merely a low surface brightness spiral galaxy.[47]

Arp number Common name Notes
194 UGC 6945 Interacting galaxy pair[48]
195 UGC 4653 Interacting galaxy triplet[49]
196 Arp 196 Interacting galaxy pair[50]
197 UGC 6503, IC 701 Interacting galaxy pair[51]
198 UGC 6073 Interacting galaxy pair[52]
199 NGC 5544, NGC 5545 Interacting galaxy pair[53]
200 NGC 1134 Spiral galaxy interacting with low surface brightness galaxy[54]
201 UGC 224 Interacting galaxy pair[55]
202 NGC 2719, NGC 2719A Interacting galaxy pair[56]
203 NGC 3712 Low surface brightness spiral galaxy[47]
204 UGC 8454 Interacting galaxy pair[57]
205 NGC 3448 Merger remnant[58]
206 UGC 5983, NGC 3432 Interacting galaxy pair[59]
207 UGC 5050 Spiral galaxy interacting with dwarf galaxy[60]
208 Arp 208 Interacting galaxy pair[61]

Irregularities, absorption, and resolution

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Starburst activity in nearby dwarf galaxy NGC 1569 (Arp 210).

Galaxies in this category have either irregular structures (irregularities), notable dust lanes (absorption), or a grainy appearance (resolution). This category contains a mix of interacting galaxies distorted by tidal interactions, nearby dwarf irregular galaxies, and spiral galaxies with unusual large amounts of gas.

Arp number Common name Notes
209 NGC 6052 Interacting galaxy pair[62]
210 NGC 1569 Dwarf galaxy[63]
211 UGCA 290 Interacting dwarf galaxies[64]
212 NGC 7625 Peculiar spiral galaxy[65]
213 IC 356 Peculiar spiral galaxy[66]
214 NGC 3718 Peculiar spiral galaxy[67]

Adjacent loops

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Nearby starburst spiral galaxy NGC 3310 (Arp 217)

These adjacent loops are another manifestation of the structures formed by gravitational interactions between galaxies. Some of these sources consist of galaxies that have nearly completed the merger process; the "adjacent loops" are merely the remnants of the interaction. Among the objects in this category is Arp 220, one of the best-studied ultraluminous infrared galaxies in the sky.

Arp number Common name Notes
215 NGC 2782 Peculiar spiral galaxy[4]
216 NGC 7679, NGC 7682 Interacting galaxy pair[68]
217 NGC 3310 Notable nearby starburst;[69] merger remnant[4]
218 Arp 218 Interacting galaxy pair[70]
219 UGC 2812 Galaxy in interaction[71]
220 IC 4553 Merger remnant; notable ultraluminous infrared galaxy

Amorphous spiral arms

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Spiral galaxy NGC 7252 (Arp 226)

Many of these galaxies are merger remnants. The "amorphous spiral arms" are the tidal debris that remains after the collision.

Arp number Common name Notes
221 Arp 221 Interacting galaxy triplet[72]
222 NGC 7727 Merger remnant[4]
223 NGC 7585 Recent inequal-mass merger[4]
224 NGC 3921 Merger remnant
225 NGC 2655 Recent inequal-mass merger
226 The Atoms for Peace Galaxy (NGC 7252) Merger remnant[4]

Concentric rings

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NGC 474, Arp 227

These are galaxies with shell-like structures. Some shell structures have been identified as the results of recent mergers.[citation needed] In other cases, however, the shell structure may represent the outer disk of an S0 galaxy. In some complicated cases, the galaxy with the rings or shells is an S0 galaxy interacting with another galaxy; the origins of the shells in such systems can be difficult to determine.

Arp number Common name Notes
227 NGC 470, NGC 474 Interacting galaxy pair with one S0 galaxy[73]
228 IC 162 S0 galaxy[74]
229 NGC 507, NGC 508 Interacting galaxy pair including one S0 galaxy and one elliptical galaxy[75]
230 IC 51 Peculiar S0 galaxy;[76] possible merger remnant
231 IC 1575
232 NGC 2911 Peculiar S0 galaxy[77]

Appearance of fission

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Interacting pair of galaxies: Arp 238 (UGC 8335).
NGC 5257 and NGC 5258 (Arp 240), interacting pair of spiral galaxies.
Arp 256, spiral galaxy pair in the early stages of colliding and merging.

Although the description of the objects in this category implies that the galaxies are separating, most of these galaxies are merging. Many of the objects have very pronounced tidal tails and bridges that have formed as a consequence of the interaction. Most objects are in the early stages of the merging process, where the galaxies still appear to have distinct nuclei and distinct (albeit distorted) disks. Among the most notable galaxies in this category are the Antennae Galaxies (NGC 4038 and NGC 4039, Arp 244) and the Mice Galaxies (NGC 4676, Arp 242).

However, not all of these objects are interacting galaxies. A few of these galaxies are simply nearby dwarf galaxies with irregular structure.

Arp number Common name Notes
233 UGC 5720 Dwarf galaxy[78]
234 NGC 3738 Dwarf galaxy[79]
235 NGC 14 Dwarf galaxy[80]
236 IC 1623 Interacting galaxy pair[81]
237 UGC 5044 Interacting galaxy pair[82]
238 UGC 8335 Interacting galaxy pair[83]
239 NGC 5278, NGC 5279 Interacting galaxy pair[84]
240 NGC 5257, NGC 5258 Interacting galaxy pair[85]
241 UGC 9425 Interacting galaxy pair[86]
242 Mice Galaxies (NGC 4676) Interacting galaxy pair[87]
243 NGC 2623 Interacting galaxy triplet[88]
244 Antennae Galaxies (NGC 4038, NGC 4039) Interacting galaxy pair[89]
245 NGC 2992, NGC 2993 Interacting galaxy pair[90]
246 NGC 7837, NGC 7838 Interacting galaxy pair[91]
247 IC 2338, IC 2339 Interacting galaxy pair[92]
248 Wild's Triplet Interacting galaxy triplet[93]
249 UGC 12891 Interacting galaxy pair[94]
250 Arp 250
251 Arp 251 Interacting galaxy triplet[95]
252 Arp 252 Interacting galaxy pair[96]
253 UGCA 173, UGCA 174 Interacting galaxy pair[97]
254 NGC 5917 Peculiar spiral galaxy[98]
255 UGC 5304 Interacting galaxy pair[99]
256 Arp 256 Interacting galaxy pair[100]

Irregular clumps

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Holmberg II/UGC 4305, dwarf galaxy (Arp 268)
Interacting galaxy pair Arp 261.

These are objects that appear to be a series of irregular clumps with no coherent structure. Many of these objects are simply nearby dwarf galaxies. Some of these objects are interacting galaxies, while others are small groups of galaxies. In both cases, many of the constituent galaxies are irregular galaxies. The superposition of two or more such irregular galaxies can easily look like a single larger irregular galaxy, which is why the Atlas of Peculiar Galaxies (and other catalogs) often classify these pairs and groups as single objects.

Arp number Common name Notes
257 UGC 4638 Interacting galaxy pair[101]
258 UGC 2140 Galaxy group[102]
259 NGC 1741 Galaxy group[103]
260 UGC 7230 Interacting galaxy pair[104]
261 Arp 261 Galaxy group[105]
262 UGC 12856 Interacting galaxy pair[106]
263 NGC 3239 Dwarf galaxy[107]
264 NGC 3104 Dwarf galaxy[108]
265 IC 3862 Interacting galaxy pair[109]
266 NGC 4861 Dwarf galaxy[110]
267 UGC 5764 Dwarf galaxy[111]
268 UGC 4305, Holmberg II Dwarf galaxy[112]

Double and multiple galaxies

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Arp originally referred to these galaxies as "double galaxies", but many of these sources are more than two galaxies. Some of the objects consist of interacting galaxies, whereas other sources are actually groups of galaxies. The difference is that interacting galaxies will be distorted, whereas galaxies in groups are simply gravitationally bound to each other but not necessarily close enough to each other to induce major structural changes.

Connected arms

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Arp 272: NGC 6050 and IC 1179, interacting spiral galaxies.

All of these galaxies are interacting pairs of galaxies except for NGC 5679 (Arp 274), which may be an interacting galaxy triplet.[113] The connected arms described here are tidal bridge features that form between interacting galaxies. These bridges form early during galaxy interactions.

Arp number Common name Notes
269 NGC 4485, NGC 4490 Interacting galaxy pair[114]
270 NGC 3395, NGC 3396 Interacting galaxy pair[115]
271 NGC 5426, NGC 5427 Interacting galaxy pair[116]
272 NGC 6050, IC 1179 Interacting galaxy pair[117]
273 UGC 1810, UGC 1813 Interacting galaxy pair[118]
274 NGC 5679 Interacting galaxy triplet[113]

Interacting galaxies

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'The Bird' is composed of two massive spiral galaxies and a third irregular galaxy. (ESO593-8)

Unlike many of the objects listed in the amorphous galaxies section, the interacting galaxies that comprise these objects are still distinguishable from each other.

Arp number Common name Notes
275 NGC 2881 Interacting galaxy pair[119]
276 NGC 935, IC 1801 Interacting galaxy pair[120]
277 NGC 4809, NGC 4810 Interacting galaxy pair[121]
278 NGC 7253 Interacting galaxy pair[122]
279 NGC 1253, NGC 1253A Interacting galaxy pair[123]
280 NGC 3769, NGC 3769A Interacting galaxy pair[124]

Infall and attraction

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Edge-on spiral galaxy NGC 4631 and dwarf elliptical NGC 4627 (below) comprise the Arp 281 pair

This category contains an odd mixture of objects. Two of the objects are edge-on disk galaxies with smaller companion galaxies nearby. Two of the objects are connected by tidal bridges. The last two objects may simply be interacting with each other over long distance.

Arp number Common name Notes
281 NGC 4627, NGC 4631 Spiral galaxy with companion dwarf elliptical galaxy[125]
282 NGC 169, NGC 169A Spiral galaxy with smaller companion galaxy[126]
283 NGC 2798, NGC 2799 Interacting galaxy pair[127]
284 NGC 7714, NGC 7715 Interacting galaxy pair[128]
285 NGC 2854, NGC 2856 Galaxy pair[129]
286 NGC 5560, NGC 5566, NGC 5569 Interacting galaxy triplet[4][130]

Wind effects

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Arp 293. NGC 6285 and 6286 interacting galaxies in 32 inch telescope.

Although included in the double galaxies category, many of these objects are individual galaxies. The "wind effects" refer to the appearance, not the actual detection of high-velocity gas (such as is found in M82). In some cases, the appearance may be the result of interaction. In other cases, particularly NGC 3981 (Arp 289), the faint, extended emission may be related to the intrinsic nature of the galaxy itself and not interactions with other objects.[4]

Arp number Common name Notes
287 NGC 2735, NGC 2735A Galaxy pair[131]
288 NGC 5221, NGC 5222 Galaxy triplet[132]
289 NGC 3981 Peculiar spiral galaxy[4][133]
290 IC 195, IC 196 Interacting galaxy pair[134]
291 UGC 5832 Irregular galaxy[135]
292 IC 575 Peculiar spiral galaxy[136]
293 NGC 6285, NGC 6286 Interacting galaxy pair[137]

Long filaments

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Arp 297: spiral (NGC 5754) /irregular galaxy (NGC 5752) pair

The long filaments in these systems are probably tidal tails or bridges that have been produced as the result of the gravitational interaction between the galaxies.

Arp number Common name Notes
294 NGC 3786, NGC 3788 Interacting galaxy pair[138]
295 Arp 295 Interacting galaxy pair[139]
296 Arp 296
297 Arp 297 Interacting galaxies within a galaxy group[140]

Unclassified objects

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IC 694 and NGC 3690 (Arp 299), interacting galaxy pair
UGC 9618 (Arp 302), a pair of a face-on and an edge-on spiral galaxy

Arp did not give a subclassification for objects 298–310 in his atlas. These objects are mostly interacting galaxy pairs.

Arp number Common name Description
298 NGC 7469, IC 5283 Galaxy pair[141]
299 Arp 299 Galaxy triplet[142]
300 Arp 300 Galaxy group[143]
301 UGC 6204, UGC 6207 Galaxy pair[144]
302 UGC 9618 Galaxy pair[145]
303 IC 563, IC 564 Galaxy pair[146]
304 NGC 1241, NGC 1242 Galaxy pair[147]
305 NGC 4016, NGC 4017 Galaxy pair[148]
306 UGC 1102 Galaxy group with two galaxy pairs[149]
307 NGC 2872, NGC 2874 Galaxy pair[150]
308 NGC 545, NGC 547 Galaxy pair[151]
309 NGC 942, NGC 943 Galaxy pair[152]
310 IC 1259 Galaxy pair[153]

Groups

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Arp number Common name Description
311 IC 1258 and Companions
312 MCG +08-31-004
313 NGC 3994 + NGC 3995
314 MCG -03-58-009 + MCG -03-58-010 + MCG -03-58-011
315 NGC 2830 + NGC 2831 + NGC 2832
316 NGC 3187 + NGC 3190 + NGC 3193
317 Leo Triplet
318 NGC 833 and companions
319 Stephan's Quintet
320 Copeland's Septet
321 Hickson 40 A-E

Chains

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Arp number Common name Description
322 UGC 6527
323 Hickson 98 A-D
324 UGC 10143
325 ESO601-G018A+B and MCG -04-52-014
326 UGC 8610
327 NGC 1875; Hickson 34 A-D
328 UGC 9532; Hickson 72
329 UGC 6514
330 I Zw 167; MCG +09-27-094
331 NGC 379 and companions (Pisces Cloud)
332 NGC 1228 + NGC 1229 + NGC 1230 + IC 1892

Miscellaneous

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Hubble Space Telescope image of Messier 82
Arp number Common name Description
333 NGC 1024
334 UGC 8498
335 NGC 3509
336 NGC 2685
337 Messier 82
338 PGC 3094767

Brightest Arp galaxies for amateur astronomers

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Maynard Pittendreigh, an amateur astronomer and occasional writer, has compiled a list of the brightest Arp Galaxies that are most easily viewed by typical amateur astronomers. The galaxies on the list can be observed visually and do not require special photographic or imaging equipment. These include:

  • Arp 26, also known as M101
  • Arp 37, also known as M77
  • Arp 76, also known as M90
  • Arp 77
  • Arp 85, also known as M51
  • Arp 116, also known as M60
  • Arp 120
  • Arp 152, also known as M87
  • Arp 153
  • Arp 168, also known as M32
  • Arp 244
  • Arp 269
  • Arp 270
  • Arp 271
  • Arp 281
  • Arp 286
  • Arp 317, also known as M65
  • Arp 313
  • Arp 337, also known as M82

See also

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Further reading

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References

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[edit]

Notes

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Atlas of Peculiar Galaxies

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The Atlas of Peculiar Galaxies is a seminal astronomical catalog compiled by Halton C. Arp and published in , featuring direct photographs of 338 galaxies selected for their unusual, asymmetrical, or interacting structures, primarily captured using the 200-inch at . Arp's atlas aimed to provide a comprehensive visual reference for studying the origins, evolution, and physical processes shaping galaxies, emphasizing empirical groupings based on morphological similarities rather than strict theoretical classifications. The selection process drew from earlier surveys, such as Vorontsov-Velyaminov's Morphological Catalogue of Galaxies, and involved high-resolution imaging with various photographic plates and filters to highlight structural details like bridges, tails, and distortions indicative of gravitational interactions. Organized into five major classes with various subclasses—ranging from peculiar spiral galaxies to amorphous forms and interacting pairs—the atlas illustrates a diverse array of peculiarities that challenge standard galaxy formation models and underscore the role of mergers and collisions in cosmic evolution. Each entry includes a photograph, coordinates, and descriptive notes, making it a foundational resource for subsequent research in . The work, published in the Astrophysical Journal Supplement Series, has influenced generations of astronomers by highlighting the dynamic nature of galaxies and inspiring deeper investigations into their compositions and the forces at play, ultimately contributing to broader understandings of the universe's structure.

Introduction

Definition and Scope

The Atlas of Peculiar Galaxies defines peculiar galaxies as those deviating from the standard morphological classifications of smooth ellipticals and symmetric spirals or disks, instead displaying distortions, asymmetries, and structural anomalies often attributable to gravitational interactions, mergers, or intrinsic evolutionary processes. These features, such as perturbed arms, bridges, tails, or multiple nuclei, set them apart from regular galaxy types and highlight dynamic phenomena in galaxy formation. Compiled by astronomer and published in 1966 in the Astrophysical Journal Supplement Series, the atlas serves as a visual reference to document 338 examples of such , facilitating research into , interaction mechanisms, and the potential role of unconventional physical forces in cosmic structures. emphasized that these peculiarities provide critical insights into the true nature of beyond idealized models, including explorations of non-standard interpretations that challenge conventional distance measures. The scope of the atlas is limited to visually striking objects selected primarily from earlier catalogs like those of Zwicky and Vorontsov-Velyaminov, with imaging obtained using the 200-inch at to emphasize qualitative morphological oddities rather than photometric or spectroscopic metrics. Each entry features direct photographic plates at various magnifications, equatorial coordinates (epoch 1970.0), and concise textual remarks describing the observed anomalies, prioritizing illustrative value for evolutionary studies over exhaustive quantitative analysis. Arp's selection criteria focused on galaxies appearing peculiar under high-resolution scrutiny from Palomar Sky Survey plates.

Historical Development

Halton Arp's interest in galaxy morphologies began during his graduate studies and early career at the (Caltech) in the 1950s. After earning his Ph.D. in 1953, Arp joined the staff of the Mount Wilson and Palomar Observatories, where he specialized in photographic observations of galaxies using the Palomar 48-inch and 200-inch telescopes. His work was motivated by the limitations of the standard Hubble classification scheme, as outlined in the Hubble Atlas of Galaxies (1961), which emphasized regular forms but overlooked the diverse anomalies present in many systems; Arp sought to systematically catalog these irregularities to better understand galaxy evolution beyond the idealized . Arp's project drew initial inspiration from earlier catalogs of unusual galaxies, particularly Boris Vorontsov-Velyaminov's 1959 Atlas and Catalogue of , which documented 355 systems of identified from Palomar Sky Survey plates. While Vorontsov-Velyaminov's work focused primarily on interactions between multiple galaxies, Arp shifted emphasis to individual peculiar features—such as distortions, multiple nuclei, and amorphous structures—within single galaxies or small groups, expanding the scope to include objects not easily explained by gravitational encounters alone. This approach built on pioneering efforts by and Vorontsov-Velyaminov but prioritized high-resolution imaging to reveal subtle morphological details. The Atlas of Peculiar Galaxies was published in as a supplement to , presenting 338 carefully selected objects based on over four years of direct photography with the 200-inch . This catalog emerged amid intense astronomical debates in the and between steady-state cosmology, which posited an eternal, unchanging , and the emerging model, supported by observations of galactic s interpreted as recession velocities. Arp, initially aligned with steady-state proponents like , later leveraged the atlas's examples of physically connected galaxies and quasars with discordant s to argue for intrinsic redshift components, challenging the assumption that all redshifts solely reflect cosmological expansion.

Creation and Methodology

Selection Process

Halton Arp's selection process for the Atlas of Peculiar Galaxies began in 1962, focusing on high-resolution photographs taken with the 200-inch at to investigate the structure and evolution of spiral galaxies. Arp manually inspected prints from the Society-Palomar Observatory Sky Survey, which comprised over 900 pairs of photographic plates covering the down to approximately -30° , along with additional sources such as catalogs by Zwicky and Vorontsov-Velyaminov. This labor-intensive review prioritized galaxies exhibiting the most striking deviations from normal morphology, excluding those with peculiarities attributable to well-understood phenomena like prominent star-forming regions or obvious projection effects. The criteria emphasized visual oddities that suggested dynamic interactions or structural anomalies, including distorted or perturbed arms, multiple nuclei, bridges and tails connecting systems, and amorphous or filamentary shapes. Arp deliberately favored conspicuous examples over considerations of distance, brightness, or completeness, aiming to compile illustrative cases that could reveal underlying physical processes. This approach resulted in 338 selected entries, numbered Arp 1 through Arp 338, with most images derived from the 200-inch plates using blue-sensitive emulsions such as 103a-O, with exposures of 30-70 minutes and magnifications from 1× to 10× to highlight faint features. Arp's methodology reflected his interest in peculiarities that might indicate non-gravitational influences, such as or plasma effects, beyond standard gravitational interactions, to probe the composition of gas, dust, and interstellar media in galaxies. The selection inherently introduced biases, including a focus on the northern sky due to the Palomar survey's coverage and a subjective emphasis on visually striking objects, which underrepresented fainter or less prominent galaxies. These aspects positioned the atlas as a curated sample rather than a statistically complete catalog, drawing later critiques for its incompleteness in representing the full population of peculiar galaxies.

Catalog Structure

The Atlas of Peculiar Galaxies employs a sequential numbering system for its entries, designated as Arp 1 through Arp 338, with galaxies loosely grouped by shared morphological characteristics to facilitate comparative study, though the arrangement is empirical rather than a rigid scheme. Each catalog entry features a black-and-white photograph reproduced from plates, centered on the within a standardized to highlight peculiar structures; accompanying data includes equatorial coordinates in the 1970 epoch (accurate to approximately ±0.2 minutes in and ±2 arcminutes in ), the angular size of the principal or system, photographic magnitude (typically pg band from the Palomar Sky Survey), and concise morphological notes describing the observed anomalies, such as arm distortions or companion interactions. The atlas was published in the Astrophysical Journal Supplement Series (volume 14, 1966) by the , comprising 57 plates of photographs, a detailed index cross-referencing entries to standard catalogs like NGC or IC, and supplementary materials; no digital edition existed at launch, but high-quality scans of the full atlas became publicly accessible online starting in the early 2000s through archives like the /IPAC Extragalactic Database. The catalog's coverage spans declinations from +90° to approximately -30°, limited by the survey's southern boundary.

Classification System

Morphological Categories

The morphological categories in the Atlas of Peculiar Galaxies organize the 338 selected objects into sections based on their visual appearances as captured on photographic plates from the Palomar 200-inch telescope. These sections—peculiar spiral galaxies (Nos. 1–102), spirals with companions attached to arms (Nos. 103–145), galaxies or groups not primarily ellipticals or spirals (Nos. 146–268, including amorphous forms and elliptical-like interactions), groups of galaxy character (Nos. 269–327), double galaxies (Nos. 328–331), and miscellaneous (Nos. 332–338)—emphasize distortions, interactions, and irregularities that deviate from standard forms, allowing researchers to study stages of galactic evolution such as tidal perturbations, bridges, and mergers. The peculiar spirals section includes galaxies with distorted arms or attached companions (totaling 145 entries), while the non-primary E/spiral section (123 objects) features interaction-induced perturbations like shells, tails, amorphous blob-like shapes indicative of advanced disruptions, and elliptical connections. Groups and doubles cover 63 interacting pairs or systems connected by material bridges or overlapping envelopes, and the miscellaneous category includes 7 unique anomalies. This classification system permits overlaps for complex galaxies that display features from multiple sections, reflecting the dynamic nature of interactions rather than rigid typology; for instance, some objects are cross-referenced in Table 1 of the atlas. The rationale stems from direct of high-resolution images, prioritizing morphological peculiarities over spectroscopic data to highlight evolutionary processes. These sections draw inspiration from the , which distinguishes smooth ellipticals from arm-structured spirals, but extend it to accommodate the 5-10% of galaxies that exhibit extreme deviations due to mergers or environmental influences. By focusing on these peculiarities, the atlas provides a framework for tracing interaction timelines, from initial arm distortions in spirals to final amorphous remnants in mergers, advancing understanding of galactic morphology beyond classical schemes. The broad sections contain numerous subclasses, such as peculiar arm shapes for spirals and jets or filaments for amorphous forms.

Evaluation Criteria

Halton Arp evaluated the peculiarities in the Atlas through qualitative assessments based on direct of high-resolution photographs taken with the 200-inch Palomar , prioritizing galaxies that exhibited the greatest deviations from standard morphological forms. These assessments included informal notes on the degree of , the apparent influence of nearby companions, and potential underlying mechanisms such as tidal interactions or material ejections, which were inferred from observed structural anomalies rather than spectroscopic data. Arp's approach emphasized conspicuous visual irregularities to facilitate studies of galactic , without employing any formalized quantitative morphology tools, as such methods were unavailable in the pre-computer era of the . Specific criteria for evaluation centered on qualitative indicators of peculiarity, including marked in galactic arms or disks, prominent filamentary structures extending from the main body, and evidence of interactions like physical connections between galaxies. Descriptions often highlighted discrepancies as secondary notes suggesting possible associations, though these were not primary selection factors and required future verification through photometry or . Arp employed descriptive terminology to characterize these features, such as "bridge" for narrow connections linking galaxies, "tail" for elongated tidal extensions, and "absorption lane" for dark lanes disrupting luminous regions, providing a for informal scoring of interaction evidence based on visual prominence. The evaluation process was inherently subjective, relying on Arp's personal judgment during plate inspections, which grouped objects by visual similarity rather than rigorous schemes. This subjectivity drew later critiques from peers, who argued that the Atlas lacked sufficient statistical rigor to represent broader galactic populations, as it sampled only a small fraction (about 1-2%) of cataloged galaxies and treated peculiarities as anecdotal "freaks" without robust analytical support. Despite these limitations, the qualitative framework laid foundational observations for subsequent quantitative studies of interacting systems.

Spiral Galaxies

Low Surface Brightness

Low spirals in the Atlas of Peculiar Galaxies comprise a distinct category of six entries (Arp 1–6), selected for their unusually faint and extended disks that challenge traditional detection methods. These galaxies feature low-contrast spiral arms that blend into the , with central typically exceeding 22 mag arcsec⁻² in the B-band, making them appear diffuse and structureless on early photographic plates. Representative examples include Arp 1 (NGC 2857), a face-on spiral requiring high-contrast to reveal its faint arms, and Arp 6 (NGC 2537, also known as the Bear Paw Galaxy), which displays a , low-density structure influenced by a nearby companion. This category underscores the diversity of spiral morphologies, where faintness highlights subtle dynamical peculiarities rather than high-contrast features. The formation of these low surface brightness spirals is attributed to gravitational interactions that tidally stretch stellar and gaseous components, redistributing material into extended, low-density configurations while potentially obscuring through increased dust content. Mergers or close encounters with companions can heat the disk, suppressing gravitational instabilities and leading to reduced rates, which further diminishes the overall . In the Atlas, several entries show signs of such interactions, such as bright knots or faint dwarfs, illustrating how dynamical processes transform ordinary spirals into faint, peculiar systems. This mechanism emphasizes the role of environmental influences in altering , distinct from self-regulated arm formation in brighter spirals. Observations of these galaxies reveal their elusive nature: on Palomar Observatory Sky Survey plates used by Arp, they manifest as vague, low-contrast patches, often requiring enhanced prints to discern spiral patterns. Modern imaging has uncovered hidden details, such as embedded H II regions and tidal distortions, in analogous low systems, demonstrating that interactions induce low star formation efficiency and reveal underlying structures invisible in ground-based data. These insights highlight the Atlas's value in identifying galaxies where peculiarity stems from photometric subtlety rather than dramatic morphology.

Split Arms

Split arms refer to a morphological peculiarity in spiral galaxies where one or more spiral arms divide into two distinct branches, creating a forked or bifurcated appearance that deviates from the standard two-armed or multi-armed symmetry. This feature is prominently displayed in the Atlas of Peculiar Galaxies under a dedicated subcategory, comprising six entries cataloged as Arp 7 through Arp 12. A representative example is Arp 10 (PGC 8802, also known as UGC 1775), which exhibits multiple bifurcated arms extending from a bright nucleus, resembling an exaggerated version of the well-known interacting system M51 but with more pronounced splitting and fragmentation along the arm lengths. These structures often appear in galaxies that are isolated or in loose groups, though many entries, such as Arp 10, are associated with nearby companion galaxies that contribute to the observed distortions. The primary mechanism driving split arms is gravitational tidal perturbations from close encounters with companion galaxies or other mass concentrations, which induce differential stresses in the galactic disk and lead to arm fragmentation. These interactions compress and shear the , causing spiral density waves to branch or fork as material is pulled into secondary streams, a process analogous to the formation of tidal tails but confined more closely to the disk. Numerical simulations of such encounters demonstrate that these bifurcated features evolve on timescales of approximately 10810^8 years, during which the arms wind up, fragment further, or dissipate before the galaxy potentially relaxes into a more symmetric form. This dynamical evolution underscores the transient nature of split arms as indicators of relatively recent gravitational disruptions. Observationally, split arms are most evident in blue-sensitive photographic plates, where the enhanced contrast from young, massive stars and recent along the bifurcated regions highlights the splits against the fainter underlying disk. The prominence in blue light suggests that these features signal ongoing or recently triggered interactions, as tidal perturbations often stimulate bursts of in the compressed arm segments, making the forks appear sharper and more defined compared to redder wavelengths that emphasize older stellar populations. In the Atlas, such details were captured using high-resolution prints from the Sky Survey, revealing the splits as straight or gently curving branches that extend outward from the primary arm paths.

Detached Segments

In the Atlas of Peculiar Galaxies, detached segments characterize spiral galaxies where portions of the spiral arms appear disconnected from the main structure, manifesting as gaps, isolated knots, or filamentary extensions that resemble separate entities. These features often give the impression of fragmented arms, potentially due to obscuration by dust lanes or actual physical separation of stellar and gaseous material. A representative example is Arp 16 (NGC 3627, also known as M66), a barred spiral in the where the northern arm segment appears notably detached, contributing to its irregular appearance. Such detached segments are typically attributed to high-velocity gravitational encounters with companion galaxies or intruding material, which strip and redistribute gas and from the arms, leading to these disruptions. These interactions suggest the galaxies are post-merger remnants or undergoing ongoing perturbations, with the detached features indicating recent dynamical . The category includes six entries in the original catalog (Arp 13 through 18), highlighting a specific subset of arm distortions among the broader morphological peculiarities observed in spirals. These systems are frequently associated with extended neutral hydrogen (H I) tails, as seen in the where M66 exhibits tidal debris linking it to neighbors like NGC 3628. Radio observations, particularly in the 21 cm H I line, resolve these detached segments by revealing the underlying gas distribution, often showing clumpy, extended structures and velocity gradients indicative of material being pulled away and redistributed during the interaction. For instance, in M66, such mapping uncovers infalling gas toward the disk alongside outflowing tails, underscoring the role of tidal forces in sculpting these features.

Three-Armed Spirals

Three-armed spirals in the Atlas of Peculiar Galaxies deviate from the standard two-armed structure of most spiral galaxies, exhibiting three distinct spiral arms that disrupt the typical bilateral symmetry observed in grand design spirals. These configurations often display unequal arm lengths or widths, with one arm frequently appearing more prominent or developed than the others, contributing to their peculiar appearance. A representative example is Arp 19 (NGC 145), an asymmetric face-on spiral where three prominent arms extend from the core, accompanied by fainter remnants suggesting additional partial structures. The formation of three-armed spirals is generally attributed to asymmetric perturbations in the galactic disk, such as those induced by minor mergers or satellite interactions, which excite non-axisymmetric density waves. In density wave theory, these features correspond to m=3 modes, where m represents the azimuthal wavenumber or number of arms, contrasting with the more stable m=2 modes dominant in isolated spirals. Higher-order modes like m=3 are less common because they experience stronger damping at Lindblad resonances, leading to reduced longevity compared to two-armed patterns. In the Atlas, the three-armed spiral category comprises a small number of entries—specifically Arp 19, Arp 20, and Arp 21—highlighting their rarity among peculiar galaxies, particularly in isolated environments where symmetric two-armed structures prevail. This scarcity underscores the role of external disturbances, such as minor mergers, in triggering these transient features, as isolated disks tend to favor even-numbered arm symmetries driven by . Observations and simulations indicate that three-armed spirals are short-lived, with dynamical models predicting persistence times on the order of 10^7 to 10^8 years before or evolution into more conventional forms.

One-Armed Spirals

One-armed spirals in the Atlas of Peculiar Galaxies represent a distinct morphological category characterized by a single dominant spiral that is significantly brighter and more extended than any faint opposing structures, often appearing tubular, straight initially, and then bent or bifurcated with secondary features like bright knots. This asymmetry contrasts with typical multi-armed spirals and highlights gravitational instabilities or external perturbations in disk dynamics. Representative examples include Arp 22 (NGC 4027), which features a prominent tubular emerging from a bright nucleus with a faint secondary , and Arp 23 (NGC 4618), where the lone sweeps around a distorted core near its companion NGC 4625. The formation of these structures is primarily driven by tidal interactions with nearby companion galaxies, where differential tidal torques align stellar and gaseous material into a coherent single arm, rendering the opposite side faint or absent. Observational studies confirm that nearly all known one-armed spirals, including those in the Atlas, exhibit of such companions, suggesting this configuration is a transient phase resulting from recent gravitational encounters rather than intrinsic stability. For instance, in NGC 4027 (Arp 22), high-resolution mapping reveals interaction signatures like a warped ring structure, indicative of ongoing material transfer. Spectroscopic observations utilizing Doppler shifts provide key insights into the of these galaxies, demonstrating that the prominent arm rotates in a direction consistent with tidal forcing from the companion, often with elevated gas velocities along the arm indicating compression and . Such data support swing-amplification models for arm development under asymmetric perturbations, while the presence of warped or ring-like features in cases like Arp 22 hints at polar accretion or infall of external gas, prolonging the peculiar morphology. The Atlas catalogs three primary examples in this category (Arp 22–24), emphasizing their rarity among peculiar spirals and their value for studying interaction-driven evolution.

One Heavy Arm

Spiral galaxies classified under the "one heavy arm" category in the Atlas of Peculiar Galaxies exhibit a distinctive where one spiral arm is significantly thickened and denser compared to the others, often appearing bulged and containing concentrations of star clusters. This category includes six entries, Arp 25 through Arp 30, which contrast sharply with typical spirals featuring uniform arms, highlighting localized structural perturbations. The heavy arm typically stands out in optical images due to its enhanced brightness and prominence, as seen in high-resolution observations from the Palomar 200-inch telescope used in the atlas compilation. A representative example is Arp 28 (NGC 7678), a approximately 164 million light-years away in , with a diameter of about 115,000 light-years, comparable to the . In this galaxy, the heavy is straight and dominant, extending prominently from the central bar, while the opposite is fainter and more diffuse, creating a lopsided appearance. Similar features appear in Arp 25 (NGC 2276), where the heavy is tubular, initially straight before bending, and includes a secondary branch. These structures differ from purely one-armed spirals by retaining multiple arms, albeit with pronounced density variations in one. The primary cause of these heavy arms is localized compression resulting from gravitational encounters with nearby galaxies, which distort the disk and enhance density in specific regions. For instance, in NGC 2276, interaction with the NGC 2300 has led to the arm's asymmetry and thickening. Such perturbations trigger elevated rates along the heavy arm, evidenced by bright knots and H II regions, as observed in Arp 26 (M101) and Arp 29 (), where the specific rate exceeds that of symmetric spirals. Observations reveal infrared excess in these arms due to lanes obscuring background and re-emitting absorbed , particularly prominent in NGC 7678's heavy arm with directional dust clouds and in 's regions showing anomalous emission at 26-40 GHz.

Integral Sign Shape

The integral sign shape refers to a class of peculiar spiral galaxies in the Atlas of Peculiar Galaxies characterized by a warped disk that bends into an S- or Z-like form, resembling the mathematical integral symbol (∫). These warps typically affect the outer regions of the disk, creating a global curvature where one side of the galaxy tilts upward and the other downward relative to the inner plane. A representative example is Arp 31 (IC 167), where the spiral arms exhibit this distinctive bending, highlighting the structural distortion without fragmentation or multiple arms. This morphology arises primarily from gravitational exerted by a passing companion or , which misaligns the disk and induces a bending mode. The interaction transfers , causing the outer disk to precess differentially and settle into the characteristic integral sign configuration over time. Such warps are often precessional in nature, maintained by the torque's ongoing influence rather than transient effects. The Atlas includes six entries in this category (Arp 31–36), underscoring their relative rarity among peculiar spirals, though similar warped structures are common in galaxy clusters where interactions are frequent. These features align with broader morphological observed in spirals, but emphasize the symmetric, global distinct from local asymmetries. Observations of these galaxies reveal kinematic twists in their velocity fields, where the axis appears to change across the disk, confirming the three-dimensional warp structure. Integral-field shows misaligned isovelocity contours in the outer regions, with gradients indicating the precessional motion and torque-induced .

Low Surface Brightness Companions

Low surface brightness companions refer to a class of peculiar spiral galaxies in the Atlas where faint, extended dwarf satellites interact with the primary galaxy, often distorting its spiral arms through gravitational . These companions are diffuse and low in surface brightness, typically appearing as irregular or amorphous structures attached to or near the arms of the host spiral, leading to asymmetries or bends in the arm structure. The original classification highlights how these faint objects, due to their extended nature, produce subtle perturbations compared to brighter companions. Representative examples include Arp 41 (NGC 1232 with its companion NGC 1232A), where the satellite is a small, low spiral wound in the same sense as the host, with visible arm splitting indicative of tidal influence. The category includes 12 representative examples (Arp 37 through Arp 48), showcasing spirals such as Arp 41, where the satellite is a small, low spiral wound in the same sense as the host, with visible arm splitting indicative of tidal influence. These systems emphasize slow, ongoing interactions where the diffuse companion accretes material gradually, contributing to the formation of extended stellar or gaseous s around the primary galaxy. The companions themselves are characteristically gas-rich dwarf galaxies with low , traits common to low dwarfs that retain significant neutral hydrogen reservoirs relative to their . Observations in the 21 cm line reveal extended H I s in these systems, often spanning beyond the optical disks and tracing tidal or infalling gas from the interaction—for instance, NGC 1232 exhibits a large neutral gas extending well beyond its visible extent, supporting models of material exchange. Such features underscore the role of these faint satellites in fueling prolonged or morphological evolution in the host spirals.

Small High Surface Brightness Companions

Small high surface brightness companions refer to compact, dense galaxies or stellar knots that or interact with spiral host galaxies, characterized by their elevated relative to the surrounding galactic structure. These companions typically appear as bright, knot-like features positioned along the spiral arms, distinguishing them from more diffuse counterparts. In Halton Arp's Atlas of Peculiar Galaxies, this morphological category highlights systems where the companions' high density leads to distinct visual prominence, often resembling nearly stellar condensations embedded in the arm. A representative example is Arp 50 (NGC 450), where a small, high surface brightness companion perturbs the spiral arm of the host galaxy. The interaction manifests as localized perturbations in the spiral arms, such as bends, brightenings, or tidal distortions confined to the vicinity of the companion, rather than affecting the overall galactic disk. These effects arise from the gravitational influence of the small companion, disrupting gas and stars in the arm on small scales. The Atlas includes 30 such entries (Arp 49–78), though focused studies often highlight subsets with pronounced starburst activity in the companions' nuclei, driven by triggered star formation during the close encounter. These starbursts contribute to the companions' high surface brightness through intense episodes of massive star birth. Additionally, the proximity of these systems implies short dynamical timescales, typically on the order of hundreds of millions of years, allowing rapid evolution of the observed features. Observations in the ultraviolet regime reveal these companions as particularly bright, owing to the emission from hot, young stars formed in the starburst regions.

Large High Surface Brightness Companions

The Atlas of Peculiar Galaxies categorizes spiral galaxies interacting with large, high surface brightness companion galaxies under entries Arp 79 through Arp 91, encompassing 13 systems where the companions are comparable in size to the primary spiral and often appear attached to or near the spiral arms. These interactions produce pronounced morphological distortions, including warped arms and enhanced star formation along the interaction interfaces, due to the strong gravitational influence of the massive companions. For instance, Arp 85 (M51 and NGC 5195) showcases the characteristic distortions from close encounters between comparable-mass galaxies. Dynamically, these systems exhibit mutual tidal responses, with the gravitational perturbations generating extended tidal tails and connecting bridges of material between the primary and companion galaxies. In Arp 85 (M51 and ), for instance, a prominent tidal bridge links the face-on spiral to its elliptical-like companion, facilitating the transfer of gas and stars that fuels ongoing starburst activity. Simulations of such encounters indicate that the companions' high arises from concentrated stellar populations, and the interactions can lead to , positioning many as future merger candidates within hundreds of millions of years. Observations of these pairs reveal symmetric bridges in several cases, such as the balanced tidal features in interacting systems within this category, where both galaxies display elongated arms curving toward each other without dominant asymmetry. Unlike interactions with smaller high companions, which often result in localized perturbations, these large-companion systems promote global restructuring of the disk. Additionally, the enhanced gas inflows in these environments correlate with increased (AGN) activity, as evidenced by elevated and radio emissions in interacting pairs compared to isolated spirals.

Elliptical Companions

In the Atlas of Peculiar Galaxies, the category of elliptical companions encompasses spiral galaxies that display apparent gravitational influences from nearby smooth elliptical galaxies, typically positioned along or adjacent to the spiral arms. These systems, cataloged as Arp 92 through 101, highlight morphological distortions in the spirals, such as bent or perturbed arms, attributed to tidal interactions with the compact, gas-deficient elliptical partners. A representative example is Arp 96, where the elliptical galaxy NGC 7547 appears to warp the arms of the interacting spiral NGC 7541, creating an asymmetric structure visible in optical images. These interactions are characterized as dry mergers, involving a gas-poor elliptical colliding or passing close to a gas-rich spiral, which can strip from the spiral and quench in affected regions. The elliptical's smooth profile contrasts sharply with the spiral's features, often accentuated by color differences: the redder, older stellar populations of the elliptical against the bluer, star-forming disk of the spiral. plays a key role, causing the elliptical companion to lose orbital energy and spiral inward, gradually slowing its relative motion and enhancing the perturbation over time. Observations of these pairs frequently reveal shell-like structures in the elliptical components, faint concentric arcs formed from stellar debris of prior minor mergers in the elliptical's history, providing of repeated interactions. There are ten such entries in the atlas, emphasizing the role of elliptical-spiral encounters in driving peculiar morphologies without prominent bridges or tails typical of gas-rich mergers.

Elliptical and Elliptical-Like Galaxies

Connections to Spirals

In the Atlas of Peculiar Galaxies, elliptical and elliptical-like galaxies physically connected to spiral galaxies are characterized by prominent tidal bridges or overlapping structures that indicate ongoing gravitational interactions. These bridges consist of stellar and gaseous debris pulled from the interacting galaxies during close encounters, often appearing as faint, elongated filaments linking the compact elliptical to the more extended spiral arms. A representative example is Arp 102 (also known as Zwicky's Connected Galaxies), where an elliptical galaxy is joined to a distorted spiral by a clear tidal bridge, demonstrating the morphological distortions induced by such mergers. The mechanism underlying these connections typically involves a recent infall event, where one approaches the other, leading to the formation of tidal debris trails as stars and gas are stripped from their orbits. In systems like Arp 105, this interaction facilitates the transfer of gas from the spiral to the elliptical component, enriching the latter with that is otherwise scarce in isolated ellipticals. Such gas inflows can trigger episodes of within the elliptical or along the bridge, as evidenced by the presence of young stellar clusters and emission-line regions. The Atlas documents seven primary examples in this subcategory (Arp 102–108), highlighting their role in understanding evolution through direct physical linkages. Spectroscopic observations of these connected systems reveal Hα emission along the tidal bridges, indicating ionized gas and ongoing star-forming activity driven by the interaction. For instance, in Arp 104, Hα and HI mapping shows clumpy emission in the bridge, consistent with compressed gas clouds collapsing to form stars. These features underscore the dynamic nature of elliptical-spiral pairs, where material exchange not only alters morphologies but also rejuvenates quiescent ellipticals with fresh fuel for nuclear activity or peripheral starbursts.

Repelling Spiral Arms

In the Atlas of Peculiar Galaxies, the class of elliptical galaxies repelling describes configurations where an elliptical or elliptical-like galaxy appears positioned such that the of an associated are bent or deflected away from it, creating an illusion of repulsion. These systems typically show the elliptical companion located near the inner region of the spiral, with the arms curving outward as if pushed aside by the companion's influence. A prominent example is Arp 112 (NGC 7805 and NGC 7806), where the NGC 7805 lies adjacent to the face-on NGC 7806, and the latter's arms exhibit clear distortions bending away from the companion. This class comprises four entries (Arp 109–112), all displaying relatively high differences between the elliptical and spiral components, often exceeding 100 km/s in some cases, which challenges simple gravitational binding models. proposed that these configurations result from ejective processes, where material or smaller galaxies are expelled from the nucleus of the parent elliptical, leading to the observed arm distortions without ongoing physical contact. Contemporary analyses attribute the apparent repulsion primarily to projection effects along the or the lingering tidal effects of a past , where the elliptical passed through or near the spiral's disk, warping the arms without current dynamical coupling. HI mapping of these systems, such as in Arp 112, reveals no detectable gas bridges connecting the galaxies, consistent with separated or stripped components following an interaction. Kinematic studies further indicate radial gas motions within the distorted spiral arms, with velocity gradients up to 50 km/s attributable to tidal shearing rather than expulsion.

Perturbing Nearby Spirals

Elliptical galaxies in close proximity to spiral galaxies, without direct physical connections, exert gravitational influence that distorts the spirals' morphology, as cataloged in Arp 113–132 of the Atlas of Peculiar Galaxies. These 20 entries highlight systems where the elliptical's tidal forces induce warps and bends in the spiral arms over extended timescales, demonstrating non-collisional perturbations that reshape galactic disks. A prominent example is Arp 114, comprising the spiral NGC 2276 and the elliptical NGC 2300, where the elliptical's gravitational pull has warped the southeastern region of the spiral, creating an asymmetric structure and enhanced along the perturbed arms. This interaction occurs within the NGC 2300 loose cluster-like environment that promotes such close encounters among member galaxies. Dynamical studies reveal velocity gradients in the spiral's disk, with Hα observations showing deviations from regular rotation, indicative of the elliptical's ongoing tidal torque without a merger. These perturbations are characteristically slow, evolving over hundreds of millions of years, and contrast with more violent encounters by preserving the elliptical's smooth appearance while subtly altering the spiral's and dust lanes. In about half of the cases, such as Arp 119, the distortions manifest as S-shaped warps, underscoring the role of differential gravitational forces in cluster settings. Spectroscopic data from affected spirals often exhibit anomalous radial velocities up to several hundred km/s, confirming the perturbing influence without evidence of material exchange.

Emanating Material

The "Emanating Material" category in the Atlas of Peculiar Galaxies highlights and elliptical-like galaxies displaying filaments or shells of material extending from their nuclei. These features suggest dynamic processes such as active galactic nuclei (AGN) activity or past mergers ejecting material. A prominent example is Arp 137 (NGC 2914), a dusty showing emanating structures interpreted as outflows or tidal debris. The origins of these emanating structures are primarily linked to AGN jets or ejecta resulting from mergers. The Atlas includes nine such entries (Arp 137–145), several of which exhibit extended radio lobes indicative of relativistic jets from the AGN. Halton Arp interpreted these configurations as evidence supporting non-cosmological redshifts, arguing for physical associations rather than vast distances. Observational evidence for these phenomena includes emissions tracing hot, outflowing gas. These features, combined with radio and optical data, underscore the role of energetic outflows in shaping the peculiar morphologies observed in this category.

Nearby Fragments

The Nearby Fragments subcategory within the Atlas of Peculiar Galaxies documents elliptical and elliptical-like galaxies featuring small, detached blobs or knots positioned near the primary structure, often appearing as compact, isolated condensations. These features are exemplified by Arp 134 (NGC 4472, also known as Messier 49), where faint companions such as UGC 7636 and UGC 7639 manifest as discrete fragments adjacent to the bright elliptical core, and Arp 135 (NGC 1023), which includes the dwarf irregular NGC 1023A as a prominent nearby knot. Similar detached structures are observed in cases like Arp 133 (NGC 541), highlighting compact, separated components that disrupt the otherwise smooth morphology of ellipticals. Such fragments are generally interpreted as products of tidal stripping during - interactions or remnants from mergers and collisions, where gravitational forces eject or strip material from the parent , leaving behind gravitationally bound clumps. In the case of NGC 1023 and its companion NGC 1023A, dynamical models suggest a recent tidal encounter has shaped the fragment's form, potentially depleting its while preserving its core. These origins underscore the dynamic evolution of elliptical systems, where interactions introduce asymmetries not seen in isolated . The Atlas catalogs four primary entries in this subcategory (Arp 133 through Arp 136), though related observations extend to additional systems resembling globular cluster-like condensations with limited dynamical coherence. Observational evidence, particularly from , reveals blue colors in these fragments indicative of recent ; for instance, NGC 1023A hosts young star clusters aged 125–500 million years, featuring strong Balmer absorption lines consistent with youthful populations. These transient features are expected to have short survival times, as their fragile structures may dissipate within a few gigayears under ongoing gravitational influences, distinguishing them from stable globular clusters in quiescent ellipticals.

Amorphous Galaxies

Associated Rings

In amorphous galaxies, associated rings manifest as nearly circular structures encircling a central core, often appearing as expansive shells detached from the main body. These features are typically observed in systems where the ring is positioned around an amorphous or early-type companion, as exemplified by Arp 147, a prominent ring galaxy pair in the constellation Cetus located approximately 440 million light-years away. The formation of these rings is generally attributed to head-on collisions or mergers, in which a smaller intruder galaxy passes through the disk of a larger host galaxy along its axis of rotation, compressing and ejecting material to form an expanding shell. This dynamical interaction disrupts the host's , creating the ring as stars and gas oscillate outward in a coherent . The Atlas of Peculiar Galaxies includes several such entries in its amorphous category, with Arp 146 through 148 serving as canonical examples; polar ring variants, where the ring orbits perpendicular to the galaxy's primary disk, remain rare overall. Expansion velocities for these rings are measured around 100–225 km/s, providing timescales for the collision events on the order of tens of millions of years. Spectroscopic and imaging observations highlight active concentrated in discrete beads or clumps along the ring, appearing as bright blue knots amid the otherwise diffuse structure. In Arp 147, these regions indicate a recent burst of with a rate of approximately 8 solar masses per year, triggered by the density waves in the expanding shell.

Jets

In the Atlas of Peculiar Galaxies, jets are prominent features observed in amorphous galaxies, manifesting as linear, often straight or slightly curved extensions projecting from the galactic nucleus. These structures appear as narrow, filamentary protrusions against the irregular, featureless body of the galaxy, distinguishing them from more diffuse emissions or tidal tails. A representative example is Arp 152 (M87, also known as Virgo A), where a prominent straight jet emanates from the nucleus, visible in optical and radio images. The origins of these jets are linked to energetic processes within the galaxy core, primarily active galactic nucleus (AGN) activity or starburst-driven superwinds. In cases like Arp 152, the jets are interpreted as outflows powered by intense star formation rates exceeding 100 solar masses per year, generating superwinds that expel material at velocities up to several hundred km/s, or by buried AGN launching relativistic plasma. Radio observations confirm counterparts for many such features, with compact nuclear sources extending into larger-scale lobes. Jet lengths typically reach several kiloparsecs, up to approximately 10 kpc in extended systems, providing evidence of large-scale feedback impacting the interstellar medium. The Atlas catalogs 4 such entries under amorphous galaxies with jets (Arp 149-152), highlighting their role in morphological peculiarities. Synchrotron emission dominates the radio signatures of these jets, arising from relativistic electrons spiraling in ordered within the outflows. In Arp 152, observations reveal synchrotron-dominated radio lobes aligned with the optical jets, extending over 10-15 kpc and indicating non-thermal processes with spectral indices around -0.7. This emission, combined with detections of hot plasma, underscores the jets' contribution to galactic evolution by regulating and dispersing enriched material.

Interior Absorption

In the context of amorphous galaxies within the Atlas of Peculiar Galaxies, interior absorption manifests as prominent dark lanes or central patches of dust that obscure the underlying stellar and gaseous components, creating a mottled or shadowed appearance in optical images. These features are highlighted in Arp's subcategory of disturbed galaxies with interior absorption, encompassing eight entries from Arp 153 (NGC 5128, ) to Arp 160 (NGC 4194). Such absorption is typically attributed to dust lanes formed during galactic mergers or close encounters, where gravitational interactions compress interstellar material into dense, optically thick structures that block visible light while allowing re-emission in the . A representative example is Arp 154 (NGC 520), a merging pair exhibiting extensive central absorption that enshrouds vigorous , with the creating silhouette-like dark regions against the galaxy's brighter on photographic plates. These systems are often infrared-luminous, signaling buried starbursts powered by the influx of gas triggered by the interaction, as the reprocesses ultraviolet radiation from young stars into longer wavelengths. Observations reveal that interior absorption not only alters the visual morphology but also indicates dynamic evolutionary processes, such as material inflow toward the nuclei, distinguishing these galaxies from more uniform amorphous forms.

Diffuse Filaments

Diffuse filaments represent a morphological subclass of amorphous galaxies in the Atlas of Peculiar Galaxies, characterized by broad, low-contrast tails that appear as fuzzy, thread-like extensions from the galaxy's irregular body. These structures exhibit low and wide extents, distinguishing them from sharper or narrower features, and are typically observed in galaxies undergoing environmental interactions. The Atlas catalogs 6 such entries (Arp 161-166), emphasizing their role in demonstrating disrupted gas distributions in peculiar systems. A prominent example is Arp 166 (NGC 750/751), where a wide, diffuse stretches prominently from the main amorphous body, illustrating the category's defining low-contrast appearance. This spans several kiloparsecs and fades gradually into the surrounding , highlighting the filament's broad and nebulous nature. These filaments originate from the stripping of interstellar gas as amorphous galaxies traverse dense galaxy clusters, where the exerts on the infalling gas reservoir. This process, first formalized in theoretical models, removes loosely bound material from the galaxy's outskirts, forming extended tails aligned with the direction of motion. The galaxies in this subclass are notably rich in neutral hydrogen (H I), with radio observations revealing extended H I envelopes that trace the diffuse structures and confirm ongoing stripping. Such extended neutral gas distributions, often spanning tens of kiloparsecs, provide key evidence for ram-pressure effects in cluster environments, where the H I content remains elevated compared to more processed cluster members.

Diffuse Counter-Tails

Diffuse counter-tails represent a subclass of amorphous galaxies in the Atlas of Peculiar Galaxies, characterized by symmetric, broad tails extending on opposite sides of the central body, often appearing as low-surface-brightness extensions indicative of disrupted morphology. These features suggest significant disturbance to the galaxy's structure, with the tails displaying diffuse, filamentary appearances that differ from more defined or unpaired structures seen in other filament types. The atlas catalogs six such examples, spanning Arp 167 through Arp 172, highlighting their rarity among peculiar systems. A representative case is Arp 171 (NGC 5718), where the amorphous galaxy shows paired diffuse tails emerging symmetrically, resolved in deep imaging as extended, low-brightness features trailing from the core. These galaxies are interpreted in modern observations as jellyfish galaxies, where the tails result from ram-pressure stripping caused by the galaxy's motion through the (ICM). The ICM flow exerts hydrodynamic forces on the , stripping gas and forming broad, opposing tails as the galaxy infalls toward a cluster core. Observations reveal that the tails trail in the direction opposite to the galaxy's motion relative to the ICM, with typical relative velocities on the order of 300 km/s driving the stripping process. This mechanism explains the symmetric appearance in some cases, as balanced flows produce counteracting extensions, and is supported by kinematic studies showing disturbed gas velocities aligned with the tail orientations.

Narrow Counter-Tails

Narrow counter-tails represent a subclass of amorphous galaxies in the , characterized by slender, paired tidal streams extending in opposing directions from the interacting systems. These narrow features arise from gravitational encounters that strip material along specific trajectories, producing thin extensions rather than broad distortions. A representative example is Arp 174 (NGC 3068), where a prominent, narrow counter-tail emanates from the primary , accompanied by a fainter opposing stream from its companion. The formation mechanism involves focused tidal stripping during close passages, where differential gravitational forces eject gas and in streamlined configurations, often enhanced by the of the . This contrasts with broader tidal features, as the narrowness suggests more directed material removal, potentially influenced by the progenitors' morphologies—such as disk galaxies yielding elongated tails. Observations indicate these tails are optically faint, with surface brightnesses typically below 25 mag/arcsec² in B-band, necessitating long-exposure imaging for detection; surveys reveal minimal ongoing in many cases, consistent with aged stellar populations. The Atlas catalogs six such systems (Arp 173–178), though focused studies highlight four prominent amorphous examples with well-defined paired tails. These galaxies frequently reside in higher-density environments like galaxy groups, where environmental effects such as may truncate tail lengths to under 50 kpc, shorter than those in isolated mergers.

Narrow Filaments

Narrow filaments represent a subclass of amorphous galaxies characterized by isolated, fine thread-like structures extending from the main body, often appearing as faint, single thin streams against the galactic disk. These features are typically narrow and elongated, distinguishing them from broader or paired extensions, and are observed in galaxies lacking clear spiral or elliptical morphology. In the Atlas of Peculiar Galaxies, this category includes 15 examples cataloged as Arp 179 through Arp 193, selected to illustrate such peculiar extensions in otherwise structureless forms. The origins of these narrow filaments are attributed to either ejective processes, where material is expelled from the galactic nucleus, or tidal interactions with companion galaxies, which stretch out thin streams of gas and stars. suggested that some filaments may arise from non-gravitational forces, challenging purely dynamical explanations and implying energetic ejections or magnetic influences in galaxy evolution. For instance, in Arp 184 (NGC 1961), two long narrow filaments extend prominently, showing faint emission consistent with ionized components. Spectroscopic observations of these filaments frequently reveal emission lines, indicative of ionized gas flows along the structures, which supports interpretations involving active outflows or shocked material. Such lines, including Hα and forbidden lines, highlight the dynamic nature of the gas, with velocities suggesting acceleration beyond simple gravitational .

Ejected Nuclear Material

In Halton Arp's Atlas of Peculiar Galaxies (), the category of galaxies with material ejected from nuclei encompasses Arp 194 through Arp 208, featuring 15 amorphous galaxies characterized by central outflows extending from their cores. These structures often manifest as filamentary or amorphous extensions, suggesting explosive or dynamic ejection events originating at the nuclear regions, with some displaying bipolar symmetry in the expelled material. Representative examples include Arp 195 (UGC 4653), where outflowing material appears as a prominent northern spike from the nucleus, and Arp 196, noted for its clear bipolar ejection pattern. Modern observations attribute these ejections to feedback processes driven by intense in nuclear starbursts or active galactic nuclei (AGN), which energize and expel gas from the galactic core. In such systems, the outflows regulate by removing molecular gas reservoirs, potentially quenching further activity. For instance, Arp 202 (NGC 7252), an amorphous merger remnant with a highly active nucleus, exemplifies this phenomenon through its central outflows powered by a compact starburst and possible buried AGN. Interferometric mapping in CO emission lines has revealed the molecular composition of these outflows, confirming bipolar ejections of dense gas in several cases within the category. High-resolution studies, such as those of Arp 202's nuclei, detect shocked molecular gas extending hundreds of parsecs, with velocities indicating energies comparable to the galaxies' rates, underscoring the role of feedback in shaping these peculiar morphologies. These observations distinguish nuclear ejections from extended radio jets by their lack of synchrotron emission and focus on thermal, gas-dominated processes.

Irregularities and Absorption

In the Atlas of Peculiar Galaxies, the subcategory of irregularities and absorption encompasses amorphous galaxies characterized by mottled surfaces and prominent absorption knots, where dark dust features interrupt the otherwise chaotic stellar distributions. These irregularities manifest as patchy, uneven brightness across the galaxy's disk, often appearing as irregular clumps or knots of obscuring material that give the galaxies a textured, disrupted appearance. A representative example is Arp 209 (NGC 6052), an interacting system displaying strong emission regions interspersed with dark absorption features, highlighting the turbulent disruption in its structure. The primary mechanism driving these features is attributed to gravitational interactions during galaxy mergers, which induce dynamical in the , leading to the formation of dust lanes and patchy absorption as gas clouds are compressed and redistributed. This process disrupts the smooth morphology, creating the observed mottled patterns through tidal forces and subsequent shock heating. In Arp's catalog, this subcategory includes six entries (Arp 209–214), though the visibility of finer irregularities is highly resolution-dependent, with high-resolution plates from large telescopes revealing details invisible in lower-quality images; additionally, plate artifacts such as artificial halos from photographic dodging techniques were noted as potential confounders in early observations. Modern multi-wavelength observations have elucidated hidden aspects of these galaxies, penetrating the optical absorption to uncover embedded star-forming regions. For instance, mid-infrared spectroscopy of Arp 209 with the detects strong emissions and silicate absorption features, indicating intense starburst activity obscured by dust in visible light, thus providing a clearer view of the underlying turbulent dynamics. These studies contrast with more centralized dark lanes seen in interior absorption cases, emphasizing the widespread surface patchiness here.

Adjacent Loops

In the Atlas of Peculiar Galaxies, adjacent loops refer to curved, loop-like extensions observed in amorphous galaxies that are not clearly classifiable as spirals or ellipticals, typically appearing as tidal features attached to or near the primary galactic structure. These loops are empirical groupings based on visual similarities in photographic plates, with six entries cataloged under this category (Arp 215–220). A representative example is Arp 215 (NGC 2782), a peculiar SABa galaxy exhibiting a prominent loop-like tidal tail extending from its distorted core, indicative of recent dynamical perturbation. The origins of these adjacent loops are attributed to gravitational interactions during galaxy mergers, where orbital motions of colliding systems produce elongated tidal debris that curves into loop structures due to differential gravitational forces. In such events, material is stripped from the disks, forming these features as remnants of the interaction. For instance, in Arp 217 (NGC 3310), a starburst spiral, the loops are interpreted as tidal remnants from a merger, enhancing in the surrounding gas-rich regions. Dynamical timescales for these loops, estimated from orbital dynamics and histories, are on the order of 10810^8 years, allowing the features to persist before fully dispersing. Observations of these systems often reveal their gaseous nature through neutral hydrogen (H I) mapping, which detects extended emission tracing the loops beyond optical limits. In NGC 2782, H I data show a prominent tidal tail containing molecular gas and young star clusters, confirming the loops as sites of ongoing triggered by the merger. Such mappings highlight the role of gas dynamics in preserving these structures, distinguishing them from stellar-only features.

Amorphous Spiral Arms

Amorphous spiral arms represent a category of peculiar galaxies in the Atlas of Peculiar Galaxies characterized by irregular, diffuse, or blobby extensions from a central disk that vaguely retain spiral patterns, often disrupted by gravitational interactions. These structures mark intermediate morphologies between well-defined spiral galaxies and fully irregular forms, where tidal forces from nearby companions distort the arms into amorphous shapes while preserving some rotational dynamics. The category comprises six entries (Arp 221 through Arp 226), selected by to illustrate evolutionary transitions driven by interactions, as peculiar features in the atlas are interpreted as evidence of galactic encounters altering morphology. Key characteristics include low-contrast, filamentary or knotty arms emanating from a bright nucleus, with residual rotation supporting the persistence of spiral-like patterns despite the disruption. For instance, Arp 222 (NGC 7727) exhibits prominent amorphous arms interpreted as tidal tails from a minor merger approximately 2 Gyr ago with a gas-rich companion, resulting in a rotating central disk (inclination ~62°, maximum ~150 km s⁻¹) and an inner lens or weak bar (~2.7 kpc) from which lanes emerge. Similarly, Arp 225 (NGC 2655) shows faint, diffuse outer arms with absorption features and a weak bar, indicative of interaction-induced perturbations transitioning the toward irregularity. Observations of these galaxies reveal weak central bars in several cases, contributing to the arm formation before interactions scatter material into blobby extensions. The amorphous nature arises from tidal shredding during encounters, with blue arcs and shells signaling recent triggered by the infall, as seen in Arp 224 (NGC 3921), where distorted arms suggest ongoing dynamical evolution from spiral to more chaotic forms. Overall, this category highlights how interactions produce hybrid morphologies, with residual rotation curves affirming their origins in differentially rotating disks.

Concentric Rings

Concentric rings in the Atlas of Peculiar Galaxies represent a class of amorphous galaxies characterized by multiple nested, ring-like shell structures that create bull's-eye patterns around a central core. These features are typically observed in early-type galaxies and manifest as faint, arc-shaped stellar envelopes with subtle density variations, often extending to several tens of kiloparsecs from the nucleus. A classic example is Arp 227, featuring the NGC 474 in Pisces, where at least seven concentric shells are visible, superimposed on a companion spiral NGC 470, highlighting the peculiar morphology without dominant spiral arms. The formation of these concentric rings is attributed to multiple shell expansions resulting from minor merger events, where a smaller satellite galaxy (mass ratio approximately 1:20 to 1:100) interacts with the host, ejecting streams of stars that phase-wrap into thin, expanding shells over gigayears. This dynamical process produces overlapping shells due to repeated pericenter passages or radial infall, with the rings representing caustics where stellar density is enhanced. The Atlas includes five such entries (Arp 227–231), though pure cases exhibiting isolated concentric rings without accompanying filaments or clumps are rare, comprising only about four well-defined instances. These structures often display age gradients, with inner shells hosting older stellar populations (around 7–8 Gyr) and outer shells showing younger stars (3–4 Gyr), indicative of phased star formation triggered by the merger. Observations reveal that these rings are low-surface-brightness features, with typical g-band magnitudes around 24–25 mag arcsec⁻², best detected in deep optical imaging from telescopes like the Canada-France-Hawaii Telescope. In wavelengths, such as those from the (), the rings appear enhanced due to far- and near-UV emission tracing shocked or bursts of young, massive stars formed in the shells, as seen in analogs like NGC 474 where NUV contours align with optical shells out to 2–3 effective radii. This UV signature underscores the recent dynamical rejuvenation, distinguishing multi-layered concentric rings from simpler single-ring morphologies.

Fission Appearance

The fission appearance category features amorphous galaxies that exhibit morphological traits resembling splitting or division, such as elongated dumbbell shapes formed by paired luminous cores separated by low-surface-brightness regions. These structures often appear as if one is bifurcating, with the components linked by tenuous bridges of indicative of dynamical disruption. This category comprises 24 entries in the Atlas, spanning Arp 233 to Arp 256, selected for their empirical similarity in suggesting internal division processes. proposed that these forms result from explosive ejections of compact from galactic nuclei, viewing them as snapshots of evolutionary fission events rather than stable configurations. Subsequent analyses have reinterpreted these appearances as optical illusions arising from the edge-on or projected views of pairs, where tidal distortions elongate and connect the systems without true splitting. In reality, they represent stages of mergers driven by gravitational encounters, a process now well-supported by simulations of tidal tail and bridge formation. Illustrative cases include Arp 239 (NGC 5278 and NGC 5279), a classic dumbbell pair connected by a faint bridge. Detailed imaging consistently shows these bridges as diffuse, ionized gas and stellar streams, confirming interaction over ejection.

Irregular Clumps

Galaxies classified under irregular clumps in the Atlas of Peculiar Galaxies are amorphous structures characterized by knotty, unresolved masses of stars and gas that appear as uneven, clumpy distributions rather than coherent shapes. These features manifest as bright, compact concentrations embedded within a diffuse , often resembling collections of dwarf-like components without clear spiral or elliptical morphology. A representative example is Arp 263 (NGC 3239), which displays prominent irregular clumps along its elongated form, contributing to its peculiar, blob-like appearance. This class comprises 12 entries (Arp 257 through Arp 268), primarily consisting of aggregates with low , making them challenging to observe in detail from ground-based telescopes. These objects typically exhibit faint, extended halos surrounding the denser knots, indicative of sparse stellar populations and minimal dust obscuration. Such low surface brightness arises from their small total spread over large angular sizes, often comparable to nearby dwarf irregulars. The origins of these irregular clumps are attributed to fragmented remnants of galaxy mergers, where gravitational interactions between dwarf irregulars disrupt coherent structures into discrete, star-forming knots. Simulations and observations suggest that minor mergers between gas-rich dwarfs can produce such clumpy morphologies, with the knots representing sites of triggered amid tidal debris. High-resolution observations with the (HST) have resolved these knotty masses into distinct clusters of young stars, revealing the underlying complexity of within the clumps. For instance, HST imaging of Arp 263 (NGC 3239) shows the irregular structure resolved into a of bright star-forming regions against a fainter background, confirming the clumps as aggregates of stellar associations rather than monolithic features. These observations highlight how interactions lead to localized bursts of star birth in low-density environments. Occasional absorption irregularities, such as dark lanes within the clumps, may further enhance their uneven appearance, though these are secondary to the dominant clumpy distribution.

Double and Multiple Galaxies

Connected Arms

In the Atlas of Peculiar Galaxies, the Connected Arms subcategory documents double galaxies where spiral arms extend between the pair members, forming visible bridges of tidal material during their initial gravitational encounter. These connections arise from the gravitational distortion in the early stages of interaction, when the galaxies are approaching each other for the first time, leading to elongated arm structures rather than full mergers. A representative example is Arp 271 (NGC 5426 and NGC 5427), a pair approximately 130 million light-years away in Virgo, where the interaction has produced arm extensions and a prominent tidal tail linking the spirals. This category includes six key entries (Arp 269 through 274), illustrating tidal arm growth driven by differential gravitational forces that amplify spiral features and promote material exchange. In these systems, gas flows along the bridges facilitate the transfer of between the galaxies, as observed in Arp 271 (NGC 5426 and NGC 5427), where neutral hydrogen mapping reveals a bridge-like feature connecting the pair and indicating ongoing dynamical evolution. Such flows contribute to enhanced rates in the interacting regions, distinguishing these from more advanced merger stages. Telescopic observations of connected arms often highlight their blue coloration, attributable to young, massive stars forming in the compressed gas of the bridges, as noted in the original catalog descriptions. For instance, Arp 272 (NGC 6050 and IC 1179) displays luminous blue extensions linking the spirals, underscoring the category's emphasis on these transient, interaction-induced features within the broader morphological classification of double galaxies.

Interacting Pairs

Interacting pairs in the Atlas of Peculiar Galaxies consist of double systems where two galaxies are in close proximity, leading to pronounced gravitational distortions without full merger. These encounters typically involve spiral or irregular galaxies with overlapping disks, resulting in warped structures and extended tidal features that reveal the dynamical interplay between the components. A prominent example is Arp 276 (NGC 935 and IC 1801), an interacting pair displaying distorted arms and tidal features. The pre-merger stages of these pairs are marked by morphological asymmetries and kinematic disruptions, such as counter-rotating gas flows and bent spiral arms, as the galaxies approach within tens of kiloparsecs. The Atlas dedicates this category to 6 such systems (Arp 275–280), selected for their clear signs of ongoing perturbation rather than resolved connections or advanced coalescence. Observations of these pairs often reveal symmetric tidal tails extending from each galaxy, a hallmark of the mutual tidal forces that strip and redistribute material symmetrically in the plane of interaction. These interacting pairs exhibit enhanced rates, with mid-infrared surveys indicating average increases of approximately 30% relative to isolated field galaxies, driven by gas compression in the disturbed disks and bridges. In field environments, the fraction of luminous galaxies in such close pairs is about 10%, underscoring their role as common evolutionary drivers outside dense clusters.

Infall and Attraction

In the Atlas of Peculiar Galaxies, the Infall and Attraction subcategory documents double galaxies exhibiting morphological signatures of mutual gravitational pull, including converging tails and bridges that point toward ongoing approach between the components. These features arise from tidal distortions during close encounters, where material is stripped and redirected by the of the companion. A prominent example is Arp 284 (NGC 7714 and NGC 7715), an interacting pair displaying prominent H I tails and bridges indicative of converging gas flows between the galaxies. The underlying dynamics involve mediated by , in which the orbiting galaxies experience drag from the gravitational wake of stars and interstellar gas, leading to gradual energy dissipation and orbital contraction. This process accelerates the inspiral, positioning the systems on trajectories toward coalescence. Representative cases in this category, such as Arp 281 (NGC 4631 and NGC 4627), demonstrate these effects through aligned distortions suggesting attractive motion. Key characteristics of these entries include low relative differences, typically under 500 km/s, which confirm the pairs are gravitationally bound and predisposed to future mergers on timescales of less than 1 Gyr. The Atlas catalogs six such systems (Arp 281–286), selected for their clear evidence of infall without advanced merger signatures. Spectroscopic observations frequently detect blue-shifted gas components, as seen in analogous interacting systems like UGC 7636 (Arp 186), where HI emission reveals infalling atomic gas approaching the companion at projected velocities of several hundred km/s.

Wind Effects

In the Atlas of Peculiar Galaxies, double galaxies classified under wind effects exhibit characteristics of diverging material streams that create an illusion of repulsion between the interacting components, often appearing as extended tails or plumes emanating from the galaxies. A prominent example is Arp 287, a system where observations reveal filamentary outflows suggesting expansive ejection rather than mere tidal distortion. These features are primarily caused by powerful galactic winds generated during intense starburst episodes triggered by the , where explosions and stellar radiation drive hot gas outward at high speeds. The atlas catalogs seven such entries (Arp 287–293), highlighting their distinct morphology as evidence of energetic outflows rather than standard gravitational interactions. These superwinds typically achieve velocities around 1000 km/s, as measured in systems like Arp 299 through kinematic analysis of emission lines, enabling them to escape the galactic potential and shape the observed peculiarities. interpreted these wind effects as indicators of non-gravitational influences, such as interactions involving charged particles and magnetic fields, challenging purely tidal explanations for the diverging appearances. Modern observations confirm these outflows through the detection of extended bubbles, which trace the hot, shocked plasma from the winds; in Arp 299, X-ray imaging reveals a ≈45 kpc with temperatures exceeding 10^7 K, supporting the starburst-driven model. Unlike the contractive motions seen in infall scenarios, these expansive winds dominate the dynamics in such pairs, enriching the and potentially influencing galaxy evolution.

Long Filaments

Long filaments represent a distinct class of peculiar features in the Atlas of Peculiar Galaxies, consisting of four entries (Arp 294-297) depicting double galaxies linked by extended bridges greater than 10 kpc in length. These structures highlight the dynamical effects of gravitational encounters, where material is drawn out into thin, thread-like connections between the primary galaxies. A representative example is Arp 295, a pair of interacting spirals separated by approximately 130 kpc, featuring a luminous bridge and extended tails visible in optical imaging. The formation of these filaments arises from the tidal stretching of interstellar gas during perigalactic passages, as demonstrated by early numerical simulations of galaxy interactions. In such events, differential gravitational forces pull gas from the disks, elongating it into bridges that can span vast distances while preserving coherent structure. Unlike the shorter arm connections observed in other interacting pairs, these filaments emphasize the scale of tidal distortion in gas-rich systems. Radio observations, particularly in the 21 cm line, confirm that these bridges are dominated by neutral hydrogen (H I), with total H I masses reaching ~10^8 M_\odot in Arp 295's eastern plume. This neutral gas composition underscores the filaments' role as repositories of unprocessed material, often showing minimal star formation and thus highlighting the preservation of pristine interstellar medium. The persistence of these H I bridges against further tidal disruption is evident in their dynamical stability over ~10^8 years, as inferred from kinematic studies and modeling of the systems.

Unclassified Objects

The unclassified objects category in the encompasses 12 entries classified as double-like galaxies that defy fitting into predefined interaction models, such as connected arms or infall dynamics. These objects, designated Arp 298 through 309, display ambiguous morphological features suggestive of binary systems but lacking clear evidence of physical connectivity or merger stages. The selection criteria emphasized visual peculiarities observed in Palomar 200-inch telescope plates, prioritizing deviations that warranted further investigation into galaxy evolution. Characteristics of these unclassified objects include irregular pairings or apparent doubles where the spatial relationship between components remains indeterminate, often appearing as compact groups without extended tidal tails or bridges. A representative example is Arp 303, an interacting pair in comprising NGC 4460 and PGC 40927, which exhibits a subtle overlap but no definitive signs of dynamical disturbance in early images. Such ambiguity arises primarily from projection effects, where unrelated galaxies align along the , or from photographic artifacts like plate defects mimicking associations. These factors complicate initial assessments, as variations and faint extensions can simulate interaction without underlying physical linkage. Subsequent spectroscopic studies, including 21-cm line observations, have reclassified many of these entries by measuring neutral gas distributions and velocities, revealing that several are chance alignments rather than bound systems. For instance, discrepancies—often exceeding 1000 km/s between apparent companions—indicate non-physical proximity for objects like Arp 299 (NGC 3690 and IC 694), supporting projection as the dominant explanation over genuine interaction. Arp noted potential associations in some cases, proposing that high- companions near these galaxies challenge standard cosmological interpretations, though later analyses attribute such alignments to statistical foreground-background overlaps. These observations underscore the category's role in highlighting the limitations of early morphological .

Galaxy Groups

Galaxy groups in the Atlas of Peculiar Galaxies refer to compact clusters comprising three or more interacting member , selected for their collective peculiar morphologies and evidence of mutual gravitational influences. These systems emphasize over individual galaxy types, distinguishing them from isolated or pairwise interactions. The category includes 11 entries (Arp 311–321), where the primary peculiarity arises from the clustered configuration rather than standalone features in ellipticals or spirals. A representative example is Arp 314, a southern triplet of (PGC 69935, PGC 70130, and PGC 1066529) exhibiting distorted structures due to close encounters, akin to Hickson compact groups. Observations of such systems reveal tidal distortions and enhanced triggered by gravitational perturbations among members. Dynamics in these groups typically involve infall motions, with galaxies accelerating toward the cluster center, fostering repeated close passages and structural evolution. Key characteristics include the influence of the intra-group medium, a hot, low-density gas enveloping the cluster that mediates interactions and ram-pressure effects on member galaxies. Compact galaxy groups like those in the atlas exhibit higher merger rates compared to field galaxies, driven by their dense environments, which increase encounter probabilities and lead to eventual coalescence. X-ray observations of these groups often detect diffuse halos from the heated intra-group gas, reaching temperatures of approximately 10^7 K and luminosities up to 10^42 erg/s. For instance, the NGC 2300 group (Arp 317) shows extended X-ray emission tracing the hot medium, which is sculpted by group infall and outflows. These features provide insights into the thermodynamic state and evolutionary paths of compact clusters, extending pairwise interaction effects to multi-member systems.

Galaxy Chains

Galaxy chains in the Atlas of Peculiar Galaxies represent linear alignments of multiple galaxies, categorized under Arp numbers 322 through 332, comprising 11 entries selected to illustrate elongated, string-like configurations. These structures differ from more compact galaxy groups by their extended, filamentary appearance, often spanning several arcminutes on the sky and involving 3 to 6 member galaxies per chain. A representative example is Arp 322, which depicts a chain of five galaxies identified as Hickson Compact Group 56, showcasing a tight linear arrangement that highlights the peculiarity of such alignments. The formation of these galaxy chains is linked to the filamentary nature of the large-scale , where gravitational instabilities lead to the coalescence of matter into elongated threads that guide evolution. Observations indicate that the galaxies within these chains exhibit aligned redshifts, typically within a few hundred km/s, confirming their physical proximity and shared dynamical history rather than mere projection effects. This alignment underscores their role as small-scale precursors to the vast filaments observed in modern cosmic web mappings, providing early insights into hierarchical . Spectroscopic and imaging studies of these chains reveal evidence of weak interactions among member galaxies, such as subtle tidal tails or distortions in outer envelopes, suggesting ongoing but gentle gravitational influences over timescales of hundreds of millions of years. Unlike intense mergers in interacting pairs, these interactions in chains appear minimal, preserving the linear morphology while contributing to gradual morphological evolution.

Scientific Significance

Key Discoveries

The Atlas of Peculiar Galaxies provided compelling visual evidence for tidal interactions as a primary driver of galaxy morphological evolution, documenting numerous cases where gravitational perturbations distorted spiral arms and triggered structural changes in galactic disks. These observations illustrated how close encounters between galaxies lead to the formation of bridges and tails, serving as distinctive signatures of ongoing or recent mergers that reshape stellar distributions and gas dynamics. The atlas's impact extended to theoretical modeling, inspiring early numerical simulations of galaxy mergers, such as those by Toomre and Toomre, which replicated the observed bridges and tails through tidal disruptions in encounters between disk galaxies. Arp's compilation underscored the rarity of such peculiarities, highlighting their role as exceptional but informative cases in galactic populations. This dataset established a foundational photographic database that facilitated subsequent quantitative analyses of interaction frequencies and evolutionary pathways. Subsequent research inspired by the atlas has emphasized the association of tidal distortions with enhanced nuclear activity, including starbursts and active galactic nuclei (AGN), where interactions funneled gas toward centers to ignite bursts of and quasar-like emissions. Its legacy influenced extensions of Zwicky's Catalogue of Galaxies and Clusters of Galaxies by providing a curated selection of interacting systems that informed broader morphological classifications and interaction studies.

Modern Interpretations

In the decades following its publication, the Atlas of Peculiar Galaxies has benefited from digital preservation efforts, notably through the /IPAC Extragalactic Database (NED), which hosts scanned plates, coordinates, and cross-references for all 338 entries, enabling systematic archival access and integration with multi-wavelength data. High-resolution reimaging by the (HST) has uncovered hidden structures in numerous Arp objects, such as tidal tails and star-forming knots in Arp 248 (Wild's Triplet), revealing dynamic interactions not visible in original Palomar plates. Similarly, (SDSS) photometry has provided deeper broadband imaging, facilitating automated detection of merger signatures in peculiar systems like Arp 335. These updates have largely rejected Arp's hypothesis of intrinsic redshifts for associated quasars and galaxies, favoring standard cosmological interpretations augmented by gravitational effects, such as orbital velocities in interacting systems, as evidenced by resolved spectroscopy showing no anomalous non-Doppler components. Contemporary simulations, including the IllustrisTNG project, have modeled the formation of Arp-like peculiarities across categories, reproducing connected arms, interacting pairs, and shells through gas-rich mergers and dynamical instabilities in a Lambda-CDM framework. These efforts demonstrate that peculiar morphologies arise primarily from interactions, with such galaxies comprising a small fraction of interacting systems, based on visual and quantitative classifications in large surveys. Recent 2020s studies further connect Arp-class objects to "galaxy harassment" in dense clusters, where repeated high-velocity encounters strip gas and transform spirals into irregulars, as evidenced in structures at z∼2.2. Additionally, as of 2024, the (JWST) has imaged several Arp galaxies, such as Arp 142 and Arp 220, revealing intricate details of dust lanes, star-forming regions, and molecular gas in interacting systems. Despite Arp's later controversies over interpretations, the atlas remains a foundational resource for studying interaction-driven evolution, underpinning modern catalogs of disturbed systems. Persistent gaps include the atlas's bias toward northern declinations (δ > -27°), leading to underrepresentation of southern peculiarities and prompting supplementary catalogs like Arp-Madore. Advances in quantitative morphology now leverage algorithms, such as convolutional neural networks trained on SDSS images, to classify peculiar features like asymmetries and tails with over 90% accuracy, surpassing traditional visual assessments.

Notable Examples

Iconic Arp Galaxies

The Atlas of Peculiar Galaxies features several iconic examples that exemplify the dramatic effects of gravitational interactions, mergers, and collisions among galaxies. These objects, often captured in high-resolution images by telescopes like Hubble and the , serve as archetypes for studying galaxy evolution and have been extensively observed due to their visual appeal and scientific insights into and dynamics. Selected for their brightness, accessibility from , and representative peculiarities, the following 7 examples highlight key features from the catalog, with coordinates (J2000 ) and apparent V-band magnitudes provided for observational reference.
Arp NumberCommon NameKey FeatureCoordinates (RA, Dec)Apparent Magnitude
87-Interacting spiral pair with a distinctive wrapped tidal arm extending from one galaxy toward its companion, illustrating gravitational distortion.11h 40m 44s, +22° 26' 26"14.1
147-Pair of colliding galaxies where an elliptical intruder has created a perfect ring structure in the companion disk through a head-on impact, demonstrating ring formation mechanics.03h 11m 19s, +01° 19' 00"14.3
148Mayall's ObjectBull's-eye ring galaxy resulting from a perpendicular collision, with the ring expanding outward from the impact site in the constellation Ursa Major.11h 03m 54s, +40° 51' 00"15.4
220-Advanced merger of two gas-rich spirals forming an ultra-luminous infrared galaxy with compact nuclei and intense starburst activity equivalent to hundreds of solar masses per year.15h 34m 57s, +23° 30' 11"13.9
242Mice GalaxiesTwo nearly equal-mass spirals locked in interaction, producing long, narrow tidal tails that give the pair a mouse-like appearance and reveal material stripped during their encounter.12h 46m 10s, +30° 44' 00"13.5
244Antennae GalaxiesClassic colliding spiral pair with elongated tidal tails resembling antennae, marking an early-stage merger rich in young star clusters and supernova remnants.12h 01m 53s, -18° 52' 00"11.2
271-Interacting spiral duo connected by a luminous bridge of gas and stars, exemplifying a gentle gravitational dance that distorts their disks without full merger.14h 03m 26s, -06° 02' 46"11.4
These galaxies are frequently imaged in astronomical media and featured in educational resources, symbolizing the chaotic beauty of cosmic interactions and inspiring public interest in galaxy dynamics. They represent broader categories such as double galaxies and colliding pairs within Arp's classification, offering windows into processes that shape the universe's structure.

Observation Tips for

astronomers can successfully observe over 50 Arp galaxies from the Atlas of Peculiar Galaxies using modest equipment under favorable conditions, with many objects becoming accessible in telescopes of 12.5 inches or larger aperture. The brightest examples, such as Arp 94 (NGC 3226/3227) at an integrated magnitude of approximately 11, are particularly rewarding targets and can be spotted as a close interacting pair in the constellation Leo. Prioritizing the roughly 20 brightest Arp galaxies, which typically range from magnitudes 8 to 12, allows beginners to build confidence before tackling fainter specimens; these include well-known spirals like Arp 26 (M101) at magnitude 7.9 and Arp 37 (M77) at magnitude 8.8. Northern declinations predominate in the catalog, with over 100 objects north of -15° , making them ideal for observers in the where higher elevations provide better visibility. Optimal viewing occurs during spring and fall seasons, when many Arp galaxies transit high in the sky away from the Milky Way's plane—spring favors members like Arp 94, while fall highlights targets in and Andromeda. Dark skies at Bortle class 3 or better are essential, as light pollution can obscure the subtle peculiar features; aim for moonless nights with high transparency to detect faint extensions. For visual observation, a 12.5-inch or larger reflector is recommended to resolve tidal tails and distorted arms, though larger apertures enhance detail on dimmer objects up to magnitude 15. filters, such as UHC or H-alpha, can selectively enhance ionized gas in spiral arms and shells, improving contrast against the sky background for certain interacting pairs. Planning sessions with software like Stellarium or Uranometria 2000.0 star atlases simplifies locating these often compact fields, which span just a few arcminutes. Challenges include discerning the eponymous peculiarities, such as faint tidal tails or bridges, which may require and 30-60 minutes of adaptation to low light; magnitudes typically fall between 12 and 15 for most accessible targets, demanding patience in suburban settings. For deeper exploration, CCD imaging with cooled cameras like the SBIG ST-2000XM captures these features more readily, allowing stacking of 5-10 minute exposures to reveal structures invisible visually. Programs from the Astronomical League encourage systematic observation of 100 northern Arp galaxies, providing certificates for verified sketches or images.

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