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Spiral galaxy
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An example of a spiral galaxy, Messier 77 (also known as NGC 1068)

Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae[1] and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them.

Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure,[2] extending from the central bulge, at the ends of which the spiral arms begin. The proportion of barred spirals relative to barless spirals has likely changed over the history of the universe, with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe (Hubble volume) have bars.[3]

The Milky Way is a barred spiral, although the bar itself is difficult to observe from Earth's current position within the galactic disc.[4] The most convincing evidence for the stars forming a bar in the Galactic Center comes from several recent surveys, including the Spitzer Space Telescope.[5]

Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in today's universe.[6] They are mostly found in low-density regions and are rare in the centers of galaxy clusters.[7]

Structure

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Tuning-fork-style diagram of the Hubble sequence

Spiral galaxies may consist of several distinct components:

The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy.

Spiral arms

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Main article: Spiral arm
Barred spiral galaxy UGC 12158

Spiral arms are regions of stars that extend from the center of barred and unbarred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies have tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so bright.

Bulge

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Main article: Galactic bulge

A bulge is a large, tightly packed group of stars. The term refers to the central group of stars found in most spiral galaxies, often defined as the excess of stellar light above the inward extrapolation of the outer (exponential) disk light.

NGC 1300 in infrared light

Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars, which are old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller[10] and are composed of young, blue Population I stars. Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity); others simply appear as higher density centers of disks, with properties similar to disk galaxies.

Many bulges are thought to host a supermassive black hole at their centers. In our own galaxy, for instance, the object called Sagittarius A* is a supermassive black hole. There are many lines of evidence for the existence of black holes in spiral galaxy centers, including the presence of active nuclei in some spiral galaxies, and dynamical measurements that find large compact central masses in galaxies such as Messier 106.

Bar

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Bar-shaped elongations of stars are observed in roughly two-thirds of all spiral galaxies.[11][12] Their presence may be either strong or weak. In edge-on spiral (and lenticular) galaxies, the presence of the bar can sometimes be discerned by the out-of-plane X-shaped or (peanut shell)-shaped structures[13][14] which typically have a maximum visibility at half the length of the in-plane bar.

Spheroid

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19 face-on spiral galaxies from the James Webb Space Telescope in near- and mid-infrared light. Older stars appear blue here, and are clustered at the galaxies’ cores. Glowing dust, showing where it exists around and between stars – appearing in shades of red and orange. Stars that have not yet fully formed and are encased in gas and dust appear bright red.[15]

The bulk of the stars in a spiral galaxy are located either close to a single plane (the galactic plane) in more or less conventional circular orbits around the center of the galaxy (the Galactic Center), or in a spheroidal galactic bulge around the galactic core.

However, some stars inhabit a spheroidal halo or galactic spheroid, a type of galactic halo. The orbital behaviour of these stars is disputed, but they may exhibit retrograde and/or highly inclined orbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Spheroidal Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.

Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I cousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters.

The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarfs close to the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830. Due to their irregular movement around the center of the galaxy, these stars often display unusually high proper motion.

Oldest spiral galaxies

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BRI 1335-0417 is the oldest and most distant known spiral galaxy, as of 2024.[dubious – discuss] The galaxy has a redshift of 4.4, meaning its light took 12.4 billion years to reach Earth.[16][17] Another contender for this title is the galaxy Zhúlóng, which according to one paper has an estimated redshift of 5.2.[18]

The oldest grand design spiral galaxy on file is BX442. At eleven billion years old, it is more than two billion years older than any previous discovery. Researchers believe the galaxy's shape is caused by the gravitational influence of a companion dwarf galaxy. Computer models based on that assumption indicate that BX442's spiral structure will last about 100 million years.[19][20]

The oldest multi-arm spiral galaxy, as of 2022, is A2744-DSG-z3. Its redshift is z=3.059, which corresponds to 11.5 billion light years to Earth.[21][22]

A1689B11 is an extremely old spiral galaxy located in the Abell 1689 galaxy cluster in the Virgo constellation.[23] A1689B11 is 11 billion light years from the Earth, forming 2.6 billion years after the Big Bang.[24][25]

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In June 2019, citizen scientists through Galaxy Zoo reported that the usual Hubble classification, particularly concerning spiral galaxies, may not be supported, and may need updating.[26][27]

Origin of the spiral structure

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Spiral galaxy NGC 6384 taken by Hubble Space Telescope
The spiral galaxy NGC 1084, home of five supernovae[28]

History

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The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem. Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure.

Since the 1970s, there have been two leading hypotheses or models for the spiral structures of galaxies:

  • star formation caused by density waves in the galactic disk of the galaxy.
  • the stochastic self-propagating star formation model (SSPSF model) – star formation caused by shock waves in the interstellar medium. The shock waves are caused by the stellar winds and supernovae from recent previous star formation, leading to self-propagating and self-sustaining star formation. Spiral structure then arises from differential rotation of the galaxy's disk.

These different hypotheses are not mutually exclusive, as they may explain different types of spiral arms.

Density wave model

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Main article: Density wave theory
Animation of orbits as predicted by the density wave theory, which explains the existence of stable spiral arms. Stars move in and out of the spiral arms as they orbit the galaxy.

Bertil Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy's stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms.[29]

Historical theory of Lin and Shu

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Exaggerated diagram illustrating Lin and Shu's explanation of spiral arms in terms of slightly elliptical orbits

The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964,[30] attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of the galaxy's gas and stars. They suggested that the spiral arms were manifestations of spiral density waves – they assumed that the stars travel in slightly elliptical orbits, and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center. This is illustrated in the diagram to the right. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.[31]

Star formation caused by density waves

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The following hypotheses exist for star formation caused by density waves:

  • As gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse (the Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars.
  • As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.
  • As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars.

More young stars in spiral arms

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Spiral arms appear visually brighter because they contain both young stars and more massive and luminous stars than the rest of the galaxy. As massive stars evolve far more quickly,[32] their demise tends to leave a darker background of fainter stars immediately behind the density waves. This make the density waves much more prominent.[29]

Spiral arms simply appear to pass through the older established stars as they travel in their galactic orbits, so they also do not necessarily follow the arms.[29] As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the local higher density. Also the newly created stars do not remain forever fixed in the position within the spiral arms, where the average space velocity returns to normal after the stars depart on the other side of the arm.[31]

Gravitationally aligned orbits

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Charles Francis and Erik Anderson showed from observations of motions of over 20,000 local stars (within 300 parsecs) that stars do move along spiral arms, and described how mutual gravity between stars causes orbits to align on logarithmic spirals. When the theory is applied to gas, collisions between gas clouds generate the molecular clouds in which new stars form, and evolution towards grand-design bisymmetric spirals is explained.[33]

Distribution of stars in spirals

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The similar distribution of stars in spirals

The stars in spirals are distributed in thin disks radial with intensity profiles such that[34][35][36]

with being the disk scale-length; is the central value; it is useful to define: as the size of the stellar disk, whose luminosity is

.

The spiral galaxies light profiles, in terms of the coordinate , do not depend on galaxy luminosity.

Spiral nebula

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Drawing of the Whirlpool Galaxy by Rosse in 1845

Before it was understood that spiral galaxies existed outside of our Milky Way galaxy, they were often referred to as spiral nebulae, due to Lord Rosse, whose telescope Leviathan was the first to reveal the spiral structure of galaxies. In 1845 he discovered the spiral structure of M51, a galaxy nicknamed later as the "Whirlpool Galaxy", and his drawings of it closely resemble modern photographs. In 1846 and in 1849 Lord Rosse identified similar pattern in Messier 99 and Messier 33 respectively. In 1850 he made the first drawing of Andromeda Galaxy's spiral structure. In 1852 Stephen Alexander supposed that Milky Way is also a spiral nebula.[37]

The question of whether such objects were separate galaxies independent of the Milky Way, or a type of nebula existing within our own galaxy, was the subject of the Great Debate of 1920, between Heber Curtis of Lick Observatory and Harlow Shapley of Mount Wilson Observatory. Beginning in 1923, Edwin Hubble[38][39] observed Cepheid variables in several spiral nebulae, including the so-called "Andromeda Nebula", proving that they are, in fact, entire galaxies outside our own. The term spiral nebula has since fallen out of use.

Milky Way

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Milky Way Galaxy's spiral arms and barred core – based on WISE data

The Milky Way was once considered an ordinary spiral galaxy. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1960s.[40][41] Their suspicions were confirmed by Spitzer Space Telescope observations in 2005,[42] which showed that the Milky Way's central bar is larger than was previously suspected.

Famous examples

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Further information: List of spiral galaxies
  • Andromeda Galaxy – Barred spiral galaxy in the Local Group
  • Black Eye Galaxy – Spiral galaxy in the constellation Coma Berenices
  • Malin 1 – Spiral galaxy in the constellation Coma Berenices
  • Milky Way – Galaxy containing the Solar System
  • Pinwheel Galaxy – Galaxy in the constellation Ursa Major
  • Sunflower Galaxy – Spiral galaxy in the constellation Canes Venatici
  • Triangulum Galaxy – Spiral galaxy in the constellation Triangulum
  • Whirlpool Galaxy – Galaxy in the constellation Canes Venatici

See also

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Classification

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Other

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Spiral galaxy

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A spiral galaxy is a type of barred or unbarred featuring a rotating disk of stars, interstellar gas, , and , from which prominent spiral arms extend outward, originating at a dense central bulge composed primarily of older stars. These galaxies are distinguished by their pinwheel-like appearance, with the spiral arms appearing bright due to ongoing triggered by density waves that compress gas and . Approximately 60% of all observed galaxies in the nearby are spirals, making them the most common galaxy type and hosting the majority of stars in the cosmos. Spiral galaxies are classified under the , which categorizes them from early-type (Sa) with tightly wound arms and large bulges to late-type (Sc or Sd) with loosely wound arms, smaller bulges, and more prominent disks rich in gas and young stars. About two-thirds of spiral galaxies are barred, featuring a linear bar of stars across the central bulge that channels gas toward the nucleus, potentially fueling supermassive black holes. The disks typically span tens to hundreds of thousands of light-years, surrounded by a spherical halo containing globular clusters and . Our own is a classic example of a , approximately 100,000 light-years in diameter, with located in a minor spiral arm about 26,000 light-years from the center. Another prominent example is the (M31), the nearest major spiral to the at about 2.5 million light-years away, destined to merge with it in roughly 4.5 billion years. Spiral galaxies form through the hierarchical merging of smaller protogalaxies and the accretion of gas from the intergalactic medium, evolving over billions of years while maintaining their disk structure through conservation. Observations from telescopes like Hubble and James Webb reveal that spirals were more common in the early but have undergone morphological , with some transitioning to lenticular or elliptical forms via mergers or gas depletion.

Physical Structure

Bulge

The bulge of a is a densely concentrated, spheroidal or ellipsoidal stellar component situated at the galaxy's center, comprising primarily older Population II stars with lower , alongside smaller quantities of interstellar gas and dust. This central region serves as the densest part of the , contrasting with the more extended disk by its three-dimensional, non-rotating dominated by random stellar motions. Bulges typically span a few kiloparsecs in diameter and contain stellar masses ranging from 10910^9 to 101110^{11} solar masses, representing a significant fraction of the galaxy's total despite their compact size. Spectroscopically, they appear redder than the surrounding disk due to the prevalence of cooler, older stars, with integrated colors reflecting low ongoing . Classical bulges, often formed through major mergers, exhibit luminosity profiles that follow the de Vaucouleurs r1/4r^{1/4} law, resembling those of elliptical galaxies, while pseudobulges, shaped by secular processes such as bar-driven instabilities, display more exponential profiles akin to disks. At the core of most bulges resides a (SMBH), with masses typically on the order of millions to billions of solar masses, exerting profound gravitational influence on the dynamics of nearby stars and gas through Keplerian orbits and potential feedback mechanisms. For instance, in the , the SMBH Sagittarius A* has a mass of approximately 4 million solar masses and shapes the motion of stars within the central , providing a key probe of bulge . These SMBHs correlate tightly with bulge mass via the M∙M_\bullet-MbulgeM_\mathrm{bulge} relation, underscoring the bulge's role in hosting these massive objects.

Disk and Spiral Arms

The galactic disk of a spiral galaxy forms a thin, rotating plane primarily composed of , interstellar gas, and , extending outward from the central bulge. This structure exhibits an exponential surface brightness profile, where the density of and light decreases radially with a characteristic scale length typically ranging from 3 to 5 kiloparsecs (kpc) in most spiral galaxies. The disk's rotation is generally flat, meaning orbital speeds remain roughly constant with increasing distance from the center, supporting the stability of this extended component. Spiral arms emerge as prominent features within the disk, manifesting as compressed regions of gas, , and that create brighter, denser lanes against the fainter interarm regions. These arms often follow logarithmic or trailing patterns, winding backward relative to the direction of galactic rotation, with pitch angles—the angle between the arm and the circumferential direction—typically measuring between 5 and 20 degrees. In barred spiral galaxies, these arms can be influenced by resonances driven by the central bar, enhancing their prominence. A key challenge in understanding spiral arms is the winding problem, arising from in the disk: inner regions orbit faster than outer ones, which should cause any initial arm pattern to tighten and wind up over time, eventually losing coherence. This observed persistence of arms suggests underlying mechanisms that maintain their structure without delving into specific theoretical resolutions here. Spiral galaxies display variations in arm morphology, broadly classified as grand design or flocculent. Grand design spirals feature two prominent, symmetric major arms extending across much of the disk, as exemplified by Messier 51 (M51), where the arms are well-defined and traceable over several scale lengths. In contrast, flocculent spirals exhibit numerous short, fragmented arms that appear patchy and less organized, lacking the continuous symmetry of grand design types. Vertically, the disk maintains a relatively constant thickness of approximately 300 parsecs (pc), measured as the full width at half maximum, but it flares outward toward the edges, increasing in scale height beyond several disk scale lengths. This flaring contributes to the disk's overall three-dimensional structure while preserving its predominantly flat appearance.

Bar

In spiral galaxies, the bar is a distinct, elongated stellar structure that protrudes from the central bulge along the major axis, forming a rectangular or boxy feature amid the disk. Composed predominantly of older, low-mass stars similar to those in the bulge, the bar exhibits a higher surface brightness and smoother morphology than the surrounding disk, with typical lengths ranging from 1 to 5 kpc depending on the galaxy's mass and type. Approximately 65% of spiral galaxies in the local universe host such bars, classified under the SB subtype in the Hubble morphological scheme, where they play a key role in channeling gas inflows toward the and facilitating the formation of spiral arms through orbital resonances. In barred spirals, the bar's ends often connect directly to the inner spiral arms, enhancing the overall non-axisymmetric structure. Dynamically, galactic bars rotate as rigid, non-axisymmetric features with a constant pattern speed, distinct from the of the disk, which establishes a corotation radius where stars orbit at the same angular speed as the bar and inner/outer Lindblad resonances where radial orbital frequencies align with the bar's perturbation. Bars are morphologically classified as strong or weak based on their prominence and extent relative to the bulge and disk scale length, often determined through isophotal analysis that traces contours of constant to measure bar ellipticity and length. Strong bars extend farther and exhibit higher ellipticity, while weak bars are shorter and more embedded. Structurally, bars arise from gravitational instabilities in the galactic disk when the Toomre stability parameter falls below 1, leading to the amplification of non-axisymmetric perturbations into a coherent bar-like feature.

Halo

The stellar halo of a spiral galaxy forms a low-density, spheroidal surrounding the central bulge and the disk, primarily composed of ancient, metal-poor classified as Population II. These , often observed as red giants, represent some of the oldest components in the galaxy, with globular clusters—dense, spherical collections of up to a million —orbiting within this structure and serving as tracers of its dynamics. The stellar halo's low makes it challenging to observe directly, but it contributes a minor fraction to the galaxy's total compared to the disk. The extent of the stellar halo typically reaches 50–100 kpc from the , though profiles often truncate sharply in some galaxies, leading to a steeper decline in stellar density beyond this radius. This truncation can result from dynamical processes that limit the halo's growth, contrasting with more extended distributions in others. In addition to the stellar component, spiral galaxies are embedded in a massive , inferred from the flattening of curves beyond approximately 10 kpc, where visible alone cannot account for the observed orbital velocities. This comprises roughly 90% of the galaxy's total mass, dominating the at large radii. Its density profile is commonly modeled by the Navarro-Frenk-White (NFW) form, ρ(r)=ρ0(r/rs)(1+r/rs)2,\rho(r) = \frac{\rho_0}{(r/r_s)(1 + r/r_s)^2}, where ρ0\rho_0 is a characteristic density, rr is the radial distance, and rsr_s is the scale radius; this profile arises from simulations of cold dark matter collapse. The dark matter halo provides the extended gravitational potential necessary for disk stability, helping to align and prevent warping of the galactic disk by counteracting misalignments or external perturbations. Observations of the halo rely on tracers such as globular clusters, which map its three-dimensional structure through their orbits, and planetary nebulae, whose reveal the underlying potential. Recent data from the mission have identified stellar streams in the Milky Way's halo, remnants of disrupted satellites that highlight the halo's hierarchical assembly, though similar features are expected in other spirals.

Formation and Evolution

Theories of Origin

Theories of spiral galaxy origin have evolved from early monolithic collapse models to modern frameworks embedded in the Lambda cold dark matter (ΛCDM) cosmology, emphasizing hierarchical assembly through mergers and accretion. In the seminal monolithic collapse model proposed by Eggen, Lynden-Bell, and Sandage in 1962, a rapidly collapsing protogalactic gas cloud forms the stellar halo first, followed by the settling of remaining gas into a rotating disk due to conservation of during the collapse. This scenario posits a single, large progenitor system undergoing rapid contraction on a dynamical timescale of about 10^8 years, leading to the differentiation between the spheroidal halo and the flattened disk components observed in spirals. In contrast, the prevailing hierarchical merging paradigm within ΛCDM cosmology describes spiral galaxies as emerging from the coalescence of smaller, gas-rich progenitors, particularly during the peak of cosmic at redshifts z ≈ 2–3. Gas-rich mergers at these epochs supply the raw material for disk formation, with subsequent dynamical settling and relaxation allowing the gas to form extended, rotationally supported disks as violent relaxation dissipates energy. This bottom-up assembly process aligns with the power spectrum, where small halos merge progressively into larger structures, fostering the growth of massive spirals through repeated interactions that redistribute . A key mechanism in disk formation is the cooling of baryonic gas within dark matter halos, where the specific angular momentum of the gas is largely preserved from the halo's spin, acquired via tidal torques during hierarchical buildup, as detailed by Fall and Efstathiou in 1980. This conservation leads to the formation of centrifugally supported disks whose sizes scale with the halo's angular momentum, explaining the observed correlation between disk scale lengths and halo virial radii without requiring excessive angular momentum loss. Despite these advances, significant challenges persist, particularly in transport during galaxy assembly, where simulations often show insufficient outward transfer of from baryons to , resulting in overly compact disks or excessive central concentrations. The bulge-disk dichotomy further complicates models, as hierarchical mergers tend to produce more bulge-dominated systems than observed in many spirals, raising questions about the efficiency of disk survival amid frequent interactions and the role of secular processes in suppressing bulge growth. Recent cosmological hydrodynamical simulations, such as IllustrisTNG and EAGLE, demonstrate that spiral galaxies form through feedback-regulated gas accretion onto halos, where stellar and feedback modulate inflow rates to enable stable disk settling without overproducing bulges. In these models, episodic gas accretion at rates of 1–10 M_⊙ yr^{-1} sustains disk growth, with supernova-driven outflows and feedback preventing catastrophic loss, thus reproducing realistic spiral morphologies. Once formed, these disks maintain their structure through mechanisms like density waves.

Density Wave Model

The density wave model addresses the persistence of spiral arms in galaxies by proposing that they arise from quasi-stationary gravitational density waves that propagate through the galactic disk, rather than being fixed material structures composed of stars and gas. This theory, developed by C.C. Lin and Frank H. Shu in 1964, resolves the "winding problem," where in a galactic disk would otherwise cause any material arm to tighten and shear apart within a few rotations, as noted in early discussions by astronomers including G. Bertil Lindblad and . In the Lin-Shu framework, the spiral pattern rotates as a whole with a constant angular speed, known as the pattern speed Ωp\Omega_p, which differs from the orbital angular speeds of stars and gas in the disk (Ω\Omega). Stars and gas clouds thus orbit at their local circular speeds but periodically pass through the denser regions of the wave, experiencing temporary gravitational perturbations that enhance density contrasts without being permanently bound to the arms. The theory relies on the stability of the galactic disk against gravitational perturbations, governed by the for density waves in a thin, differentially rotating disk. For tightly wound spiral waves, this relation approximates σ2=κ2−4π2GΣ+⋯\sigma^2 = \kappa^2 - 4\pi^2 G \Sigma + \cdots, where σ\sigma is the velocity dispersion, κ\kappa is the epicyclic frequency, GG is the , and Σ\Sigma is the surface density. Disk stability is further quantified by Toomre's criterion, Q=σκ3.36GΣ>1Q = \frac{\sigma \kappa}{3.36 G \Sigma} > 1, which ensures that local overdensities neither fragment into stars nor disperse too quickly, allowing wave modes to persist. Wave amplification occurs primarily near corotation, where Ω=Ωp\Omega = \Omega_p, and at Lindblad resonances, defined by Ω±κm=Ωp\Omega \pm \frac{\kappa}{m} = \Omega_p (with mm the number of arms), where stars enter and exit the spiral arms, leading to shocks in the gas and enhanced density in the stellar component. Modern refinements to the Lin-Shu theory incorporate nonlinear effects, such as wave steepening and shock formation, which allow for more realistic, non-axisymmetric patterns observed in galaxies. Additionally, stochastic forcing from central bars or interactions with companion galaxies can excite and sustain these waves, providing a dynamical driver for arm maintenance over gigayears.

Evolutionary Processes

Spiral galaxies undergo secular evolution primarily through internal dynamical processes that redistribute mass and over gigayears, without requiring major external events. In barred spiral galaxies, the central bar structure exerts gravitational torques on the interstellar gas, driving inflows toward the . These inflows fuel intense in nuclear regions, known as starbursts, and contribute to the growth of supermassive black holes via enhanced accretion. Over time, this process leads to the formation of pseudobulges, which are centrally concentrated stellar components built gradually from disk material rather than through violent mergers. Bars play a dominant role in this secular redistribution, transforming gas-rich disks into more bulge-dominated systems while maintaining the overall spiral morphology. Mergers and interactions with companion galaxies represent another key driver of evolutionary change in spiral systems. Minor mergers, involving satellites with mass ratios less than 1:4, can dynamically heat the stellar disk by injecting energy through tidal perturbations, thereby increasing the vertical thickness without fully disrupting the spiral structure. In contrast, major mergers between comparable-mass spirals often lead to the complete reconfiguration of the galaxy, transforming the ordered disk into a spheroidal elliptical remnant through violent relaxation and loss of , as proposed in early simulations of interacting systems. This stability threshold relates to the Toomre parameter, which governs disk vulnerability to perturbations during such encounters. Environmental influences become prominent for spirals infalling into dense galaxy clusters, where interactions with the hot trigger quenching of . Ram-pressure stripping occurs as the galaxy's relative velocity through the medium creates a that exceeds the gravitational binding of the interstellar gas, efficiently removing molecular clouds and atomic from the disk outskirts. This process truncates the gas reservoir, halting new and transitioning the galaxy toward a red, quiescent state, with the extent of stripping depending on the galaxy's orbital trajectory and cluster density. Observations and models confirm that such stripping primarily affects spirals in cluster environments, reducing their specific rates by orders of magnitude. Internal disk heating further shapes the long-term structure of spirals by gradually increasing the random motions of . Scattering events with giant molecular clouds and transient spiral density waves impart to stellar orbits, elevating the dispersion in radial, azimuthal, and vertical directions over billions of years. Molecular clouds, with their high and transient nature, dominate vertical heating, while spiral arms contribute more to in-plane dispersion, collectively causing the disk to thicken from an initially thin configuration to observed scales of several kiloparsecs. This secular heating aligns with age-velocity dispersion relations observed in nearby galaxies, reflecting cumulative dynamical evolution since disk formation. Recent observations from the James Webb Space Telescope (JWST) have revealed mature spiral galaxies at redshifts z ≳ 3, indicating that disk structures and bar features formed rapidly within the first 2 billion years after the Big Bang. For instance, the galaxy CEERS-2112, a barred spiral at z ≈ 3, exhibits well-defined arms and a central bar, suggesting efficient angular momentum transport and disk settling much earlier than predicted by traditional hierarchical models of slow, merger-driven buildup. These findings challenge paradigms assuming prolonged disk instability and heating phases, implying accelerated evolutionary pathways possibly enhanced by high gas fractions in the early universe. As of 2025, even earlier examples, such as the grand-design spiral Zhúlóng at z ≈ 5.2 (~1 billion years after the Big Bang), confirm the prevalence of ordered disk structures in the primordial cosmos.

Ancient Spiral Galaxies

Observations of ancient spiral galaxies, particularly those at high redshifts, have revealed surprisingly mature structures in the early universe. Among the earliest known examples is Zhúlóng, an ultra-massive grand-design spiral galaxy observed at a photometric redshift of z ≈ 5.2, corresponding to approximately 1 billion years after the Big Bang. This galaxy exhibits prominent spiral arms and a central bulge, with a stellar mass comparable to the Milky Way, indicating rapid assembly of its disk components. Another notable case is CEERS-2112, a barred spiral at z ≈ 3 (~2 billion years after the Big Bang), with a mass-weighted stellar age of about 620 million years, demonstrating quick formation of bar and arm features within roughly 400 million years. Additionally, J0107a, a massive barred spiral at z ≈ 2.5 (~2.6 billion years after the Big Bang), shows intense star formation and dynamical stability, forming stars at rates 300 times that of the modern Milky Way. Such findings, enabled by JWST, challenge expectations of chaotic, merger-driven morphologies in the young universe. At redshifts z ∼ 2–3, spiral galaxies constitute approximately 10–20% of the galaxy population, a fraction that decreases at higher redshifts but remains detectable up to z ≈ 5. JWST imaging has identified spiral features in a higher proportion of galaxies than previously detected by the , suggesting that disk structures were more common earlier than anticipated. For instance, in samples from fields like CEERS and PANORAMIC, spirals show elevated rates and larger sizes compared to non-spiral counterparts at these epochs. The existence of these early spirals implies rapid disk formation mechanisms, such as accretion of gas from the cosmic web, which could build ordered in under a billion years and contrasts with models emphasizing violent mergers as the dominant driver. In Zhúlóng, the high gas fraction and efficient settling facilitated grand-design spiral formation without prolonged instability, supporting scenarios where enable fast rotationally supported disks. This rapid evolution aligns with observations of dynamically stellar populations, hinting at efficient transfer in the primordial environment. In J0107a, the barred and gas dynamics further illustrate how early feedback and accretion sustained complex morphologies. Structurally, ancient spirals at z ∼ 2–5 feature compact disks with elevated gas fractions, often exceeding 50% of the total baryonic mass—far higher than the 5–10% typical in present-day spirals. These characteristics are probed through from HST and JWST, revealing turbulent yet organized gas dynamics and clumpy along arms. High gas content contributes to disk thickness and against perturbations, but also fuels the sustained spiral patterns observed. The persistence of these early spirals to the present day underscores their stability against disruptive processes like bar buckling or vertical heating. Dynamically cold disks in examples such as CEERS-2112 and Zhúlóng resist such instabilities due to their ordered rotation and gas damping effects. Similarly, the barred spiral J0107a at z ∼ 2.5 demonstrates robust structural integrity, with the bar and arms maintaining coherence over despite environmental perturbations. This suggests that density waves may have played a role in preserving arm features from the early onward.

Stellar Content and Dynamics

Star Distribution

In spiral galaxies, the hosts the majority of young, metal-rich Population I stars, which are primarily concentrated along the spiral arms due to ongoing activity there. These stars exhibit a small vertical of approximately 300 pc, reflecting their confinement to a flattened structure embedded within the . The , in contrast, comprises older intermediate-age stars with a broader vertical distribution and higher dispersion, typically around 40 km s⁻¹, indicating a more dynamically heated population. These stars are generally more metal-poor than those in the thin disk, with metallicities ranging from [Fe/H] ≈ -0.5 to -1.0, and they extend to scale heights of about 1 kpc. The bulge and halo populations consist predominantly of ancient, metal-poor stars, with the bulge often following an exponential density profile and the halo characterized by a power-law profile such as ρ ∝ r⁻³.⁵. These components represent the oldest stellar generations in spiral galaxies, with ages exceeding 10–12 Gyr and low metallicities that decrease outward. gradients across spiral galaxies, measured through the analysis of emission lines in H II regions, show a general decrease from the center to the outer disk, typically on the order of -0.02 to -0.07 dex kpc⁻¹, highlighting the radial variation in chemical enrichment. In terms of mass contributions to the baryonic content, the disk accounts for approximately 70% of the total, the bulge contributes 10–20%, and the stellar halo makes up less than 1%, underscoring the disk's dominance in the visible budget of spiral galaxies.

Star Formation Processes

in spiral galaxies is predominantly driven by the of dense gas clouds, with processes significantly enhanced within the spiral arms due to dynamical interactions. The spiral structure funnels interstellar gas into compressed regions, promoting the formation of molecular clouds where can birth. This enhancement is crucial, as the arms account for a disproportionate share of the galaxy's activity despite comprising only a fraction of the disk area. Observations indicate that while the interarm regions contribute to a baseline level of , the arms trigger bursts that dominate the overall rate. A key empirical relation governing is the Schmidt-Kennicutt , which describes the star formation rate surface (Σ_SFR) as proportional to the gas surface (Σ_gas) raised to the power of approximately 1.4, expressed as ΣSFR∝Σgas1.4\Sigma_{\mathrm{SFR}} \propto \Sigma_{\mathrm{gas}}^{1.4}. This power- relation, derived from integrated measurements across numerous galaxies, highlights the nonlinear dependence of on available gas, with steeper exponents observed in denser environments typical of spiral arms. The underscores that even modest increases in gas can lead to substantial boosts in efficiency. In spiral arms, density waves play a pivotal role in triggering collapse by compressing ambient gas, which reduces the —the minimum required for gravitational —and facilitates fragmentation into star-forming cores once densities reach a threshold of about 104 cm−310^4 \, \mathrm{cm}^{-3}. This compression occurs as gas enters the potential, shocking and piling up to form elongated structures conducive to formation. Giant molecular s (GMCs), with typical masses ranging from 10510^5 to 106 M⊙10^6 \, M_\odot, emerge as the primary sites of this activity, hosting clusters of young . However, feedback from supernovae and stellar within these GMCs regulates the process, limiting the overall efficiency to around 1-2% of the cloud's gas before dispersal. Globally, spiral galaxies exhibit rates of 1 to 10 M⊙ yr−1M_\odot \, \mathrm{yr}^{-1}, with the rate declining radially outward due to diminishing gas densities and dynamical influences. This radial reflects the concentration of molecular gas and young stars toward the inner disk, where conditions favor higher efficiency. Recent observations using Hα emission, which traces ionized gas from massive stars, and light from hot young stars reveal prominent concentrations along spiral arms, confirming the localized nature of enhanced formation. Complementing these, Atacama Large Millimeter/submillimeter Array (ALMA) mappings of CO emissions have resolved molecular gas distributions, showing dense complexes aligned with arms and providing direct evidence of the gas reservoirs fueling these processes.

Orbital Mechanics

In spiral galaxies, the orbital velocities of stars and gas maintain remarkably flat rotation curves, where the rotational speed v(r)v(r) remains approximately constant at around 200 km/s beyond radial distances of about 5 kpc from the . This behavior deviates sharply from the Keplerian decline expected under Newtonian dominated by visible matter, implying the presence of extensive halos that provide the additional gravitational pull to sustain these speeds. An alternative explanation, (MOND) proposed by Milgrom, modifies the laws of at low accelerations to reproduce flat rotation curves without invoking , though it faces challenges in explaining broader cosmological observations. The epicyclic approximation describes the motion of stars on nearly circular orbits perturbed slightly from circular paths in the galactic potential. In this framework, stars undergo small radial oscillations around a guiding center, characterized by the epicyclic frequency κ=4Ω2+dΩ2dln⁡R\kappa = \sqrt{4 \Omega^2 + \frac{d \Omega^2}{d \ln R}}
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