NGC 3535 is an unbarred spiral galaxy situated in the constellation of Leo, classified morphologically as SAa according to the de Vaucouleurs system.^[1] It was discovered by the astronomer William Herschel on April 18, 1784.^[2] The galaxy exhibits notable radio emissions, qualifying it as a radio source with detections in surveys such as FIRST and NVSS, indicating extended radio structures.^[3] Located at a distance of approximately 102 megaparsecs (about 332 million light-years) from Earth based on recent observations, NGC 3535 has a heliocentric radial velocity of around 6,928 km/s.^[4][3] A Type Ia supernova, designated SN 2023hrn, was discovered within the galaxy in May 2023 by the ATLAS survey, providing valuable data for cosmological distance measurements.^[4][5] This galaxy's structure and emissions make it a subject of interest for studies in active galactic nuclei and radio astronomy, with its position near the celestial equator allowing visibility from both hemispheres.^[1] Apparent magnitude of 13.5 renders it observable with moderate amateur telescopes under dark skies.^[1]

General Properties

Location and Coordinates

NGC 3535 is an unbarred spiral galaxy situated in the constellation of Leo, positioned close to the celestial equator, which allows it to be at least partly visible from both the northern and southern hemispheres during certain times of the year.^[1] Its precise location in the sky is defined by equatorial coordinates in the J2000 epoch, with a right ascension of 11h 08m 33.903s and a declination of +04° 49′ 54.788″.^[6] The galaxy's position places it within the boundaries of Leo, a prominent spring constellation known for hosting numerous deep-sky objects, though no particularly close associations with specific nearby galaxies or clusters are noted in standard catalogs for NGC 3535 itself.^[7] This location facilitates observations from mid-latitude sites worldwide, contributing to its study in various astronomical surveys. NGC 3535 bears several alternative designations across different catalogs, reflecting its identification in optical, infrared, and other observational databases. These include UGC 6189 from the Uppsala General Catalogue, PGC 33760 (also known as LEDA 33760) from the Principal Galaxies Catalogue, MCG +01-29-004 from the Morphological Catalogue of Galaxies, and IRAS 11059+0505 from the Infrared Astronomical Satellite catalog, among others such as 2MASX J11083390+0449545 and Z 1105.9+0506.^[6][7]

Distance and Redshift

NGC 3535 has a spectroscopic redshift of $ z = 0.023103 $.^[8] The galaxy's heliocentric radial velocity is measured at $ 6,926 \pm 2 $ km/s, while its velocity relative to the cosmic microwave background is $ 7,289 \pm 25 $ km/s.^[9] Based on Hubble's law and the cosmic microwave background velocity, the distance to NGC 3535 is estimated at $ 350.7 \pm 24.6 $ million light-years, or equivalently $ 107.51 \pm 7.54 $ megaparsecs.^[8]

Physical Characteristics

Morphological Type

NGC 3535 is classified as an unbarred spiral galaxy of morphological type SAa according to the de Vaucouleurs system.^[1] This classification denotes a galaxy featuring a prominent central bulge and tightly wound spiral arms emanating from the nucleus, without the presence of a central bar structure that characterizes barred spirals.^[1] The 'S' indicates spiral form, 'A' signifies the absence of a bar, and the 'a' subtype reflects its early-stage spiral characteristics with a relatively large bulge compared to the disk.^[1] Additionally, database entries confirm the type as SA, emphasizing its unbarred nature.^[10] Some classifications note it as Sa, highlighting its spiral morphology with continuous arms.^[11]

Size and Structure

NGC 3535 exhibits a physical diameter of approximately 162,300 light-years, equivalent to about 49.77 kiloparsecs, making it a moderately sized spiral galaxy. This measurement encompasses the extent of its stellar disk and associated components, derived from integrated photometric data across multiple wavelengths. The galaxy's apparent angular size, as observed from Earth, measures 1.7 arcminutes by 0.8 arcminutes, reflecting its elongated appearance in optical images due to the inclination of its disk. This angular dimension highlights the projected structure visible in surveys like the Sloan Digital Sky Survey, where the galaxy appears as a faint, extended object. Internally, NGC 3535 features a prominent central bulge surrounded by a thin stellar disk, with spiral arms that trace the distribution of stars, gas, and dust throughout the galaxy. The bulge contributes significantly to the galaxy's luminosity in the inner regions, while the disk extends outward with tightly wound spiral patterns that show evidence of ongoing star formation. These structural elements are consistent with its unbarred spiral morphology, providing a framework for understanding its dynamical evolution.

Observational Details

Visibility and Magnitude

NGC 3535 has an apparent visual magnitude of 13.5, making it a faint object that is not visible to the naked eye and requires a telescope for observation.^[2] This magnitude indicates that it appears relatively dim from Earth, suitable for detection with amateur telescopes of moderate aperture under dark sky conditions.^[1] Due to its position close to the celestial equator, NGC 3535 is visible from both the Northern and Southern Hemispheres at certain times of the year. It is best observed in the morning sky from northern latitudes, where it becomes accessible as it rises in the east.^[12][1]

Imaging and Spectroscopy

Imaging and spectroscopy of NGC 3535 have been conducted through several modern astronomical surveys, providing detailed photometric and kinematic data for this galaxy. The Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys offer multi-band optical imaging coverage of NGC 3535, enabling precise photometric measurements across filters such as g, r, i, and z.^[13] These surveys, which combine data from the Dark Energy Camera Legacy Survey (DECaLS), the Beijing-Arizona Sky Survey (BASS), and the Mayall z-band Legacy Survey (MzLS), support target selection for DESI's spectroscopic observations and provide high-quality images for morphological analysis.^[14] Data from the NASA/IPAC Extragalactic Database (NED) and the SIMBAD astronomical database compile extensive observational records for NGC 3535, including cross-referenced photometry and positions from various telescopes.^[15] SIMBAD, maintained by the Centre de Données astronomiques de Strasbourg, aggregates imaging data from surveys like the Sloan Digital Sky Survey (SDSS), reporting angular sizes of approximately 1.66 arcminutes by 0.75 arcminutes in the optical band.^[15] These databases facilitate access to calibrated images that reveal the galaxy's unbarred spiral structure, with apparent magnitudes in the AB system ranging from u = 15.485 to z = 12.234 mag, consistent with its brightness in visible light.^[15] Spectroscopic measurements for NGC 3535, primarily from the SDSS, contribute key data on its velocity and redshift, essential for distance estimates. The 16th Data Release of the SDSS provides a heliocentric radial velocity of 6928.2 ± 3.0 km/s and a spectroscopic redshift of z = 0.02311 ± 0.00001, obtained through optical spectroscopy.^[16][15] These values, integrated into SIMBAD and referenced in NED compilations, support refined cosmological models by confirming the galaxy's recession velocity.^[15]

Discovery and History

Discovery

NGC 3535 was discovered by the German-born British astronomer William Herschel on 18 April 1784 during one of his systematic sweeps of the night sky using a homemade 18.7-inch f/13 speculum metal reflector telescope.^[7] Herschel described the object as "vF, vS, r" (very faint, very small, round) and verified it with 240 power magnification.^[7] This observation was part of his extensive program of cataloging nebulae, clusters, and other deep-sky objects, which aimed to systematically map the heavens beyond the known stars.^[2] Herschel's sweeps represented a pioneering effort in observational astronomy, involving methodical scans of the sky with his large telescopes to identify and record previously unknown celestial phenomena.^[17] Between 1783 and 1802, he and his sister Caroline amassed thousands of observations, leading to the publication of several catalogs that laid the foundation for modern deep-sky astronomy. NGC 3535, initially cataloged as WH III 111 in Herschel's third list, was later included in John Herschel's General Catalogue of Nebulae and Clusters of Stars (GC) as GC 2310.^[2] In 1888, John Louis Emil Dreyer incorporated GC 2310 into the New General Catalogue (NGC) as NGC 3535, confirming its position and description based on Herschel's original notes.^[2] This cataloging process ensured the object's place in astronomical records, linking Herschel's early visual discovery to subsequent systematic compilations.

Historical Observations

Following its discovery on 18 April 1784, NGC 3535 was observed by John Herschel on 10 April 1828 during his survey of nebulae, confirming its position and appearance as a faint, compact object.^[2] It was formally cataloged in John Louis Emil Dreyer's New General Catalogue of Nebulae and Clusters of Stars, published in 1888, where Dreyer described it as "considerably faint, very small, round, brighter middle, mottled but not resolved," based on earlier visual observations that did not resolve its structure.^[2] In the early 20th century, NGC 3535 appeared in several foundational galaxy catalogs as part of broader efforts to systematize extragalactic objects. It was included in the Morphological Catalogue of Galaxies (MCG), compiled by Vorontsov-Velyaminov and colleagues between 1961 and 1974, as MCG+01-29-004, with an early morphological assessment emphasizing its spiral nature amid the catalog's focus on detailed photographic classifications.^[3] The Uppsala General Catalogue of Galaxies (UGC), published in 1973 by P. Nilson, listed it as UGC 6189; this catalog drew primarily from photographic plates of the Palomar Observatory Sky Survey (POSS) conducted in the 1950s, enabling the first systematic measurements of its size, position angle, and basic morphology for northern declination galaxies brighter than magnitude 14.5.^[18][12] These inclusions marked a shift from visual to photographic observations, revealing NGC 3535's extended disk beyond its brighter core noted in 19th-century descriptions.^[2] The evolution of NGC 3535's classification reflects advancing observational techniques and cataloging refinements. Early 19th-century accounts portrayed it as a mottled, unresolved nebula, consistent with visual telescope limits of the era.^[2] By the mid-20th century, photographic surveys like POSS allowed for recognition of its spiral arms, leading to an initial classification as an Sa spiral in the UGC, indicating a tightly wound spiral structure with a prominent bulge.^[19] This was further consolidated in the Principal Galaxies Catalogue (PGC) of 1989, where it was designated PGC 33760 and classified as SAa, following the de Vaucouleurs revised system that emphasized unbarred spirals.^[19][1] Notable historical imaging prior to the modern era included exposures from the POSS in the 1950s, which provided the blue-sensitive plates used for UGC measurements, capturing NGC 3535's faint disk at approximately 1.7 by 0.8 arcminutes.^[18] Additional early photographic surveys, such as those contributing to the MCG, utilized Schmidt telescope plates from the 1960s to confirm its unbarred spiral form and measure its inclination, laying groundwork for later spectroscopic studies without resolving finer details like arm segments.^[3] These pre-1990 efforts, reliant on film-based photography, established NGC 3535 as a relatively isolated, early-type spiral in Leo, influencing its placement in environmental studies of galaxy evolution.^[19]

Scientific Studies

Radio Emission Properties

NGC 3535 is classified as a radio galaxy (rG) in the SIMBAD astronomical database, indicating significant non-thermal radio emission characteristic of this class of objects.^[15] This classification arises from its inclusion in a catalog of low-power radio galaxies analyzed by Lin et al. (2018), which focuses on galaxies with 1.4 GHz radio luminosities between $2 \times 10^{23}$ and $3 \times 10^{25}$ W Hz$^{-1}$.^[20][15] Radio galaxies exhibit regions of radio emission extending well beyond the visible structure of the host galaxy.^[21] The galaxy has been detected as a radio source in key surveys, including the Faint Images of the Radio Sky at Twenty-cm (FIRST) as FIRST J110833.9+044956 and the NRAO VLA Sky Survey (NVSS) as NVSS J110834+045000, with positional data confirming radio-wavelength observations at 20 cm.^[15] These detections highlight extended radio structures, typical of radio lobes formed by synchrotron emission from relativistic particles.^[22] The radio properties of NGC 3535 imply the presence of an active galactic nucleus (AGN) at its core, where a supermassive black hole accretes material and launches relativistic jets that power the observed emissions.^[23] In low-power radio galaxies such as this, the nuclear activity is often triggered by gas accretion from the intracluster medium in massive halos, influencing properties like ICM entropy and cooling time.^[20] These jets can extend over large scales, shaping the radio lobes and providing insights into AGN feedback processes in galaxy clusters.^[24]

Stellar and Gaseous Content

NGC 3535, as an unbarred spiral galaxy of morphological type SAa, hosts a stellar population dominated by older stars in its bulge and a mix of ages in the disk, though detailed age distributions are not well-characterized in available studies. Spectroscopic observations indicate the presence of neutral hydrogen gas in the interstellar medium, consistent with typical spiral galaxies at similar redshifts.^[3]

Notable Events

Supernovae

A Type Ia supernova, SN 2023hrn, was discovered in NGC 3535 on 8 May 2023 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) at an apparent magnitude of 18.4.^[4][25] This event, designated ATLAS23kbm or AT2023hrn, was identified as a candidate supernova in the galaxy at a distance of 102 Mpc.^[4][25] Type Ia supernovae arise from the thermonuclear explosion of a white dwarf star that accretes mass from a companion until reaching the Chandrasekhar limit, resulting in consistent peak luminosities that make them reliable standard candles for astronomical distance measurements.^[26][27] These events exhibit a characteristic light curve with a well-defined decline rate, allowing astronomers to calibrate their absolute brightness and infer distances by comparing observed and intrinsic magnitudes.^[28][29] Analysis of SN 2023hrn provided a distance estimate of 102 Mpc for NGC 3535, consistent with the galaxy's heliocentric distance of approximately 332 million light-years derived from other methods.^[4][25] This measurement leverages the standardized properties of Type Ia supernovae to refine cosmological distance scales in the local universe.^[26][27]

Other Transient Phenomena

As part of broader efforts to monitor distant galaxies for short-lived phenomena, the ATLAS survey routinely scans fields including NGC 3535 to identify and report such candidates to the Transient Name Server, facilitating rapid community follow-up. No other confirmed variable stars or additional transient events have been reported in NGC 3535 from systematic surveys as of January 2026, though ongoing ATLAS observations continue to provide high-cadence coverage for potential future detections in this region.^[30]