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Messier 87

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Messier 87
Messier 87 Hubble WikiSky.jpg
Messier 87 as seen by the Hubble Space Telescope.
Observation data
Epoch J2000
Constellation Virgo
Right ascension 12h 30m 49.42338s[1]
Declination +12° 23′ 28.0439″[1]
Apparent dimension (V) 7.2 × 6.8 moa[2]
Apparent magnitude (V)9.59[3]
Characteristics
TypeE+0-1 pec, NLRG Sy[3]
Astrometry
Heliocentric radial velocity 1307 ± 7[3] km/s
Redshift 0.004360 ± 0.000022[3]
Galactocentric velocity 1254 ± 7[3] km/s
Distance 53.5 ± 1.63 Mly (16.40 ± 0.50 Mpc)[4]
Other designations
Virgo A, Virgo X-1, NGC 4486, UGC 7654, PGC 41361, VCC 1316, Arp 152, 3C 274,[3] 3U 1228+12.[5]
Database references
SIMBAD data
See also: Galaxy, List of galaxies

Messier 87 (also known as Virgo A or NGC 4486, and generally abbreviated to M87) is a supergiant elliptical galaxy in the constellation Virgo. One of the most massive galaxies in the local universe, it is notable for its large population of globular clusters—M87 contains about 12,000 compared to the 150–200 orbiting the Milky Way—and its jet of energetic plasma that originates at the core and extends outward at least 1,500 parsecs (4,900 light-years), travelling at relativistic speed. One of the brightest radio sources in the sky, it is a popular target for both amateur astronomy observations and professional astronomy study.

French astronomer Charles Messier discovered M87 in 1781, cataloguing it as a nebulous feature while searching for objects that would confuse comet hunters. The second brightest galaxy within the northern Virgo Cluster with many satellite galaxies, M87 is located about 16.4 million parsecs (53.5 million light-years) from Earth. Unlike a disk-shaped spiral galaxy, M87 has no distinctive dust lanes. Instead, it has an almost featureless, ellipsoidal shape typical of most giant elliptical galaxies, diminishing in luminosity with distance from the centre. Forming around one sixth of M87's mass, the stars in this galaxy have a nearly spherically symmetric distribution, their population density decreasing with increasing distance from the core. At the core is an active supermassive black hole, which forms the primary component of an active galactic nucleus. This object is a strong source of multiwavelength radiation, particularly radio waves. M87's galactic envelope extends out to a radius of about 150 kiloparsecs (490,000 light-years), where it has been truncated—possibly by an encounter with another galaxy. Between the stars is a diffuse interstellar medium of gas that has been chemically enriched by elements emitted from evolved stars.

Observation history[edit]

In 1781, French astronomer Charles Messier published a catalogue of 103 objects that had a nebulous appearance. This list was intended to identify objects that might be confused with comets. In subsequent use, each item in the catalogue was prefixed with an "M". Thus, M87 was the eighty seventh member of Messier's catalogue.[6] During the 1880s, this object was included in the New General Catalogue as NGC 4486. This compilation of nebulae and star clusters was assembled by the Danish-Irish astronomer John Dreyer based primarily on the observations of English astronomer John Herschel.[7]

In 1918, American astronomer Heber Curtis of Lick Observatory observed that there was no spiral structure in M87 and he noticed a "curious straight ray ... apparently connected with the nucleus by a thin line of matter." The ray appeared brightest at the inner end.[8] The following year, a supernova within M87 reached a peak photographic magnitude of 21.5, although this event was not reported until photographic plates were examined by the Russian astronomer Innokentii A. Balanowski in 1922.[9][10]

Identification as galaxy[edit]

 Hubble classified galaxies according to their shape: ellipticals, lenticulars and spirals. Ellipticals and spirals have further categories
Hubble galaxy classification scheme: M87 is an E0 galaxy

In 1922, American astronomer Edwin Hubble categorized M87 as one of the brighter globular nebulae, as it lacked any spiral structure, but just like spiral nebulae, appeared to belong to the family of non-galactic nebulae.[11] In 1926 he produced a new categorization, distinguishing extragalactic nebulae from galactic nebulae, with the former being independent star systems. M87 was classified as a type of elliptical extragalactic nebula with no apparent elongation (class E0).[12] By 1931, Hubble had identified M87 as a member of the Virgo Cluster, for which he gave a provisional estimate of 1.8 million parsecs from Earth. At the time it was the only known example of an elliptical nebula for which individual stars could be resolved, although it was pointed out that globular clusters would be indistinguishable from individual stars at such great distances.[13] In his 1936 work The Realm of the Nebulae, he discusses the terminology of the day; some labeling extragalactic nebulae as external galaxies on the grounds that they were stellar systems far away from our own galaxy, while others preferred term extragalactic nebulae in following the convention, as galaxy at that time was analogue to Milky Way.[14] The term nebula persisted for some time thereafter and M87 continued to be called an extragalactic nebula at least until 1954.[15] The term was replaced by galaxy in later literature.[16]

Images showing helical flow of matter in M87 jet
Hubble follows spiral flow of black-hole-powered jet.[17]

Modern research[edit]

In 1947, a prominent radio source was identified overlapping the location of M87, and this was labeled Virgo A.[18] This source was confirmed to be M87 by 1953, and the linear relativistic jet emerging from the core of the galaxy was suggested as the cause. This jet extended from the core at a position angle of 260° to an angular distance of 20″ with an angular width of 2″.[15] In 1969–70, a strong component of the radio emission was found to closely align with the optical source of the jet.[5]

In April 1965, the United States Naval Research Laboratory group launched an Aerobee 150 rocket equipped with a pair of Geiger counters.[19] This flight discovered seven candidate X-ray sources, including the first extragalactic X-ray source; Virgo X-1 was designated as the first X-ray source detected in Virgo.[20] A later Aerobee rocket launched from White Sands Missile Range on 7 July 1967, yielded further evidence that the source of Virgo X-1 was the radio galaxy M87.[21] Subsequent X-ray observations by the HEAO 1 and Einstein Observatory showed a complex source that included the active galactic nucleus of M87.[22] However, there is little central concentration of the X-ray emission.[5]

M87 has been an important testing ground for techniques to measure the masses of central supermassive black holes in galaxies. In 1978, stellar-dynamical modeling of the mass distribution in M87 demonstrated evidence for a central mass of 5 billion solar masses.[23] After the installation of the COSTAR corrective-optics module in the Hubble Space Telescope in 1993, the Hubble Faint Object Spectrograph (FOS) was used to measure the rotation velocity of the ionized gas disk in the center of M87, as an "early release observation" designed to test the scientific performance of the post-repair Hubble instruments. The FOS data indicated a central black hole mass of 2.4 billion solar masses, with 30% uncertainty.[24]

Visibility[edit]

The location of M87 (upper right) in Virgo constellation
Cercle rouge 100%.svg
The location of M87 in Virgo

M87 is located near the high declination border of the Virgo constellation, next to the constellation of Coma Berenices. It lies along the line between the stars Epsilon Virginis and Denebola.[a] At an apparent magnitude of 9.59, this galaxy can be readily observed using a small telescope with a 6 cm (2.4 in) aperture, extending across an angular area of 7.2 × 6.8 arcminutes with a bright, 45 arcsecond core.[2] Viewing the jet is a challenge without the aid of photography.[25] Before 1991, Russian-American astronomer Otto Struve was the only person known to have seen the jet visually, using the 254 cm (100 in) Hooker telescope.[26] In recent years, however, it has been observed in larger amateur telescopes under excellent conditions.[27]

Properties[edit]

Central bright region of M87 is surrounded by diffuse, fainter halo
Huge halo around Messier 87.[28]

In the modified Hubble sequence galaxy morphological classification scheme of French astronomer Gérard de Vaucouleurs, M87 is categorized as an E0p galaxy. The E0 designation is used for an elliptical galaxy that displays no flattening—that is, it appears spherical.[29] A "p" suffix indicates a peculiar galaxy that does not fit cleanly into the classification scheme; in this case, the peculiarity denotes the presence of the jet emerging from the core.[29][30] In Yerkes (Morgan) scheme, M87 is considered a type-cD galaxy, which is a supergiant D class galaxy.[31][32] The latter category, first proposed by American astronomer William W. Morgan in 1958, is a galaxy that has an elliptical-like nucleus surrounded by an extensive, dustless, diffuse envelope.[33][34]

The distance to M87 has been estimated using several independent techniques. These include measuring the luminosity of planetary nebulae, comparison with nearby galaxies whose distance has been estimated using standard candles such as cepheid variables, the linear size distribution of globular clusters,[b] and the tip of the red-giant branch method using individually resolved red giant stars.[c] These measurements are consistent with each other, and their weighted average yields a distance estimate of 16.4 ± 0.5 megaparsecs (53.5 ± 1.63 million light-years).[4]

Enclosed mass
Mass
×1012 M
Radius
kpc
2.4[35] 32
3.0[36] 44
5.7[37] 47
6.0[38] 50

This galaxy is one of the most massive in the local Universe;[37] it spans a diameter of 120,000 light-years, about the same as the Milky Way. But M87 is a spheroid, not a flat spiral. The mass of M87 within a radius of 9–40 kiloparsecs (29–130 thousand light-years) from the core steadily increases roughly in proportion to r1.7, where r is the radius from the core.[36] Within a radius of 32 kiloparsecs (100 thousand light-years), the mass is (2.4 ± 0.6) × 1012 times the mass of the Sun,[35] which is double the mass of the Milky Way galaxy.[39] Only a fraction of this mass is in the form of stars, as M87 has an estimated mass to luminosity ratio of 6.3 ± 0.8. That is, about one part in six of the galaxy's mass is in the form of stars that are radiating energy.[40] The total mass of M87 may be 200 times that of the Milky Way.[41]

stellar velocities in M87 are somewhat random, as opposed to more circular velocities in spirals
Stellar velocity map of the central region of M87, showing the motion of stars relative to Earth. Blue patches represent motion towards the Earth and red ones away from the Earth, while yellow and green are in between the two extremes. The map signifies random motion of the stars.[42][43]

Gas is infalling into the galaxy at the rate of two to three solar masses per year, most of which may be accreted onto the core region.[44] The extended stellar envelope of this galaxy reaches a radius of about 150 kiloparsecs (490 thousand light-years),[45][46] compared to about 100 kiloparsecs (330 thousand light-years) for the Milky Way.[47] Beyond that distance the outer edge of the galaxy has been truncated by some means; possibly by an earlier encounter with another galaxy.[45][46] There is some evidence of linear streams of stars to the northwest of the galaxy, which may have been created by tidal stripping of orbiting galaxies, or by small satellite galaxies falling in toward M87,[48] as well as a filament of hot, ionized gas in the northeastern outer part of this galaxy that has been proposed to be the remnant of a small, gas-rich galaxy that was disrupted by M87 and that could be feeding its active nucleus.[49] The number of satellite galaxies has been estimated to be at least 50, with NGC 4486B and NGC 4478 among them.[50][51]

The spectrum of nuclear region of M87 shows emission lines of various ions including HI, HII, HeI, OI, OII, OIII, NI, MgII and SII. The line intensities for weakly ionized atoms (such as OI) are stronger than those of strongly ionized atoms (such as OIII). Galactic nuclei with such spectral properties are termed as LINERs for "Low Ionization Nuclear Emission-line Region".[52][53] The mechanism and ionizing source for such behavior of LINERs and M87 is under debate. Shock induced excitation[52][53] and photoionization[54] powered by the jet have been claimed as the causes in M87.

Active elliptical galaxies of a form similar to M87 are believed to form as a result of one or more mergers between smaller galaxies.[55] There is now little dust remaining to form the diffuse nebulae where new stars were created, so the stellar population is dominated by old, population II stars that contain relatively low abundances of elements other than hydrogen and helium. M87's elliptical shape is maintained by random orbital motions of its member stars, in contrast to the more orderly rotational motions found in a spiral galaxy such as the Milky Way.[56] Using the Very Large Telescope to study the motions of about 300 planetary nebulae, it has been shown that M87 has absorbed a medium-sized star-forming spiral galaxy over the last billion years. This has resulted in addition of some younger, bluer stars to M87. The distinctive spectral properties of the planetary nebulae allowed astronomers to discover a chevron-like structure in M87's halo which was produced by the incomplete phase-space mixing of a disrupted galaxy.[28][57]

Components[edit]

Supermassive black hole[edit]

Supermassive black holes are surrounded by massive accretion disks and generate jets of plasma
Artist's concept of a supermassive black hole and its accretion disk

At the core of this galaxy is a supermassive black hole estimated to weigh billions of times the Sun's mass: from (3.5 ± 0.8) × 109 M[58] or (6.6 ± 0.4) × 109 M[58] up to the latest measurement 7.22+0.34
−0.40
×109
M.[59] This is one of the highest masses known for such an object. Surrounding the black hole is a rotating disk of ionized gas, oriented roughly perpendicular to the relativistic jet. This disk rotates at velocities of up to roughly 1,000 km/s,[60] and spans a maximum diameter of 0.12 parsecs (0.39 light-years).[61] Gas accretes onto the black hole at an estimated rate equal to the mass of the Sun every ten years (about 91 earth masses per day).[62]

Observations suggest that the black hole in M87 may be displaced from the galaxy center by a distance of about 25 parsecs (82 light-years).[63] The displacement is in the opposite direction from the one-sided jet, which may indicate that the black hole was accelerated away by the jet. Another possibility is that the displacement occurred during the merger of two supermassive black holes.[63][64] The study, however, does not include any spectroscopic discrimination between the stellar and active galactic nucleus components. It is, therefore, possible that the seeming position of the galaxy center with respect to the black hole is misinterpreted by an optical flare of the jet itself. In 2011, an analysis of M87 did not find any statistically significant displacement.[65]

Interstellar medium[edit]

The space between the stars in the M87 galaxy is filled with a diffuse interstellar medium of gas, which has been chemically enriched by the elements ejected from stars as they passed beyond the end of their main sequence lifetime. Carbon and nitrogen are continuously supplied by intermediate mass stars as they pass through the asymptotic giant branch.[66][67] The heavier elements from oxygen to iron are primarily produced by supernova explosions within the galaxy. About 60% of the abundance of these heavy elements was produced by core-collapse supernovae, while the remainder came from Type Ia supernovae.[66] The distribution of oxygen is roughly uniform throughout, while iron is concentrated more near the center.[67][68] Since oxygen is produced mainly by core-collapse supernovae, which occur during early stages of galaxies and mostly in outer star forming regions,[66][67][68] such distribution of these elements suggests an early enrichment of the interstellar medium of M87 from core-collapse supernovae and continuous contribution from Type Ia supernovae throughout the history of the galaxy.[66] The contribution from these sources was much lower in abundance than in the Milky Way.[66]

Examination of M87 at far infrared wavelengths shows an excess emission at wavelengths longer than 25 μm. Normally, such emission may be an indication of thermal emission by warm dust.[69] However, in the case of M87, the emission appears to be fully explained by synchrotron radiation from the jet. Within the galaxy, silicate grains are expected to survive for no more than 46 million years because of the X-ray emission from the core.[70] This dust may be destroyed by the hostile environment or expelled from the galaxy.[71] The combined mass of dust in this galaxy is no more than 70,000 times the mass of the Sun.[70] By comparison, the Milky Way contains about a hundred million (108) solar masses worth of dust.[72]

Within a 4 kpc (13×10^3 ly) radius of the core, the abundance of elements other than hydrogen and helium—what astronomers term the metallicity—is about half the abundance in the Sun. Outside this radius, the abundance of metals steadily decreases with increasing distance from the core.[73] Although M87 is classified as an elliptical galaxy and therefore lacks the dust lanes of a spiral galaxy, optical filaments have been observed in M87. These filaments have an estimated mass of about 10,000 times the mass of the Sun.[44] Surrounding the galaxy is an extended corona with hot, low density gas.[73]

Globular clusters[edit]

M87 has an abnormally large population of globular clusters. A 2006 survey out to an angular distance of 25′ from its core estimates that there are 12,000 ± 800 globular clusters in orbit around M87, as compared to the Milky Way's 150–200.[74] These clusters are similar in size distribution to the globular clusters of the Milky Way, with most having an effective radius between 1 and 6 parsecs. The size of the M87 clusters shows a gradual increase with distance from the galactic center.[75] In 2014, the first hypervelocity globular cluster, HVGC-1, was discovered escaping at 2300 km/s from M87. Several possibilities have been suggested for such a high velocity, including close encounter with and subsequent gravitational kick by a supermassive black hole binary.[76] If correct, this interpretation would mean that the core of M87 has two supermassive black holes, resulting from an ancient collision between two galaxies, which merged into a single giant galaxy.[77]

Ultra-compact dwarfs resemble globular clusters but have a diameter of 10 parsecs (33 light-years) or more, much larger than the 3-parsec (10 light-years) maximum of globular clusters. Their nature is unclear as to whether they are dwarf galaxies captured by M87 or a new class of massive globular cluster. Almost a hundred have been identified from the thousands of globular clusters in M87.[78]

Jet[edit]

M87 jet extends up to 5000 light years from the core.
This Hubble Space Telescope photograph shows the jet of matter ejected from M87 at nearly the speed of light, as it stretches 1.5 kpc (5 kly) from the galactic core.
In X-ray image, blue appearing hot matter from cluster falls to M87 center and cools, thus fading in brightness. Jet (appearing orange in radio) hinders this infall and lifts the falling matter up.
In this X-ray (Chandra) and radio (VLA) composite image, hot matter (blue in X-ray) from the Virgo cluster falls toward the core of M87 and cools, where it is met by the relativistic jet (orange in radio), producing shock waves in the galaxy's interstellar medium.

The relativistic jet of matter emerging from the core extends at least 1.5 kiloparsecs (5 thousand light-years) from the nucleus of M87 and is made up of matter ejected from the galaxy by a supermassive black hole. This jet is highly collimated, appearing constrained to an angle of 60° within 0.8 parsecs (2.6 light-years) of the core, about 16° at a distance of 2 parsecs (6.5 light-years) and an angle of 6–7° at a distance of 12 parsecs (39 light-years).[79] The jet's base has the diameter of 5.5 ± 0.4 Schwarzschild radii. The jet is probably powered by a prograde accretion disk around a spinning supermassive black hole.[79] German-American astronomer Walter Baade found that the light from the jet is plane polarized, which suggested that the energy was being generated by the acceleration of electrons moving at relativistic velocities in a magnetic field. The total energy output of these electrons was estimated as 5.1 × 1056 ergs[80] (or 5.1 × 1049 joules or 3.2 × 1068 eV). For comparison, the entire Milky Way galaxy output is estimated at 5 × 1036 joules per second (watts).[81] It is surrounded by a lower-velocity non-relativistic component. There is evidence of a counter jet, but this feature remains unseen from the Earth due to relativistic beaming.[82][83] The jet is precessing, causing the outflow to form a helical pattern out to a distance of 1.6 parsecs (5.2 light-years).[61] Lobes of matter from the jet extend out to a distance of 77 kiloparsecs (250 thousand light-years).[84]

In pictures taken by the Hubble Space Telescope in 1999, the motion of M87's jet was measured at four to six times the speed of light. This phenomenon, called superluminal motion, is an illusion caused by the relativistic velocity of the jet. This is because the time interval between any two light pulses emitted by the jet is, as registered by the observer, less than actual interval between the pulses due to relativistic speed of the jet moving in the direction of the observer. This results in perceived faster than light speeds. Detection of such motion is believed to support the theory that quasars, BL Lacertae objects and radio galaxies may all be the same phenomenon, known as active galaxies, viewed from different perspectives.[85][86] It has been proposed that M87 could actually be a BL Lacertae object (with a low-luminosity nucleus compared with the brightness of its host galaxy) seen from an unfavorable angle to appreciate the properties of that kind of galaxy.[87][88]

M87 black hole is a strong source of radio waves
Radio wavelength image of M87 showing strong radio emission from the core

Observations indicate that the rate at which material is ejected from the supermassive black hole in jets is variable. These variations produce pressure waves in the hot gas surrounding M87. Chandra X-ray Observatory has detected loops and rings in the gas. The distribution of loops suggests that minor eruptions occur every few million years. One of the rings, caused by a major eruption, is a shock wave 26 kiloparsecs (85 thousand light-years) in diameter around the black hole. Other features observed include narrow X-ray emitting filaments up to 31 kiloparsecs (100 thousand light-years) long, and a large cavity in the hot gas caused by a major eruption 70 million years ago. The regular eruptions prevent a huge reservoir of gas from cooling and forming stars, implying that M87's evolution may have been seriously affected, preventing it from becoming a large spiral galaxy. These observations also indicate that these variable eruptions produce sound waves of about 56 to 59 octaves below middle C in the medium.[89]

M87 is a very strong source of gamma rays, which are the most energetic rays of the electromagnetic spectrum. Gamma rays coming from M87 have been observed since the late 1990s. In 2006, using the High Energy Stereoscopic System Cherenkov telescopes, scientists measured the variations of the gamma ray flux coming from M87, and found that the flux changes over a matter of days. This short period makes the immediate vicinity of the supermassive black hole in M87 the most promising source for these gamma rays.[90] In general, the smaller the diameter of the emission source, the faster the variation in flux, and vice versa.

A knot of matter in the jet, designated HST-1, has been tracked by the Hubble Space Telescope and the Chandra X-ray Observatory. This knot is about 65 parsecs (210 light-years) from the core. By 2006, the X-ray intensity of this knot had increased by a factor of 50 over a four-year period.[91] This X-ray emission has since been decaying in a variable manner.[92]

Interaction of relativistic jets of plasma emanating from the core with the surrounding medium gives rise to radio lobes in active galaxies. The lobes occur in pairs and are often symmetrical.[93] Observations have revealed the radio lobes of M87 with a combined span of about 80 kiloparsecs. Inner part, extending upto 2 Kiloparsecs, is extremely bright in radio. Two flows of material emerge from this region, with one aligned with the jet itself and the other in the opposite direction. Flows are, however, asymmetrical and deformed. This implies that flows encounter dense intracluster medium. At larger distances both flows diffuse into two lobes. The lobes are surrounded by a fainter halo of radio emitting gas.[94][95]

Environment[edit]

Visible wavelength image of Virgo cluster with M87 located near lower left
Messier 87 is a member of the Virgo Cluster, and can be seen to lower left of this cluster image.

This supergiant elliptical galaxy is located near the center of the Virgo Cluster.[32] This rich cluster has about 2,000 members[96] and it forms the core of the larger Virgo Supercluster, of which the Local Group, and hence the Milky Way galaxy, is an outlying member.[45] The cluster is organized into at least three distinct subsystems that are associated with the three galaxies M87, M49 and M86, with the one centered around M87 known as Virgo A and the one around M49 as Virgo B.[97] There is a preponderance of elliptical and S0 galaxies around M87, with a chain of elliptical galaxies aligned with the jet.[98] In terms of mass, M87 is a dominant member of the cluster, and hence appears to be moving very little relative to the cluster as a whole.[45] Indeed, M87 is defined as the cluster center. The cluster has a sparse gaseous atmosphere that emits X-rays that decrease in temperature toward the middle, where M87 is located.[69] The combined mass of the cluster is estimated to be (0.15–1.5) × 1015 solar masses.[96]

Measurements of the motion of intracluster planetary nebulae between M87 and M86 suggest that these two galaxies are moving toward each other and this may be their first encounter. M87 may have encountered M84 in the past, as evidenced by the truncation of the outer halo of the former from tidal effects. However, another possible cause of this truncation is a contraction due to an unseen mass falling into M87 from the rest of the cluster, which, in particular, may be the hypothesized dark matter. A third possibility is that the halo formation was truncated as a result of early feedback from the active galactic nucleus at the core of M87.[45]

Notes[edit]

  1. ^ Epsilon Virginis is at celestial coordinates α=13h 02m, δ=+10° 57′; Denebola is at α=11h 49m, δ=+14° 34′. The midpoint of the pair is at α=12h 16m, δ=12° 45′. Compare to the coordinates of Messier 87: α=12h 31m, δ=+12° 23′.
  2. ^ this yields a distance of 16.4 ± 2.3 megaparsecs (53.5 ± 7.50 million light-years).[4]
  3. ^ this yields a distance of 16.7 ± 0.9 megaparsecs (54.5 ± 2.94 million light-years).[4]

References[edit]

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  2. ^ a b Luginbuhl, C. B.; Skiff, B. A. (1998). Observing Handbook and Catalogue of Deep-Sky Objects (2nd ed.). Cambridge, United Kingdom: Cambridge University Press. p. 266. ISBN 0-521-62556-4. 
  3. ^ a b c d e f "Results for NGC 4486". NASA/IPAC Extragalactic Database. California Institute of Technology. Retrieved 21 March 2018.  In "Source List", click "Row no. 1" OR click "Home", Select the "By Name" cell, then enter "NGC 4486" in the "Object Name:" field of the query form.
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  6. ^ Basu, B.; Chattopadhyay, T.; Biswas, S. N. (2010). An Introduction to Astrophysics (2nd ed.). New Delhi: PHI Learning Pvt. Ltd. p. 278. ISBN 978-81-203-4071-8. 
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External links[edit]

Coordinates: Sky map 12h 30m 49.4s, +12° 23′ 28″