|Messier 87 as seen by the Hubble Space Telescope.|
|Right ascension||12h 30m 49.42338s|
|Declination||+12° 23′ 28.0439″|
|Apparent dimension (V)||7.2 × 6.8 moa|
|Apparent magnitude (V)||9.59|
|Type||E+0-1 pec, NLRG Sy|
|Heliocentric radial velocity||1307 ± 7 km/s|
|Redshift||0.004360 ± 0.000022|
|Galactocentric velocity||1254 ± 7 km/s|
|Distance||53.5 ± 1.63 Mly (16.40 ± 0.50 Mpc)|
|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. It is one of the brightest radio sources in the sky, and 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 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.
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. 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.
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. 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.
Identification as galaxy
American astronomer Edwin Hubble categorized M87 as one of the brighter globular nebulae, as it lacked any spiral structure, but appeared to belong to the same family of non-galactic nebulae as spiral nebulae. In 1926 he produced a new categorization, distinguishing extra-galactic nebulae from galactic nebulae, with former being independent star systems. M87 was classified as a type of elliptical extra-galactic nebula with no apparent elongation (class E0). 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. 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. The term nebula persisted for some time thereafter and M87 continued to be called an extragalactic nebula at-least until 1954. The term was replaced by galaxy in later literature.
In 1947, a prominent radio source was identified overlapping the location of M87, and this was labeled Virgo-A. 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″. In 1969–70, a strong component of the radio emission was found to closely align with the optical source of the jet.
In April 1965, the United States Naval Research Laboratory group launched an Aerobee 150 rocket equipped with a pair of Geiger counters. 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. A later Aerobee rocket launched from White Sands Missile Range on July 7, 1967, yielded further evidence that the source of Virgo X-1 was the radio galaxy M87. Subsequent X-ray observations by the HEAO 1 and Einstein Observatory showed a complex source that included the active galactic nucleus of M87. However, there is little central concentration of the X-ray emission.
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. 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.
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. 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. Viewing the jet is a challenge without the aid of photography. 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. In recent years, however, it has been observed in larger amateur telescopes under excellent conditions.
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. 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. In Yerkes (Morgan) scheme, M87 is considered a type-cD galaxy, which is a supergiant D class galaxy. 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.
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,[a] and the tip of the red-giant branch method using individually resolved red giant stars.[b] 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).
This galaxy is one of the most massive in the local Universe; 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. Within a radius of 32 kiloparsecs (100 thousand light-years), the mass is (2.4 ± 0.6) × 1012 times the mass of the Sun, which is double the mass of the Milky Way galaxy. 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. The total mass of M87 may be 200 times that of the Milky Way.
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. The extended stellar envelope of this galaxy reaches a radius of about 150 kiloparsecs (490 thousand light-years), compared to about 100 kiloparsecs (330 thousand light-years) for the Milky Way. Beyond that distance the outer edge of the galaxy has been truncated by some means; possibly by an earlier encounter with another galaxy. 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, 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. The number of satellite galaxies has been estimated to be at least 50, with NGC 4486B and NGC 4478 among them.
Active elliptical galaxies of a form similar to M87 are believed to form as a result of one or more mergers between smaller galaxies. 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. 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 period of 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.
Supermassive black hole
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☉ or (6.6 ± 0.4) × 109 M☉ up to the latest measurement +0.34
−0.40×109 M☉. 7.22 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, and spans a maximum diameter of 0.12 parsecs (0.39 light-years). 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).
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). 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. 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.
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. 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. The distribution of oxygen is roughly uniform throughout, while iron is concentrated more near the center. Since oxygen is produced mainly by core-collapse supernovae, which occur during early stages of galaxies and mostly in outer star forming regions, 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. The contribution from these sources was much lower in abundance than in the Milky Way.
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. 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. This dust may be destroyed by the hostile environment or expelled from the galaxy. The combined mass of dust in this galaxy is no more than 70,000 times the mass of the Sun. By comparison, the Milky Way contains about a hundred million (108) solar masses worth of dust.
Within a 4 kpc (13×103 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. Although this 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. Surrounding the galaxy is an extended corona with hot, low density gas.
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. 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. The first hypervelocity globular cluster, HVGC-1, was discovered escaping M87. The discovery of HVGC-1 suggests that the core of M87 held not one but two supermassive black holes. A supermassive black hole binary can result from a long-ago collision between two galaxies, which merged to form a single giant galaxy.
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.
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). 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. 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 (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). 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. The jet is precessing, causing the outflow to form a helical pattern out to a distance of 1.6 parsecs (5.2 light-years). Lobes of matter from the jet extend out to a distance of 77 kiloparsecs (250 thousand light-years).
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 motion is presumably an optical illusion caused by the relativistic velocity of the jet, and not true superluminal motion. However, detection of such motion supports the theory that quasars, BL Lacertae objects and radio galaxies may all be the same phenomenon, known as active galaxies, viewed from different perspectives. It has been proposed 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.
Observations made by Chandra X-ray Observatory indicate the presence of loops and rings in the hot X-ray emitting gas that permeates the cluster and surrounds M87. These loops and rings are generated by pressure waves. The pressure waves are caused by variations in the rate at which material is ejected from the supermassive black hole in jets. The distribution of loops suggests that minor eruptions occur every six 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. The observations also imply the presence of sound waves, 56 octaves below middle C for the minor eruptions and 58 to 59 octaves below middle C for the major eruptions.
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, but in 2006, using the High Energy Stereoscopic System Cherenkov telescopes, scientists have 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. 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. This X-ray emission has since been decaying in a variable manner.
This supergiant elliptical galaxy is located near the center of the Virgo Cluster. This rich cluster has about 2,000 members 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. 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. There is a preponderance of elliptical and S0 galaxies around M87, with a chain of elliptical galaxies aligned with the jet. 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. 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. The combined mass of the cluster is estimated to be (0.15–1.5) × 1015 solar masses.
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.
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