Messier 87

Observation data Characteristics Epoch J2000 The Messier 87 galaxy. Constellation Virgo Right ascension 12h 30m 49.42338s[1] Declination +12° 23′ 28.0439″[1] Apparent dimension (V) 8.3 × 6.6 moa[2] Apparent magnitude (V) 9.59[2] Type E+0-1 pec, NLRG Sy[2] 1307 ± 7[2]km/s 0.004360 ± 0.000022[2] 1254 ± 7[2] km/s 53.5 ± 1.63 Mly (16.40 ± 0.50 Mpc)[3] Virgo A, Virgo X-1, NGC 4486, UGC 7654, PGC 41361, VCC 1316, Arp 152, 3C 274,[2] 3U 1228+12.[4] SIMBAD Search M87 data

Messier 87 (also known as M87, Virgo A or NGC 4486) is a supergiant elliptical galaxy. It was discovered in 1781 by French astronomer Charles Messier, who cataloged it as a nebulous feature. The second brightest galaxy within the northern Virgo Cluster, it is located about 16.4 million parsecs (53.5 million light-years) from Earth.[3][5] Unlike a disk-shaped spiral galaxy, Messier 87 has no distinctive dust lanes and it has an almost featureless, ellipsoidal shape that diminishes in luminosity with distance from the center. At the core is a 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. A jet of energetic plasma originates at the core and extends outward at least 1,500 parsecs (5,000 light-years).[6]

The stars in this galaxy form about one sixth of Messier 87's mass. They have a nearly spherically symmetric distribution, while the density of stars decreases with increasing distance from the core. The galactic envelope extends out to a radius of about 150 kpc (490 kly), 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. Orbiting the galaxy is an abnormally large population of about 12,000 globular clusters, compared to 150-200 globular clusters orbiting the Milky Way.

As one of the most massive[7] giant elliptical galaxies near our galaxy and one of the brightest radio sources in the sky, Messier 87 is a popular target for both amateur astronomy observations and professional astronomy study.

Observation history

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.[8] During the 1880s, this nebula 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.[9]

In 1918, American astronomer Heber Curtis of Lick Observatory observed that there was no spiral structure in Messier 87 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.[10] The following year, a supernova within Messier 87 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.[11][12]

American astronomer Edwin Hubble categorized Messier 87 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.[13] In 1926 he produced a new categorization of nebulae, with Messier 87 being classified as a type of elliptical extra-galactic nebula with no apparent elongation (class E0).[14] By 1931, Hubble had identified Messier 87 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.[15] Messier 87 continued to be called an extragalactic nebula for many years thereafter, but by 1956 it had been identified as an E0-type galaxy.[16]

In 1947, a prominent radio source was identified overlapping the location of Messier 87, and this was labeled Virgo-A.[17] This source was confirmed to be Messier 87 by 1953, and the linear jet emerging from the core of this extragalactic nebula 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″.[6] German-American astronomer Walter Baade found that the light from this 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[18] (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).[19] In 1969-70, a strong component of the radio emission was found to closely align with the optical source of the jet.[4]

Hubble follows spiral flow of black-hole-powered jet.[20]

In April 1965, the US Naval Research Laboratory group launched an Aerobee 150 equipped with a pair of geiger counters.[21] 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.[22] A later Aerobee rocket launched from White Sands Missile Range on July 7, 1967, yielded further evidence that the source Virgo X-1 was the radio galaxy Messier 87.[23] Subsequent X-ray observations by the HEAO 1 and Einstein Observatory showed a complex source that included the active galactic nucleus of Messier 87.[24] However, there is little central concentration of the X-ray emission.[4]

Properties

The location of M87 (upper right) in Virgo

Messier 87 is located near the high declination border of the Virgo, next to the constellation of Coma Berenices. It lies along a line between the stars Epsilon Virginis and Denebola.[25] 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. Visually, the galaxy extends across an angular area of 7′.2 × 6′.8, with a bright, 45″ core.[26] Viewing the jet feature is a challenge without the aid of photography.[27] Up through 1990, the only known visual observation of the jet had been by Russian-American astronomer Otto Struve using the 254 cm (100 in) Hooker telescope.[28] In recent years, however, it has been observed in larger amateur telescopes under excellent conditions.[29]

In the modified Hubble sequence galaxy morphological classification scheme of French astronomer Gérard de Vaucouleurs, Messier 87 is categorized as an E0p galaxy. The E0 designation is used for an elliptical galaxy that displays no flattening—that is, it appears spherical in profile.[30] A 'p' suffix indicates a peculiar galaxy that does not fit cleanly into the classification scheme; in this case, the peculiarity is caused by the presence of a relativistic jet emerging from the core.[30][31] Messier 87 is considered a type-cD galaxy, which is a supergiant D class galaxy.[32][33] 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.[34][35]

The distance to Messier 87 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, and the tip of the red giant branch method using individually resolved red giant stars. These measurements are consistent with each other, and their weighted average yields a distance estimate of 16.7 ± 0.5 Mpc (54.5 ± 1.63 Mly).[3]

 Mass ×1012 M☉ Radius kpc 2.4[36] 32 3.0[37] 44 5.7[7] 47 6.0[38] 50

This galaxy is one of the most massive in the local Universe.[7] This galaxy spans a diameter of 120,000 light years, about the same as the Milky Way. But M87 is a spheroid, not a flat spiral. So it contains close to 2.7 trillion solar masses by some estimates.[39] The mass of Messier 87 within a radius of 9–40 kpc (29–130 kly) from the core steadily increases roughly in proportion to r1.7, where r is the radius from the core.[37] Within a radius of 32 kpc (100 kly), the mass is (2.4 ± 0.6) × 1012 times the mass of the Sun,[36] which is double the mass of the Milky Way galaxy.[40] Only a fraction of this mass is in the form of stars, as Messier 87 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.[41] The total mass of Messier 87 may be 200 times that of the Milky Way.[42]

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.[43] The extended stellar envelope of this galaxy reaches a radius of about 150 kpc (490 kly),[44][45] compared to about 100 kpc (330 kly) for the Milky Way.[46] Beyond that distance the outer edge of the galaxy has been truncated by some means; possibly by an earlier encounter with another galaxy.[44][45] 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 Messier 87.[47] 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 Messier 87 and that could be feeding its active nucleus.[48]

Components

Artist's concept of a supermassive black hole and its accretion disk

At the core of this galaxy is a supermassive black hole (SMBH) with an estimated (3.5 ± 0.8) × 109 times the mass of the Sun.[49] This is one of the highest masses known for such an object. Surrounding the black hole is a rotating disk of ionized gas that is oriented roughly perpendicular to a relativistic jet. This disk is rotating at velocities of up to roughly 1,000 km/s,[50] and spans a maximum diameter of 0.12 pc (0.39 ly).[51] Gas is accreting onto the black hole at an estimated rate equal to the mass of the Sun every ten years.[52]

Observations suggest that the black hole in M87 may be displaced from the galaxy center by a distance of about 25 pc (82 ly).[53] The displacement is in the opposite direction from the one-sided jet, which may indicate that the black hole has been accelerated away from the center by the jet. Another possibility is that the displacement occurred during the merger of two SMBH.[53][54] However care has to be taken with these findings. The study does not include any spectroscopic discrimination between the stellar and active galactic nucleus component. 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.[55]

Active elliptical galaxies of a form similar to Messier 87 are believed to form as a result of one or more mergers between smaller galaxies.[56] There is now little dust remaining to form the diffuse nebulae where new stars are created, so the stellar population is dominated by old, population II stars that contain relatively low abundances of elements other than hydrogen and helium. The elliptical shape of this galaxy 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.[57]

The space between the stars in the Messier 87 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 is being 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 these elements suggests that early enrichment was from core-collapse supernovae. The contribution from these sources was much lower in abundance than in the Milky Way. Type Ia supernovae have provided a continuous contribution to the interstellar medium of Messier 87 throughout the history of the galaxy.[58]

Colors represent the motion of the stars that form the giant elliptical galaxy Messier 87.[59]

Examination of Messier 87 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.[60] However, in the case of Messier 87, 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.[61] This dust may be destroyed by the hostile environment or expelled from the galaxy.[61] The combined mass of dust in this galaxy is no more than 70,000 times the mass of the Sun.[61] By comparison, the Milky Way contains about a hundred million (108) solar masses worth of dust.[62]

Within a 4 kpc (13 kly) 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.[63] Although this is classified as an elliptical galaxy and therefore lacks the dust lanes of a spiral galaxy, optical filaments have been observed in Messier 87. These filaments have an estimated mass of about 10,000 times the mass of the Sun.[43] Surrounding the galaxy is an extended corona with hot, low density gas.[63]

Messier 87 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 Messier 87, as compared to the Milky Way's 150–200.[64] 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 Messier 87 clusters shows a gradual increase with distance from the galactic center.[65] The first hypervelocity globular cluster, HVGC-1, was discovered escaping M87.[66] The discovery of HVGC-1 suggests that the core of M87 holds not one but two supermassive black holes. The two supermassive black holes are the result of a long-ago collision between two galaxies, which merged to form a single giant galaxy.[67]

Jet

This Hubble Space Telescope photograph shows the jet of matter ejected from M87 at nearly light speed, as it stretches 1.5 kpc (5 kly) from the galactic core.
In this Chandra X-ray Observatory image, cold matter from the Virgo cluster falls toward the core of M87, where it's met by the relativistic jet, producing shock waves in the galaxy's interstellar medium.

The relativistic jet of matter emerging from the core extends at least 1.5 kpc (5 kly) from the nucleus of Messier 87 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 pc (2.6 ly) of the core, about 16° at a distance of 2 pc (6.5 ly) and an angle of 6–7° at a distance of 12 pc (39 ly).[68] 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.[68] 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.[69][70] The jet is precessing, causing the outflow to form a helical pattern out to a distance of 1.6 pc (5.2 ly).[51] Lobes of matter from the jet extend out to a distance of 77 kpc (250 kly).[71]

In pictures taken by the Hubble Space Telescope in 1999, the motion of Messier 87's jet was measured at four to six times the speed of light. This motion may be 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 Lac objects and radio galaxies may all be the same phenomenon, known as active galaxies, viewed from different perspectives.[72][73] It has been proposed Messier 87 could actually be a BL Lacertae object (with a low-luminosity nuclei compared with the brightness of its host galaxy) seen from an unfavorable angle to appreciate the properties of that kind of galaxies.[74][75]

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 Messier 87. 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 kpc (85 kly) in diameter around the black hole. Other features observed include narrow X-ray emitting filaments up to 31 kpc (100 kly) 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 below middle C for the major eruptions.[76]

Messier 87 is a very strong source of gamma rays, which are the most energetic rays of the electromagnetic spectrum. Gamma rays coming from Messier 87 have been observed since the late 1990s, but in 2006, using the HESS Cherenkov telescopes, scientists have measured the variations of the gamma ray flux coming from Messier 87, and found that the flux changes over a matter of days. This short period makes the immediate vicinity of the supermassive black hole in Messier 87 the most promising source for these gamma rays.[77] 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 pc (210 ly) from the core. By 2006, the X-ray intensity of this knot had increased by a factor of 50 over a four-year period.[78] This X-ray emission has since been decaying in a variable manner.[79]

Cluster membership

The Virgo Cluster showing the diffuse light between the galaxies belonging to the cluster. Messier 87 is to lower left.

This supergiant elliptical galaxy is located near the center of the Virgo Cluster.[33] This rich cluster has about 2,000 members[80] 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.[44] The cluster is organized into at least three distinct subsystems that are associated with the three galaxies Messier 87, Messier 49 and Messier 86, with the one centered around M87 known as Virgo A and the one around M49 as Virgo B.[81] In terms of mass, Messier 87 is a dominant member of the cluster, and hence appears to be moving very little relative to the cluster as a whole.[44] Indeed, Messier 87 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 Messier 87 is located.[60] The combined mass of the cluster is estimated to be (0.15–1.5) × 1015 solar masses.[80]

Measurements of the motion of intracluster planetary nebulae between Messier 87 and Messier 86 suggest that these two galaxies are moving toward each other and this may be their first encounter. Messier 87 may have encountered Messier 84 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 Messier 87 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 Messier 87.[44]

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