Messier 87
Messier 87 | |
---|---|
arcmin[8] | |
Other designations | |
Virgo A, Virgo X-1, NGC 4486, UGC 7654, PGC 41361, VCC 1316, Arp 152, 3C 274,[5] 3U 1228+12.[9] |
Messier 87 (also known as Virgo A or NGC 4486, generally abbreviated to M87) is a supergiant elliptical galaxy in the constellation Virgo that contains several trillion stars. One of the largest and most massive galaxies in the local universe,[b] it has a large population of globular clusters—about 15,000 compared with the 150–200 orbiting the Milky Way—and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs (4,900 light-years), traveling at a relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.
The French astronomer
The galaxy is a strong source of multi-wavelength radiation, particularly
Observation history
In 1781, the French astronomer Charles Messier published a catalogue of 103 objects that had a nebulous appearance as part of a list intended to identify objects that might otherwise be confused with comets. In subsequent use, each catalogue entry was prefixed with an "M". Thus, M87 was the eighty-seventh object listed in Messier's catalogue.[15] During the 1880s, the object was included as NGC 4486 in the New General Catalogue of nebulae and star clusters assembled by the Danish-Irish astronomer John Dreyer, which he based primarily on the observations of the English astronomer John Herschel.[16]
In 1918, the American astronomer Heber Curtis of Lick Observatory noted M87's lack of a spiral structure and observed a "curious straight ray ... apparently connected with the nucleus by a thin line of matter." The ray appeared brightest near the galactic center.[17] The following year, supernova SN 1919A within M87 reached a peak photographic magnitude of 11.5, although this event was not reported until photographic plates were examined by the Russian astronomer Innokentii A. Balanowski in 1922.[18][19]
Identification as a galaxy
In 1922, the American astronomer Edwin Hubble categorized M87 as one of the brighter globular nebulae, as it lacked any spiral structure, but like spiral nebulae, appeared to belong to the family of non-galactic nebulae.[20] In 1926 he produced a new categorization, distinguishing extragalactic from galactic nebulae, the former being independent star systems. M87 was classified as a type of elliptical extragalactic nebula with no apparent elongation (class E0).[21]
In 1931, Hubble described M87 as a member of the Virgo Cluster, and gave a provisional estimate of 1.8 million parsecs (5.9 million light-years) from Earth. It was then the only known 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 distances.[22] In his 1936 The Realm of the Nebulae, Hubble examines the terminology of the day; some astronomers labeled extragalactic nebulae as external galaxies on the basis that they were stellar systems at far distances from our own galaxy, while others preferred the conventional term extragalactic nebulae, as galaxy was at that time a synonym for the Milky Way.[23] M87 continued to be labelled as an extragalactic nebula at least until 1954.[24][25]
Modern research
In 1947, a prominent
M87 has been an important testing ground for techniques that measure the masses of central supermassive black holes in galaxies. In 1978,
M87 was observed by the Event Horizon Telescope (EHT) during much of 2017.[35] The event horizon of the black hole at the center was directly imaged by the EHT,[36] then revealed in a press conference on the issue date stated, filtering out from this the first image of a black hole's shadow.[37]
Visibility
M87 is near a high
Properties
In the modified Hubble sequence galaxy morphological classification scheme of the French astronomer Gérard de Vaucouleurs, M87 is categorized as an E0p galaxy. "E0" designates an elliptical galaxy that displays no flattening—that is, it appears spherical.[41] A "p" suffix indicates a peculiar galaxy that does not fit cleanly into the classification scheme; in this case, the peculiarity is the presence of the jet emerging from the core.[41][42] In the Yerkes (Morgan) scheme, M87 is classified as a type-cD galaxy.[43][44] A D galaxy has an elliptical-like nucleus surrounded by an extensive, dustless, diffuse envelope. A D type supergiant is called a cD galaxy.[45][46]
The distance to M87 has been estimated using several independent techniques. These include measurement of the luminosity of planetary nebulae, comparison with nearby galaxies whose distance is estimated using standard candles such as cepheid variables, the linear size distribution of globular clusters,[d] and the tip of the red-giant branch method using individually resolved red giant stars.[e] 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).[3]
Radius kpc |
Mass ×1012 M☉ |
32 | 2.4[47] |
44 | 3.0[48] |
47 | 5.7[49] |
50 | 6.0[50] |
M87 is one of the most massive galaxies in the local Universe. Its diameter is estimated at 132,000 light-years, which is approximately 51% larger than that of the Milky Way.[5][6] As an elliptical galaxy, the galaxy is a spheroid rather than a flattened disc, accounting for the substantially larger mass of M87. Within a radius of 32 kiloparsecs (100,000 light-years), the mass is (2.4±0.6)×1012 times the mass of the Sun,[47] which is double the mass of the Milky Way galaxy.[53] As with other galaxies, only a fraction of this mass is in the form of stars: M87 has an estimated mass to luminosity ratio of 6.3 ± 0.8; that is, only about one part in six of the galaxy's mass is in the form of stars that radiate energy.[54] This ratio varies from 5 to 30, approximately in proportion to r1.7 in the region of 9–40 kiloparsecs (29,000–130,000 light-years) from the core.[48] The total mass of M87 may be 200 times that of the Milky Way.[55]
The galaxy experiences an infall of gas at the rate of two to three solar masses per year, most of which may be accreted onto the core region.
The spectrum of the nuclear region of M87 shows the
Elliptical galaxies such as M87 are believed to form as the result of one or more mergers of smaller galaxies.[66] They generally contain relatively little cold interstellar gas (in comparison with spiral galaxies) and they are populated mostly by old stars, with little or no ongoing star formation. M87's elliptical shape is maintained by the random orbital motions of its constituent stars, in contrast to the more orderly rotational motions found in a spiral galaxy such as the Milky Way.[67] Using the Very Large Telescope to study the motions of about 300 planetary nebulae, astronomers have determined that M87 absorbed a medium-sized star-forming spiral galaxy over the last billion years. This has resulted in the 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.[68][69]
Components
Supermassive black hole M87*
The core of the galaxy contains a
A 2010 paper suggested that the black hole may be displaced from the galactic center by about seven parsecs (23 light-years).[79] This was claimed to be in the opposite direction of the known jet, indicating acceleration of the black hole by it. Another suggestion was that the offset occurred during the merger of two supermassive black holes.[79][80] However, a 2011 study did not find any statistically significant displacement,[81] and a 2018 study of high-resolution images of M87 concluded that the apparent spatial offset was caused by temporal variations in the jet's brightness rather than a physical displacement of the black hole from the galaxy's center.[82]
This black hole is the first to be imaged. Data to produce the image were taken in April 2017, the image was produced during 2018 and was published on 10 April 2019.[37][83][84] The image shows the shadow of the black hole,[85] surrounded by an asymmetric emission ring with a diameter of 690 AU (103 billion km; 64 billion mi). The shadow radius is 2.6 times that of the black hole's Schwarzschild radius. The asymmetry in the brightness of the ring is due to relativistic beaming, whereby material moving towards the observer at relativistic velocities appears brighter. The visible material around the black hole rotates mostly clockwise with respect to the observer, which due to the direction of the axis of rotation causes the bottom part of the emission region to have a component of velocity toward the observer.[86] The rotation parameter was estimated at , corresponding to a rotation speed ≈ 0.4 c.[87]
After the black hole had been imaged, it was named Pōwehi, a Hawaiian word meaning "the adorned fathomless dark creation", taken from the ancient creation chant Kumulipo.[89]
On 24 March 2021, the Event Horizon Telescope collaboration revealed a unprecedented unique view of the M87 black hole shadow: how it looks in polarized light.[90] Polarization is a powerful tool which allows astronomers to probe physics behind the image in more detail. Light polarization informs us about the strength and orientation of magnetic fields in the ring of light around the black hole shadow.[91] Knowing those is essential to understand how M87's supermassive black hole is launching jets of magnetized plasma, which expand at relativistic speeds beyond the M87 galaxy.
On 14 April 2021, astronomers further reported that the M87 black hole and its surroundings were studied during Event Horizon Telescope 2017 observing run also by many multi-wavelength observatories from around the world.[clarification needed][92]
In April 2023, a team developed a new principal-component interferometric modeling (PRIMO) technique to produce sharper image reconstructions from EHT data. They applied this to the original EHT observations of the M87 black hole, yielding a crisper final image and allowing closer testing of the alignment of observations to theory.[93][94]
Jet
The
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
Observations indicate that the rate at which material is ejected from the supermassive black hole is variable. These variations produce pressure waves in the hot gas surrounding M87. The Chandra X-ray Observatory has detected loops and rings in the gas. Their distribution suggests that minor eruptions occur every few million years. One of the rings, caused by a major eruption, is a shock wave 26 kiloparsecs (85,000 light-years) in diameter around the black hole. Other features observed include narrow X-ray-emitting filaments up to 31 kiloparsecs (100,000 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.
M87 is a very strong source of gamma rays, the most energetic rays of the electromagnetic spectrum. Gamma rays emitted by 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 indicates that the most likely source of the gamma rays is a supermassive black hole.[105] In general, the smaller the diameter of the emission source, the faster the variation in flux.[105][106]
A knot of matter in the jet (designated HST-1), about 65 parsecs (210 light-years) from the core, has been tracked by the Hubble Space Telescope and the Chandra X-ray Observatory. By 2006, the X-ray intensity of this knot had increased by a factor of 50 over a four-year period,[108] while the X-ray emission has since been decaying in a variable manner.[109]
The interaction of relativistic jets of plasma emanating from the core with the surrounding medium gives rise to
Interstellar medium
The space between the stars in M87 is filled with a diffuse interstellar medium of gas that has been chemically enriched by the elements ejected from stars as they passed beyond their main sequence lifetime. Carbon and nitrogen are continuously supplied by stars of intermediate mass as they pass through the asymptotic giant branch.[113][114] The heavier elements from oxygen to iron are produced largely by supernova explosions within the galaxy. Of the heavy elements, about 60% were produced by core-collapse supernovae, while the remainder came from type Ia supernovae.[113]
The distribution of oxygen is roughly uniform throughout, at about half of the solar value (i.e., oxygen abundance in the Sun), while iron distribution peaks near the center where it approaches the solar iron value.[114][115] Since oxygen is produced mainly by core-collapse supernovae, which occur during the early stages of galaxies, and mostly in outer star-forming regions,[113][114][115] the distribution of these elements suggests an early enrichment of the interstellar medium from core-collapse supernovae and a continuous contribution from type Ia supernovae throughout the history of M87.[113] The contribution of elements from these sources was much lower than in the Milky Way.[113]
Element | Abundance (solar values) |
C | 0.63 ± 0.16 |
N | 1.64 ± 0.24 |
O | 0.58 ± 0.03 |
Ne | 1.41 ± 0.12 |
Mg | 0.67 ± 0.05 |
Fe | 0.95 ± 0.03 |
Examination of M87 at far infrared wavelengths shows an excess emission at wavelengths longer than 25 μm. Normally, this may be an indication of thermal emission by warm dust.[116] In the case of M87, the emission can 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.[117] This dust may be destroyed by the hostile environment or expelled from the galaxy.[118] The combined mass of dust in M87 is no more than 70,000 times the mass of the Sun.[117] By comparison, the Milky Way's dust equals about a hundred million (108) solar masses.[119]
Although M87 is an elliptical galaxy and therefore lacks the dust lanes of a spiral galaxy, optical filaments have been observed in it, which arise from gas falling towards the core. Emission probably comes from shock-induced excitation as the falling gas streams encounter X-rays from the core region.[120] These filaments have an estimated mass of about 10,000 M☉.[56][120] Surrounding the galaxy is an extended corona with hot, low-density gas.[121]
Globular clusters
M87 has an abnormally large population of globular clusters. A 2006 survey out to an angular distance of 25
Almost a hundred ultra-compact dwarfs have been identified in M87. They resemble globular clusters but have a diameter of ten parsecs (33 light-years) or more, much larger than the three-parsec (9.8-light-year) maximum of globular clusters. It is unclear whether they are dwarf galaxies captured by M87 or a new class of massive globular cluster.[125]
Environment
M87 is near (or at) the center of the Virgo Cluster,
Measurements of the motion of those intracluster starburst ("planetary") nebulae between M87 and M86 suggest that the two galaxies are moving toward each other and that this may be their first encounter. M87 may have interacted with M84, as evidenced by the truncation of M87's outer halo by tidal interactions. The truncated halo may also have been caused by contraction due to an unseen mass falling into M87 from the rest of the cluster, which may be the hypothesized dark matter. A third possibility is that the halo's formation was truncated by early feedback from the active galactic nucleus.[7]
See also
- List of Messier objects
Notes
- ^ Size quoted refers to the diameter directly measured by the 25.0 mag/arcsec2 isophote at the B-band. The galaxy has a much diffuse and extensive halo extending up to 300 kpc (980,000 ly).[7]
- ^ "Local universe" is not a strictly defined term, but it is often taken as that part of the universe out to distances between about 50 million to a billion light-years.[10][11][12]
- ^ Epsilon Virginis is at celestial coordinates α=13ʰ02ᵐ, δ=+10°57′; Denebola is at α=11ʰ49ᵐ, δ=+14°34′. The midpoint of the pair is at α=12ʰ16ᵐ, δ=12°45′. Compare to the coordinates of Messier 87: α=12ʰ31ᵐ, δ=+12°23′ .
- ^ This yields a distance of 16.4 ± 2.3 megaparsecs (53.5 ± 7.50 million light-years).[3]
- ^ This yields a distance of 16.7 ± 0.9 megaparsecs (54.5 ± 2.94 million light-years).[3]
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External links
- Messier 87, SEDS Messier pages
- ESA/Hubble images of M87
- Messier 87 on
- Solar elemental abundances