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|<ref name="Ghez">{{cite journal|last=Ghez|first=A.M.|coauthors=Hornstein, S.D., Tanner, A., Lu, J.R., Morris, M., Becklin, E.E., Duchêne, G.| title=Stellar Orbits Around the Galactic Center Black Hole| journal=The Astrophysical Journal|issue=620|month=February 20|year=2005|pages=744–757}} </ref>
|<ref name="Ghez">{{cite journal|last=Ghez|first=A.M.|coauthors=Hornstein, S.D., Tanner, A., Lu, J.R., Morris, M., Becklin, E.E., Duchêne, G.| title=Stellar Orbits Around the Galactic Center Black Hole| journal=The Astrophysical Journal|issue=620|month=February 20|year=2005|pages=744–757}} </ref>
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Revision as of 23:26, 20 November 2011

Sagittarius A

Sgr A* (centre) and two light echoes from a recent explosion (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension 17h 45m 40.045s
Declination −29° 0′ 27.9″[citation needed]
Details
Mass(4.31 ± 0.38) × 106[1] M☉
or
(4.1 ± 0.6) × 106[2] M☉
Astrometry
Distance25,900 ± 1,400 ly
(7,940 ± 420[3] pc)
Chandra X-ray Observatory image of Sgr A*.

Sagittarius A* (pronounced "Sagittarius A-star", standard abbreviation Sgr A*) is a bright and very compact astronomical radio source at the center of the Milky Way Galaxy, near the border of the constellations Sagittarius and Scorpius. It is part of a larger astronomical feature known as Sagittarius A. Sagittarius A* is believed to be the location of a supermassive black hole,[4] which are now generally accepted to be at the centers of many spiral and elliptical galaxies. Observations of the star S2 in orbit around Sagittarius A* have been used to show the presence of, and produce data about, the Milky Way's central supermassive black hole, and have led to the conclusion that Sagittarius A* is the site of that black hole.[5]

Observation and description

Astronomers have been unable to observe Sgr A* in the optical spectrum because of the effect of 25 magnitudes of extinction between the source and Earth.[6] Several teams of researchers have attempted to image Sagittarius A* in the radio spectrum using Very Long Baseline Interferometry (VLBI). The current highest-resolution measurement, made at a wavelength of 1.3 mm, indicated an angular diameter for the source of 37 μas.[7] At a 26,000 light-year distance, this yields a diameter of 44 million kilometers. For comparison, the Earth is 150 million kilometers from the Sun, and Mercury is 46 million kilometers from the Sun at its perihelion. The proper motion of Sgr A* is approximately −2.70 mas per year for the right ascension and −5.6 mas per year for the declination.[8]

History

Sgr A* was discovered on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown using the baseline interferometer of the National Radio Astronomy Observatory.[9]

On October 16, 2002, an international team led by Rainer Schödel of the Max Planck Institute for Extraterrestrial Physics reported the observation of the motion of the star S2 near Sagittarius A* over a period of ten years. According to the team's analysis, the data ruled out the possibility that Sgr A* contains a cluster of dark stellar objects or a mass of degenerate fermions, strengthening the evidence for a massive black hole.[10] The observations of S2 used near-infra red (NIR) interferometry (in the K-band, e.g. 2.2 μm) because of reduced interstellar extinction in this band. SiO masers were used to align NIR images with radio observations, as they can be observed in both NIR and radio bands. The rapid motion of S2 (and other nearby stars) easily stood out against slower-moving stars along the line-of-sight so these could be subtracted from the images.

The VLBI radio observations of Sagittarius A* could also be aligned centrally with the images so S2 could be seen to orbit the galactic centre. From examining the Keplerian orbit of S2, they determined the mass of Sagittarius A* to be 2.6 ± 0.2 million solar masses, confined in a volume with a radius no more than 17 light-hours (120 AU).[11] Later observations determined the mass of the object to be about 4.1 million solar masses within a volume with radius no larger than 6.25 light-hours (45 AU) or about 6.7 billion kilometres.[2] They also determined the distance to the galactic centre, which is important in calibrating astronomical distance scales, as 8.0 ± 0.6 × 103 parsecs.

In November 2004 a team of astronomers reported the discovery of a potential intermediate-mass black hole, referred to as GCIRS 13E, orbiting three light-years from Sagittarius A*. This black hole of 1,300 solar masses is within a cluster of seven stars. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.

After monitoring stellar orbits around Sagittarius A* for 16 years, Gillessen et al. estimate the object's mass at 4.31 ± 0.38 million solar masses. The result was announced in 2008 and published in The Astrophysical Journal in 2009.[1] Reinhard Genzel, team leader of the research, said the study has delivered "what is now considered to be the best empirical evidence that super-massive black holes do really exist. The stellar orbits in the galactic centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt." [12]

Central black hole

Inferred orbits of 6 stars around supermassive black hole candidate Sagittarius A* at the Milky Way galactic centre.[13]

If the apparent position of Sagittarius A* was exactly centered on the black hole, it would be possible to see it magnified beyond its actual size, because of gravitational lensing. According to general relativity, this would result in a minimum observed size of at least 5.2 times the black hole's Schwarzschild radius, which, for a black hole of around 4 million solar masses, corresponds to a minimum observed size of approximately 52 μas. This is much larger than the observed size of 37 μas and so suggests that the Sagittarius A* radio emissions are not centered on the hole but arise from a bright spot in the region around the black hole, close to the event horizon, possibly in the accretion disc or a relativistic jet of material ejected from the disc.[7]

The mass of Sagittarius A* has been estimated in two different ways. (1) Two groups—in Germany and the U.S.—monitored the orbits of individual stars very near to the black hole and used Kepler's laws to infer the enclosed mass. The German group found a mass of 4.31 ± 0.38 million solar masses[1] while the American group found 4.1 ± 0.6 million solar masses.[2] Given that this mass is confined inside a 44 million km diameter sphere, this yields a density ten times higher than previous estimates. (2) More recently, measurement of the proper motions of a sample of several thousand stars within approximately one parsec from the black hole, combined with a statistical technique, has yielded both an estimate of the black hole's mass, and also of the distributed mass in this region. The black hole mass was found to be consistent with the values measured from individual orbits; the distributed mass was found to be 1.0 ± 0.5 million solar masses.[14] The latter is believed to be composed of stars and stellar remnants.

Astronomers are confident that these observations of Sagittarius A* provide good empirical evidence that our own Milky Way galaxy has a supermassive black hole at its center, 26,000 light-years from the Solar System[5] because:

  • The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 light hours (1.8×1013 m) from the center of the central object.[15]
  • From the motion of star S2, the object's mass can be estimated as 4.1 million solar masses.[2]
  • The radius of the central object must be significantly less than 17 light hours, because otherwise, S2 would either collide with it or be ripped apart by tidal forces. In fact, recent observations[16] indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit, leading to density limit 8.55*10^36kg/1.288*10^39m^3=0.0066kg/m^3
  • The only widely hypothesized type of object which can contain 4.1 million solar masses in a volume that small is a black hole.

While, strictly speaking, there are other mass configurations that would explain the measured mass and size, such an arrangement would collapse into a single supermassive black hole on a timescale much shorter than the life of the Milky Way.[7]

The comparatively small mass of this black hole, along with the low luminosity of the radio and infrared emission lines, imply that the Milky Way is not a Seyfert galaxy.[6]

Ultimately, what is seen is not the black hole itself, but observations that are consistent only if there is a black hole present near Sgr A*. In the case of such a black hole, the observed radio and infrared energy emanates from gas and dust heated to millions of degrees while falling into the black hole. Although other possibilities exist for how these gases emanate energy, such as radiation pressure and interaction with other gas streams, interaction with a massive source of gravity is the simplest explanation.[17] The black hole itself is believed to emit only Hawking radiation at a negligible temperature, on the order of 10−14 kelvin.

Orbital parameters of stars orbiting Saggitarius A*[18]
Star Alias a (″) a (AU) e P (years) T0 (date) Reference
S1 S0-1 0.412±0.024 3300±190 0.358±0.036 94.1±9.0 2002.6±0.6 [13]
S2 S0-2 0.1226±0.0025 980±20 0.8760±0.0072 15.24±0.36 2002.315±0.012 [13]
919±23 0.8670±0.0046 14.53±0.65 2002.308±0.013 [19]
S8 S0-4 0.329±0.018 2630±140 0.927±0.019 67.2±5.5 1987.71±0.81 [13]
S12 S0-19 0.286±0.012 2290±100 0.9020±0.0047 54.4±3.5 1995.628±0.016 [13]
1720±110 0.833±0.018 37.3±3.8 1995.758±0.050 [19]
S13 S0-20 0.219±0.058 1750±460 0.395±0.032 36±15 2006.1±1.4 [13]
S14 S0-16 0.225±0.022 1800±180 0.9389±0.0078 38±5.7 2000.156±0.052 [13]
1680±510 0.974±0.016 36±17 2000.201±0.025 [19]
  1. ^ a b c Gillessen et al. 2009
  2. ^ a b c d Ghez et al. 2008 Cite error: The named reference "Ghez08" was defined multiple times with different content (see the help page).
  3. ^ Eisenhauer et al. 2003, § 3.1
  4. ^ Reynolds 2008
  5. ^ a b Henderson, Mark (December 9, 2008). "Astronomers confirm black hole at the heart of the Milky Way". Times Online. Retrieved 2009-05-17.
  6. ^ a b Osterbrock and Ferland 2006, p. 390
  7. ^ a b c Doeleman et al. 2008
  8. ^ Backer and Sramek 1999, § 3
  9. ^ Melia 2007, p. 2
  10. ^ Schödel et al. 2002
  11. ^ Ghez et al. 2003
  12. ^ O'Neill 2008
  13. ^ a b c d e f g Eisenhauer, F. (2005). "SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month". The Astrophysical Journal (628): 246–259. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  14. ^ Schödel et al. 2009
  15. ^ Schödel, R. (17 October 2002). "A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way". Nature. 419 (6908): 694–696. arXiv:astro-ph/0210426. Bibcode:2002Natur.419..694S. doi:10.1038/nature01121. PMID 12384690. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  16. ^ Ghez, A. M. (2005). "Stellar Orbits around the Galactic Center Black Hole". The Astrophysical Journal. 620 (2): 744–757. arXiv:astro-ph/0306130. Bibcode:2005ApJ...620..744G. doi:10.1086/427175. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  17. ^ Wheeler 2007, p. 224
  18. ^ "Orbital Parameters of Stars Orbiting Sgr A*". The Astrophysics Spectator (4.10). July 11, 2007.
  19. ^ a b c Ghez, A.M. (2005). "Stellar Orbits Around the Galactic Center Black Hole". The Astrophysical Journal (620): 744–757. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)

Notes

References

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