Sagittarius A* - Central Black Hole

Central Black Hole

If the apparent position of Sagittarius A* were 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.

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 while the American group found 4.1 ± 0.6 million solar masses. 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. 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 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.
• From the motion of star S2, the object's mass can be estimated as 4.1 million solar masses. (The corresponding Schwarzschild radius is 0.08 AU; 17 times bigger than Sun.)
• The radius of the central object must be significantly less than 17 light hours, because otherwise, S2 would collide with it. In fact, recent observations indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit, leading to density limit 8.55×1036 kg / 1.288×1039 m3 = 0.0066 kg/m3.
• 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.

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.

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. The black hole itself is believed to emit only Hawking radiation at a negligible temperature, on the order of 10−14 kelvin.

The European Space Agency's gamma-ray observatory INTEGRAL has observed gamma rays interacting with the nearby giant molecular cloud Sagittarius B2, causing x-ray emission from the cloud. This energy was emitted about 350 years before by Sgr A*. The total luminosity from this outburst (L≈1,5×1039 erg/s) is an estimated million times stronger than the current output from Sgr A* and is comparable with a typical AGN. This conclusion has been supported in 2011 by Japanese astronomers observed the Galaxy center with Suzaku satellite.