The black hole in the center of the galaxy

posted by @ulaulaman about #astronomy #SagittariusA #BlackHole #MilkyWay

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,^{(1, 2)} like those that are now generally accepted to be at the centers of most 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⁽³⁾.

In the image there is an x-ray photo of Sgr A* from the paper by Wang et al. published on Science⁽⁴⁾.

the x-ray emission from Sgr A* can be described as the superposition of a pointlike source from the black hole itself, and a much larger extended cloud of emission about 2″ across. Within this cloud, we can identify over a hundred individually resolved bright stars, and infer thousands more that are too dim to detect.⁽⁵⁾

They also infer that

the temperature and density profile of the gas cloud surrounding Sgr A*. They show that over 99% of the gas never reaches the central black hole, but rather is ejected from the system⁽⁵⁾

There are also some unresolved questions: for example if the observed accretion rate is dued exclusively by Sgr A* or if there is another source for the data; or his low luminosity, orders of magnitude below its theoretical potential⁽⁵⁾.

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(1) Christopher S. Reynolds (2008). Bringing black holes into focus, Nature, 455 (7209) 39-40.
(2) Doeleman S.S., Weintroub J., Rogers A.E.E., Plambeck R., Freund R., Tilanus R.P.J., Friberg P., Ziurys L.M., Moran J.M. & Corey B. & (2008). Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre, Nature, 455 (7209) 78-80. DOI: 10.1038/nature07245 (arXiv)
(3) Hamaus N., Paumard T., Müller T., Gillessen S., Eisenhauer F., Trippe S. & Genzel R. (2009). Prospects for testing the nature of Sgr A*'s near-infrared flares on the basis of current very large telescope - and future very large telescope interferometer - observations, The Astrophysical Journal, 692 (1) 902-916. DOI: 10.1088/0004-637X/692/1/902 (via Mark Henderson)
(4) Wang Q.D., Nowak M.A., Markoff S.B., Baganoff F.K., Nayakshin S., Yuan F., Cuadra J., Davis J., Dexter J. & Fabian A.C. & (2013). Dissecting X-ray-Emitting Gas Around the Center of Our Galaxy, Science, 341 (6149) 981-983. DOI: 10.1126/science.1240755
(5) Jeremy D. Schnittman (2013). The Curious Behavior of the Milky Way's Central Black Hole, Science, 341 (6149) 964-965