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Sagittarius A* (RX 174540.04-29027.9) & The Milky Way’s Central Super-Massive Black Hole


The central light year of the Milky Way is a hotbed of stellar activity. This comparatively small region of our Galaxy contains a dense and luminous star cluster with extremely fast stellar proper motions as well as several components of neutral, ionized and extremely hot gas (Genzel, Hollenbach & Townes 1994). This central light year, which is known simply as the Galactic centre, also contains a very compact radio source: Sagittarius A* (Sgr A*), which is located at the aforementioned star cluster’s centre of mass (Balick & Brown 1974). This source appears to be slinging many of the cluster’s constituent members at incredible speeds; so fast that Sag A* being a supermassive black hole (SMBH)  is now paradigm (Ozernoi 1979; Lo 1986; Genzel & Eckart 1998; Eckart et al. 2002).

Fig. 1: Right ~ A diffraction limited image of Sgr A* from the 8m ESO VLT, taken with the NACO AO-camera and an infrared wavefront sensor at 1.6/2.2/3.7μm. (Credit: ESO VLT). Left ~ Positions on the sky as a function of time for the central stars orbiting the compact radio source Sgr A*. (Credit: Genzel, R. & Karas, V. 2007 The Galactic Centre. IAUS, 238 pp.173-178.)

Short-wavelength centimeter and millimeter VLBI observations have established that its intrinsic radio size is a mere 10 light minutes (Bower et al 2004; Shen et al 2005). This central region of our Galaxy is believed to be gravitationally bound to an extremely massive object: an object so massive it is dubbed a “super-massive black hole” (SMBH) massing in at almost a million times the mass of our own Sun (Reynolds 2008). The mass of this mysterious object is well known (see below for further details). Such determinations conclude that this object is, as you would expect, massive. It is therefore, under current theoretical models of compact stellar remnants, possible to place an object in the black hole camp.

Sgr A* is also an X-ray emission source, albeit of only modest luminosity (Baganoff et al 2001). Most recently, Aharonian et al (2004) have discovered a source of TeV γ-ray emission within 10arcsec of Sgr A*. It is not yet clear whether these most energetic γ -rays come from Sgr A* itself or whether they are associated with the nearby supernova remnant, Sgr A East.

The mass of Sagittarius A* has been estimated in two ways, critically by simply applying Kepler’s Laws (4.31±0.38×106: Ghez et al. 2008; 4.10±0.60×106: Gillessen et al. 2009) and by a statistical analysis of the proper motions of a sample of stars within ~1pc of the central object (Schödel et al. 2009). From both studies using the Keplerian orbits method, a mass estimate in the range of 4.1±0.6×106. Given that this mass is confined inside a 44 million km diameter sphere, this yields a density ten times higher than previous estimates. The second method applied by Schödel et al. (2009) involved a statistical analysis of the proper motion of a large sample of stars within a 1pc cubic region from the central object. This method yielded a result of 1.0±0.6×105.

As the reader may well be aware, masses this large can mean only one thing: a black hole. A normal stellar-sized black hole usually has a mass raging from ~3-20 (Bombaci 1996; Celotti, Miller & Sciama 1999). Hence, as such objects are millions of times more massive, they have been described as ‘super massive’.

Such intense energy regimes provide evidence that Sgr A* may be a supermassive black hole analogous to many quasi-stellar objects (QSOs), albeit of much lower mass and luminosity. Because of its proximity, the distance to the Galactic centre is about 105 times closer than the nearest quasars (Genzel & Karas 2007), high resolution observations of the Milky Way nucleus offer the unique opportunity of stringently testing the current super massive black hole paradigm and of studying stars and gas in the immediate vicinity of a black hole, at a level of detail that will not be accessible in any other galactic nucleus in the foreseeable future.

Journal References:

  • Backer, D. C. & Sramek, R. A. (1999). Proper Motion Of The Compact, Non-Thermal Radio Source At The Galactic Center: Sagittarius A*The Astrophysical Journal 524 (2) pp.805–815.
  • Schödel, R. et al. (2002). A Star In A 15.2-Year Orbit Around The Supermassive Black Hole At The Centre Of The Milky WayNature, 419 (6908) pp.694–696.
  • Gillessen, S. et al. (2009). Monitoring Stellar Orbits Around The Massive Black Hole In The Galactic CenterThe Astrophysical Journal, 692 (2) pp.1075–1109.
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