Bulletin of the American Physical Society
APS April Meeting 2012
Volume 57, Number 3
Saturday–Tuesday, March 31–April 3 2012; Atlanta, Georgia
Session B3: Invited Session: Homing in on the Galactic Center Black Hole |
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Sponsoring Units: DAP Chair: Clifford Will, Washington University in St. Louis Room: Hanover CDE |
Saturday, March 31, 2012 10:45AM - 11:21AM |
B3.00001: Evidence for a Super-massive Black Hole at the Center of the Milky Way Invited Speaker: Mark Reid While the concept of a black hole formed by the explosive collapse of a dying star is astounding, the possibility that matter from billions of stars can condense into a single super-massive black hole (SMBH) is even more fantastic. Yet astronomers are now confident that they exist at the centers of most galaxies and hold more than 0.01\% of the baryonic mass of the Universe. Early evidence for SMBHs came from ``radio galaxies'' with two lobes symmetrically placed about the parent galaxy. These lobes are immense and {\it minimum} energy estimates require the total conversion of $10^7$ stars to energy! The source of energy was traced to galaxy's center and observed to vary on time scales $<1$ year. Since nuclear reactions convert less than 1\% of mass to energy, this would require channeling $>10^9$ stars through a region smaller than that between the Sun and the nearest star. A very compact radio source was discovered toward the center of the Milky Way and named Sgr A*, leading to speculation that it might be a SMBH. Infrared observations of stars on elliptical orbits give clear evidence of an unseen gravitational source of $4\times10^6$ solar masses. One star has been seen moving at 5000 km/s in its 16 year eccentric orbit. Sgr A* has been located at the position of the gravitational focus of the stellar orbits. However, in contrast to the rapidly moving stars, Sgr A* is motionless ($<1$ km/s), requiring the source to be extremely massive. For comparison, gravitational ``Brownian motion'' of a SMBH at the center of a dense stellar cluster would be comparable to the measured limits. Recent radio interferometric observations show that the radio emission from Sgr A* comes from a region comparable in size to the Schwarzschild radius ($2GM/c^2$) of 0.1 AU ($1.5\times10^7$ km)! Placing any known concentration of $4\times10^6$ solar masses within this tiny volume would rapidly condense to a black hole. [Preview Abstract] |
Saturday, March 31, 2012 11:21AM - 11:57AM |
B3.00002: Accretion Mechanisms Invited Speaker: Ramesh Narayan |
Saturday, March 31, 2012 11:57AM - 12:33PM |
B3.00003: Imaging Black Holes Invited Speaker: Avery Broderick Black holes are simultaneously powerful astrophysical engines and an exotic prediction of general relativity. As a consequence, understanding both the phenomenology of the interactions between astronomical black holes and their environments as well as the peculiar properties of strong gravity and its implications for physics beyond Einstein, is of great interest. Both of these, however, require probing black holes on scales comparable to their event horizon. With the advent of millimeter-wave Very Long Baseline Interferometry, it has now become possible to image a handful of black holes on sub-horizon scales. I will discuss some of the implications that existing observations have already had upon our understanding of accretion, jet formation, and strong field gravity, and how constraints upon these will dramatically improve in the coming years. [Preview Abstract] |
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