Astronomy Colloquium

The Galactic center is one of the most fascinating and extreme places in the Galaxy. Harboring a supermassive black hole with a mass of order 4 million solar masses it experiences cycles of activity and star formation, separated by periods of quiescence that last of order a million years. The Milky Way's SMBH currently is inactive. However a small, diffuse gas cloud (G2) has recently been detected (Gillessen et al. 2012) on an orbit almost straight into the Galactic SMBH. Within the next 2 years, G2 will pass the SMBH at a small distance of just 3000 Schwarzschild radii, corresponding to 4e15 cm. Depending on its nature it might break up and feed the SMBH, triggering a phase of AGN activity. The next years will therefore provide a unique opportunity to investigate directly the processes that drive and regulate gas accretion onto the Galactic SMBH as well as the onset of activity cycles in the Galactic center.

This talk will summarize the observations of G2 and current models about its nature. The existence of such a tiny, cold gas cloud in the hostile vicinity of the SMBH raises numerous fascinating questions. Is G2 a diffuse gas clump that originates from winds of high-mass stars in the surrounding stellar disk or is it the atmosphere of an evaporating, invisible protostellar disk, planet or star? Or is it something completely different? Where did it come from and where will it go? Why is it on such a highly eccentric orbit? Which physical processes constrain its properties like its size, mass, density, temperature and geometrical shape? How many clouds like G2 are currently orbiting Sgr A* and how do they affect its activity and gas accretion rate? Like comet Shoemaker Levy's 1994 collision with Jupiter, the big challenge has started for astrophysicists to predict the outcome of G2's close encounter with the SMBH in the year 2013 and beyond. Their models will be validated directly by observations within the next couple of years.