The Galactic Center

The Galactic Center is an ideal laboratory to study the properties of stars and gas in a complex environment in detail. The Galactic Center is very quite compared to other nuclei of spiral galaxies whose luminosities outshine that of the whole galaxy (Active Galactic Nuclei = AGN). For such a large energy output three ingredients must be in place:
(i) a massive black hole, (ii) a gas reservoir, and (iii) a relatively high mass accretion rate.
The Galactic Center harbours a black hole of about 3 million solar masses and there are several tens of thousand solar masses of gas within a radius of 10 pc around the central black hole in form of a circumnuclear ring. Thus, conditions (i) and (ii) are fullfilled. However, the mass accretion rate onto the central black hole is about one million times too low to trigger a central activity. The central black hole is thus extremely sub-Eddington.

In order to understand the fueling mechanisms of the central engine, there is an inevitable need for understanding the gas physics and dynamics in the inner ~50 pc of the Galaxy. Gas that flows radially into the Galactic Centre has to pass several barriers. At large scales (<kpc) the gas has to cross the resonance of the inner Lindblad radius. According to the gravitational potential of the Galactic Bulge region, there might be a second inner Lindblad radius that the gas has to overcome. When the gas finally arrives in the inner 200 pc of the Galaxy, it is very clumpy and has a volume filling factor of a few percent. In this environment five different environmental effects determine the structure of the ISM: (i) the stellar radiation field, (ii) stellar winds, (iii) the shear due to differential rotation, (iv) instabilities due to self-gravitation, and (v) supernovae. We modeled analytically the properties of the gas located in the inner 20 pc including the effects (i)--(iv). In a second step we investigated the collision of an external gas cloud falling onto an existing disk structure in the Galactic Centre. Finally, the line-of-sight distribution of the giant molecular clouds in the inner 50 pc of the Galaxy were reconstructed using their NIR absoprtion.

From the present modeling of the gas in the inner 50 pc of the Galaxy we have learned about crucial aspects of the gas physics and dynamics. We found that the mass accretion rate into the central parsec is highly variable and a period of almost no mass accretion is conceivable. This period of starvation might last about ten thousand years, which is the cloud-cloud collision time within the circumnuclear disk. We are now in the position to use the acquired knowledge to determine the temporal behaviour of the mass accretion rate in the past and how it might change in the future. Remaining questions are:

• Can we find clear signs of past interactions of an external cloud with a pre-existing circumnuclear disk?
• Will the accreted mass exclusively come from the CND in the near future?
• Will there be a major accretion event in the near future, when a part of a massive giant molecular cloud interacts with the CND?

We already have the keys in our hand to unravel the exciting history and future of the gas dynamics in the Galactic Centre.