Abstract
The black hole in our own Galaxy is the best known, being the closest, and allows fundamental discoveries to be made about supermassive black holes in general. The distribution of gas and stars towards the center of the Galaxy is very complex; it seems that within a ring (the CMZ or Central Molecular Zone), ionized gas falls towards the center, in a three-armed spiral structure. But this nuclear structure is decoupled from the whole, for example, it is not seen from the edge, like the whole Galaxy, but more from the front. Very close to the center, less than a parsec away, several star clusters have been observed. The closest stars have been tracked over time in infrared, and their own motions indicate speeds of several thousand km/s. The trajectories are Keplerian, and have quantified the mass of the black hole at 4 million solar masses. The core of the Galaxy is not very active. It is a weak radio source (Sagittarius A*) whose radiation is several orders of magnitude below that of the usual active cores. Yet our black hole was probably active 10 or 100 million years ago, as ejections of matter are still visible at comparable distances. The Sagittarius A* source is observed with X-ray and infrared bursts, some of which have a quasi-periodicity of 15 minutes. This could correspond to matter rotating on the accretion disk, at a distance corresponding to the 25-minute rotation period (last stable orbit).
In the near future, it will be possible to observe the shadow of the black hole: the phenomenon is due to light rays deflected by the black hole, as in a gravitational lens. The size of the shadow is slightly larger than the black hole's horizon. By combining radio telescopes around the world, we can achieve spatial resolution in the fraction of a milli-arcsecond, using long-baseline interferometry (including ALMA, the Atacama Large Millimeter Array). Together, they form what is known as the EHT(Event Horizon Telescope). The Gravity instrument will also enable comparable resolutions in the infrared, on the ESO (European Southern Observatory) VLT (Very Large Telescope).
