1. The Orbit of S2 in the Galactic Center
Over the last decades, it has become a standard paradigm that massive black holes are located at the centers of most, if not all, galaxies. They are believed to be the central engines powering the enormous energy ouput and luminosity of active galactic nuclei and of quasars, the most distant and most luminous objects observed in our universe. Observations of other galactic nuclei have already provided strong evidence that these massive black holes exist. However, this evidence has always been indirect and did not exclude alternative explanations (such as the neutrino ball model). By showing in a direct and straightforward way that such a massive black hole does indeed exist in the heart of our own galaxy, the Milky Way, we provide a firm and compelling fundament for this key hypothesis of modern astronomy.
In 2002 we have observed 2/3 of a complete orbit of the star currently closest to the enigmatic radio source Sagittarius A*, which is thought to mark the location of our Milky Way's massive central black hole. This orbit provides overwhelming evidence that SgrA* is indeed a supermassive black hole of more than 2 million solar masses.
Observations by our group of the velocities and accelerations of stars in the Galactic Center over the course of the last decade have already provided strong evidence that SgrA* is in fact a black hole of about 2.6 million solar masses. However, these measurements could not rule out some alternatives to the black hole model. Two such alternatives are a ball of massive, degenerate fermions, like neutrinos, or a cluster of dark astrophysical objects, such as stellar mass black holes or neutron stars. These two alternative explanations can now be excluded by analyzing the orbit of S2.
In spring 2002 S2 was passing with the extraordinary velocity of more than 5000 km/s at a mere 17 light hours distance through the perinigricon, the point of closest approach to the black hole. By combining all measurements of the position of S2 made between spring 1992 and summer 2002, we have obtained enough data in order to determine a unique keplerian orbit for this star. It has a period of 15.2 years. From Kepler's 3rd law we can determine the enclosed mass to be 3.7±1.5 million solar masses.
Enclosed mass derived from different studies of gas dynamics and stellar dynamics at various distances from the Galacic Center. The point at the shortest distance is the mass measurement added with the aid of the newly determined orbit. The fascinating feature of this plot is that the best fit curve for the enclosed mass is flat inside of about 0.2 parsecs. From this we draw the compelling conclusion that a massive black hole must be present at the center of the Milky Way.
published as: Schödel et al., Nature 419, 694 (2002)
R. Schödel, T. Ott, R. Genzel