2004ApJ...604L..45M

The discovery of hundreds of young, bright stars within a parsec from the massive black hole Sagittarius A* at the center of the Galaxy presents a challenge to star formation theories. The requisite Roche densities for the gravitational collapse of gas clouds are most naturally achieved in accretion disks. The water maser sources in Keplerian rotation in the nuclei of NGC 4258, NGC 1068, and the Circinus galaxy indicate the presence of warm, extended, molecular accretion disks around black holes similar in mass to Sgr A*. Here we argue that the current conditions in the maser nuclei, and those near the Galactic center, represent two consecutive, recurrent phases in the life cycle of the nucleus of a typical gas-rich spiral bulge. The warm molecular disks that give rise to the observed maser emission fragment into stellar-size objects. The stellar masses, their orbital geometry, and the total number of stars thus formed are consistent with the values identified at the Galactic center. The stars tend to form in compact groups resembling the IRS 13 complex that dominates the stellar light in the neighborhood of Sgr A*.