THE NEED FOR A SECOND BLACK HOLE AT THE GALACTIC CENTER

摘要

Deep infrared observations and long-term monitoring programs have provided dynamical evidence of a su-permassive black hole of mass 3 x 10~6 solar mass associated with the radio source Sagittarius A~* at the center of our Galaxy. The brightest stars orbiting within 0.1 pc of the black hole appear to be young, massive main-sequence stars, in spite of an environment near the black hole that is hostile to star formation. We discuss mechanisms by which stars born outside the central parsec can sink toward the black hole and conclude that the drag coming from plausible stellar populations does not operate on the short timescales required by the stellar ages. We propose that these stars were dragged in by a second black hole of mass ～10~3―10~4 solar mass, which would be classified as an intermediate-mass black hole. We discuss the implications for the stellar populations and the kinematics in the Galactic center. Finally, we note that continued astrometric monitoring of the central radio source offers us the prospect for a direct detection of such objects.