Are Ultraluminous X‐Ray Sources Intermediate‐Mass Black Holes Accreting from Molecular Clouds?

The origin and nature of Ultra-Luminous X-ray sources (ULXs) is a contentiousand controversial topic. There are ongoing debates about the masses of theobjects responsible, their sources of mass for accretion, and their relation tostellar populations in galaxies. A new picture of these objects is proposed inwhich they are intermediate-mass black holes confined to the disks of theirhost galaxies and accreting from the interstellar medium. They are thenpreferentially found in or near molecular clouds. This model correctly predictsthe shape of the observed luminosity function and requires only a very smallfraction of the baryonic mass of a galaxy to be in the form ofintermediate-mass black holes. Because the X-rays they produce strongly heatnearby interstellar gas and because they move relatively rapidly in and out ofdense regions, ULXs are predicted to have brief episodes of high luminosity,perhaps of order 10^5 yr in duration, but they may recur many times.Comment: 16 pages, to appear in 1 November 2004 Ap