Intermediate-Mass Black Holes

We describe ongoing searches for intermediate-mass black holes with M-BH approximate to 10-10(5) M-circle dot. We review a range of search mechanisms, both dynamical and those that rely on accretion signatures. We find the following conclusions: Dynamical and accretion signatures alike point to a high fraction of 10(9)-10(10) M-circle dot galaxies hosting black holes with M-BH similar to 10(5) M-circle dot. In contrast, there are no solid detections of black holes in globular clusters. There are few observational constraints on black holes in any environment with M-BH approximate to 100-10(4) M-circle dot. Considering low-mass galaxies with dynamical black hole masses and constraining limits, we find that the M-BH-sigma* relation continues unbroken toMBH similar to 10(5) M-circle dot, albeit with large scatter. We believe the scatter is at least partially driven by a broad range in black hole masses, because the occupation fraction appears to be relatively high in these galaxies. We fold the observed scaling relations with our empirical limits on occupation fraction and the galaxy mass function to put observational bounds on the black hole mass function in galaxy nuclei. We are pessimistic that local demographic observations of galaxy nuclei alone could constrain seeding mechanisms, although either highredshift luminosity functions or robust measurements of off-nuclear black holes could begin to discriminate models.