Intermediate-Mass Black Holes
Abstract
We describe ongoing searches for intermediate-mass black holes with MBH ~ 100-105 Msun. We review a range of search mechanisms, both dynamical and those that rely on accretion signatures. We find that dynamical and accretion signatures alike point to a high fraction of 109-1010 Msun galaxies hosting black holes with MBH<105 Msun. 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 MBH ~ 100-104 Msun. Considering low-mass galaxies with dynamical black hole masses and constraining limits, we find that the MBH-sigma* relation continues unbroken to MBH~105 Msun, albeit with large scatter. We believe the scatter is at least partially driven by a broad range in black hole mass, since 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 high-redshift luminosity functions or robust measurements of off-nuclear black holes could begin to discriminate models.
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