Intermediate-Mass Black Holes

We describe ongoing searches for intermediate-mass black holes with M_BH≈ 10-10⁵M⊙. 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 10⁹-10¹⁰M⊙ galaxies hosting black holes with M_BH∼ 10⁵M⊙. 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≈ 100-10⁴M⊙. ▪Considering low-mass galaxies with dynamical black hole masses and constraining limits, we find that the M_BH-σ_∗relation continues unbroken to M_BH∼10⁵M⊙, 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 high-redshift luminosity functions or robust measurements of off-nuclear black holes could begin to discriminate models.