Intermediate-Mass Black Holes

We describe ongoing searches for intermediate-mass black holes with M_BH ≈ 10-10⁵ M_⊙. 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 10⁹-10¹⁰ M_⊙ galaxies hosting black holes with M_BH∼ 10⁵ M_⊙. 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 ≈ 100-10⁴ M_⊙. 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 ∼10⁵ M_⊙, 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.