Intermediate-Mass Black Holes

<jats:p>We describe ongoing searches for intermediate-mass black holes with M<jats:sub>BH</jats:sub>≈ 10–10<jats:sup>5</jats:sup>M<jats:sub>⊙</jats:sub>. 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<jats:sup>9</jats:sup>–10<jats:sup>10</jats:sup>M<jats:sub>⊙</jats:sub>galaxies hosting black holes with M<jats:sub>BH</jats:sub>∼ 10<jats:sup>5</jats:sup>M<jats:sub>⊙</jats:sub>. 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<jats:sub>BH</jats:sub>≈ 100–10<jats:sup>4</jats:sup>M<jats:sub>⊙</jats:sub>. ▪  Considering low-mass galaxies with dynamical black hole masses and constraining limits, we find that the M<jats:sub>BH</jats:sub>–σ<jats:sub>*</jats:sub>relation continues unbroken to M<jats:sub>BH</jats:sub>∼10<jats:sup>5</jats:sup>M<jats:sub>⊙</jats:sub>, 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.</jats:p>