Precise black hole masses from megamaser disks: Black hole-bulge relations at low mass

The black hole (BH)-bulge correlations have greatly influenced the last decade of efforts to understand galaxy evolution. Current knowledge of these correlations is limited predominantly to high BH masses (M_BH≳ 10⁸M_⊙) that can be measured using direct stellar, gas, and maser kinematics. These objects, however, do not represent the demographics of more typical L < L * galaxies. This study transcends prior limitations to probe BHs that are an order of magnitude lower in mass, using BH mass measurements derived from the dynamics of H₂O megamasers in circumnuclear disks. The masers trace the Keplerian rotation of circumnuclear molecular disks starting at radii of a few tenths of a pc from the central BH. Modeling of the rotation curves, presented by Kuo et al., yields BH masses with exquisite precision. We present stellar velocity dispersion measurements for a sample of nine megamaser disk galaxies based on long-slit observations using the B&C spectrograph on the Dupont telescope and the Dual Imaging Spectrograph on the 3.5 m telescope at Apache Point. We also perform bulge-to-disk decomposition of a subset of five of these galaxies with Sloan Digital Sky Survey imaging. The maser galaxies as a group fall below the M _BH-σ_* relation defined by elliptical galaxies. We show, now with very precise BH mass measurements, that the low-scatter power-law relation between M_BH and σ_* seen in elliptical galaxies is not universal. The elliptical galaxy M _BH-σ_* relation cannot be used to derive the BH mass function at low mass or the zero point for active BH masses. The processes (perhaps BH self-regulation or minor merging) that operate at higher mass have not effectively established an M_BH-σ_* relation in this low-mass regime.