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 (MBH≳ 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 MBH 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 MBH-σ* relation in this low-mass regime.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science
- Galaxies: active, galaxies: bulges, galaxies: nuclei, galaxies: Seyfert