Understanding the Nuclear Gas Dispersion in Early-Type Galaxies in the Context of Black Hole Demographics

(abridged) We analyze and model HST /STIS observations of a sample of 27galaxies; 16 Fanaroff & Riley Type I radio galaxies and 11 (more) normalearly-type galaxies. We focus here on what can be learned from the nuclearvelocity dispersion (line width) of the gas as a complement to the many studiesdealing with gas rotation velocities. We find that the dispersion in a STISaperture of ~0.1''-0.2'' generally exceeds the large-scale stellar velocitydispersion of the galaxy. This is qualitatively consistent with the presence ofcentral BHs, but raises the question whether the excess gas dispersion is ofgravitational or non-gravitational origin and whether the implied BH masses areconsistent with our current understanding of BH demography(as predicted by theM-sigma relation between BH mass and stellar velocity dispersion). To addressthis we construct dynamical models for the gas, both thin disk models andmodels with more general axis ratios and velocity anisotropies. For the normalgalaxies the nuclear gas dispersions are adequately reproduced assuming disksaround BHs with masses that follow the M-sigma relation. In contrast, the gasdispersions observed for the radio galaxies generally exceed those predicted byany of the models. We attribute this to the presence of non-gravitationalmotions in the gas that are similar to or larger than the gravitationalmotions. The non- gravitational motions are presumably driven by the activegalactic nucleus (AGN), but we do not find a relation between the radiativeoutput of the AGN and the non-gravitational dispersion. It is not possible touniquely determine the BH mass for each galaxy from its nuclear gas dispersion.However, for the sample as a whole the observed dispersions do not provideevidence for significant deviations from the M-sigma relation.Comment: accepted for publication in the Astronomical Journa