Low-Luminosity Active Galaxies and Their Central Black Holes

Abstract

Central black hole masses for 117 spiral galaxies representing morphological stages S0/a through Sc and taken from the large spectroscopic survey of Ho et al. are derived using Ks-band data from the Two Micron All Sky Survey. Black hole masses are found using a calibrated black hole-Ks bulge luminosity relation, while bulge luminosities are measured by means of a two-dimensional bulge-disk decomposition routine. The black hole masses are correlated against a variety of parameters representing properties of the nucleus and host galaxy. Nuclear properties such as line width (FWHM [N II]), as well as emission-line ratios (e.g., [O III]/Hβ, [O I]/Hα, [N II]/Hα, and [S II]/Hα), show a very high degree of correlation with black hole mass. The excellent correlation with line width supports the view that the emission-line gas is in virial equilibrium with either the black hole or bulge potential. The very good emission-line ratio correlations may indicate a change in ionizing continuum shape with black hole mass in the sense that more massive black holes generate harder spectra. Apart from the inclination-corrected rotational velocity, no excellent correlations are found between black hole mass and host galaxy properties. Significant differences are found between the distributions of black hole masses in early-, mid-, and late-type spiral galaxies (subsamples A, B, and C) in the sense that early-type galaxies have preferentially larger central black holes, consistent with observations that Seyfert galaxies are found preferentially in early-type systems. The line width distributions show a marked difference among subsamples A, B, and C in the sense that earlier type galaxies have larger line widths. There are also clear differences in line ratios between subsamples A+B and C that likely are related to the level of ionization in the gas. Finally, a Ks-band Simien & de Vaucouleurs diagram shows excellent agreement with the original B-band relation, although there is a large dispersion at a given morphological stage.