Bulge and Halo Kinematics Across the Hubble Sequence

Abstract

The correlation between the maximum rotational velocity of the disk (vm) and the central stellar velocity dispersion of the bulge (σ0) offers insights into the relationship between the halo and the bulge. We have assembled integrated H I line widths and central stellar velocity dispersions to study the vm-σ0 relation for 792 galaxies spanning a broad range of Hubble types. Contrary to earlier studies based on much smaller samples, we find that the vm-σ0 relation exhibits significant intrinsic scatter and that its zero point varies systematically with galaxy morphology, bulge-to-disk ratio, and light concentration, as expected from basic dynamical considerations. Nucleated but bulgeless late-type spiral galaxies depart significantly from the vm-σ0 relation. While these results render questionable any attempt to supplant the bulge with the halo as the fundamental determinant of the central black hole mass in galaxies, the observed distribution of vm/σ0, which depends on both the density profile and kinematic structure of the galaxy, offers a useful constraint on galaxy formation models. With the aid of a near-infrared Tully-Fisher relation, we identify a population of otherwise normal, luminous galaxies that have exceptionally low values of vm/σ0. We argue that a significant fraction of the H I gas in these kinematically anomalous objects is dynamically unrelaxed, having been acquired externally either through capture from tidal interactions or through cold accretion from the intergalactic medium.