ABSTRACT

During the stage of galaxy formation, or because of subsequent accretion events, cold, rotationally supported matter carrying substantial angular momentum is deposited and organized in disks/rings at the outer parts of many types of galaxies. Examples are warped HI disks in spirals, disks of gas and dust in ellipticals, and polar rings around SOs/ellipticals. The observed nuclear activity in many galaxies has its origin in mass transfer from large radii into the centers of galaxy potential wells. We describe two stages of such a mass transfer process and present related numerical multidimensional hydrodynamical simulations. In the first stage, gas still carrying some angular momentum flows toward the nuclear region as a moderately inclined outer disk is attempting to settle toward an energetically “preferred orientation.” In the second stage, a nuclear accretion disk, influenced by the gravitational potential well of a central massive black hole and possibly by a coherent weak magnetic field, suffers one of several known dynamical nonaxisymmetric instabilities that drives gas deeper into the potential well and regulates its accretion onto the black hole.

INTRODUCTION

Although gas is predominantly found in spiral galaxies distributed in rotationally supported large-scale disks (∼ 10 kpc), recent observations reveal the existence of counter-rotating gaseous disks in the central regions of many elliptical and SO galaxies (Franx and Illingworth 1988; Bertola et al. 1990; Bertola, Buson, and Zeilinger 1992). The accepted interpretation of such observations is that gas in ellipticals and SOs must be of external origin.