Excitation of Activity in Galaxies by Minor Mergers

Mergers between gas-rich disks and less massive dwarf galaxies are studied using numerical simulation. As the orbit of a satellite decays through dynamical friction, the primary disk develops large-amplitude spirals in response to its tidal forcing. While these features arise in both the stars and the gas in the disk, the nonaxisymmetric structures in the gas differ slightly from those in the stars. In particular, as a consequence of the formation of strong shocks in the gas and the effects of radiative cooling, the gas response tends to lead the stellar response, enabling the stars to strongly torque the gas. These torques deprive the gas of its angular momentum, forcing a significant fraction of it into the inner regions of the disk. Depending on the detailed treatment of the gas physics and the structure of the primary galaxy, the gas can also condense into dense knots in reaction to the gravitational perturbation of the dwarf, which later sink to the center of the primary by dynamical friction against the stellar background.

The radial inflows induced by these mergers accumulate large quantities of interstellar gas in the nuclear regions of the host disks. In some cases, nearly half of all the gas initially distributed throughout the disk winds up in a dense "cloud" extending several hundred parsecs. The models reported here do not include star formation, and so we cannot determine the ultimate fate of the gas. Nevertheless, given the high densities in the nuclear gas, it is plausible to identify these concentrations of dense gas in the remnants with those accompanying intense starbursts in some active galaxies. Therefore, the calculations here provide a framework for interpreting the origin of nuclear activity in otherwise quiescent disk galaxies. To the extent that galaxy formation is a chaotic process in which large structures are built up by the accretion of smaller fragments, our models may also be relevant to starbursts and the onset of nuclear activity in protogalaxies at high red shifts.