Encounters of Disk/Halo Galaxies

Observations and simplified models have long suggested that disk galaxies interact remarkably often and that some of these systems even merge and perhaps form elliptical galaxies, but detailed numerical evidence on such questions has been slow to accumulate. Here I present the first numerical models of interacting galaxies in which all dynamical components of the participants-bulges, disks, and massive halos-are modeled as fully three-dimensional N-body systems with thousands of particles each, and the equations of motion are integrated by a self-consistent algorithm without arbitrary restrictions on the geometry or spatial extent of the mass distribution. Several encounters between multicomponent galaxy models are described, starting with a look-alike for the "Antennae," NGC 4038/9. The influence of massive halos is exhibited by contrasting models run with and without dark matter. The present study shows that encounters involving pairs of disk/halo galaxies can produce impressive tidal tails while permitting the visible galaxies to merge on a short time scale, mimicking "double-tailed" objects such as NGC 7252. It turns out that extensive dark halos are very effective at soaking up binding energy and angular momentum from other components, significantly altering the distribution and dynamics of the luminous material. The collisions of model disk/halo galaxies described here produce slowly rotating remnants with surface brightness profiles approximating de Vaucouleurs law. The effective radii, shapes, and velocity dispersions of these remnants are generally consistent with elliptical galaxies.