Transformations of Galaxies. I. Mergers of Equal-Mass Stellar Disks

This paper presents self-consistent numerical results for a small sample of merging encounters between equal-mass disk galaxies. These calculations illustrate how self-gravitating disks respond to tidal perturbations and suggest an improved point of view on orbital decay in multicomponent systems. Preexisting spheroidal components merge rather gently, but the incomplete violent relaxation of the disks themselves is accompanied by a large drop in coarse-grained phase-space density. A detailed analysis of the orbital structure of these merger remnants shows how their shapes and kinematic properties are related to the initial disk spin vectors and other encounter parameters. Many of these remnants exhibit significant misalignment between their minor and rotation axes, a result which may constrain the number of elliptical galaxies formed by purely stellar-dynamical mergers.