Head-on collision of spherical galaxies

Abstract

N-body simulations of head-on collision of a pair of identical spherical galaxies have been performed with a set of initial conditions covering both merging and non-merging types of collisions. The merging of the galaxies takes place when the relative velocity of collision V is less than about four times the internal velocity dispersion traction, i.e., V < 4.1 traction. The transition between merging and non-merging of galaxies occurs in a narrow range in collision velocity. The energy change, the mass-loss, the half-mass radius, the central concentration etc. reach maximum value when V = 4.1 traction. Mass transfer dominates. when the collision velocity is small and consequently the merger remnants show negligible mass-loss. There is evidence for the formation of a tighter core and more extended envelope than that of the initial model. The estimates of energy transfer using impulse approximation show large discrepancy with numerical results in merging cases. In non-merging cases, the analytic formula overestimates the energy change by more than a factor of two.