Comparison of N-Body Simulations to Statistical Observations of Galaxy Pairs

Abstract

N-body simulations were conducted of pairs of galaxies with a 3:1 mass ratio on parabolic orbits, in order to quantify the effect of dynamical friction. The effects of varying the ratio of the dark matter halo size to the distance of closest approach were explored. Once the dark matter halos are fully overlapping, the more massive simulated galaxies achieve a larger maximum separation after the first encounter despite the increased dynamical friction caused by the more extended halos. Projected separation and radial velocity histograms were generated by "observing" the simulation results at various times and from various orientations. These histograms were compared with observations of galaxy pairs (Charlton & Salpeter 1991; Chengalur, Salpeter, & Terzian 1993) with the result that large halo radii (~200 - 600 kpc) and wide distances of closest approach are generally favored. It is difficult to reconcile the small radial velocity differences that have been observed (median of ~ 30 km/s, Chengalur et al. 1993) with the simulations when we sample all parts of the orbits equally. Including an additional population of wide pairs that have just recently reached "turnabout" from the Hubble flow would lower the median velocity differences. Models suggest that additional data for pairs at intermediate separations should have a somewhat larger median velocity difference than the wide pairs. Very narrow pairs include galaxies which are interacting, and whose gaseous components respond to forces other than gravity. If consistently small $\Delta v$ are measured from neutral hydrogen velocities in a larger sample of narrow pairs, pressure forces and dissipation effects on the gaseous components could be responsible.