  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.
