Dark matter, mergers, and morphology: clues from galaxy pairs

Abstract

We have examined the distributions of radial velocity differences Δv and projected separations rp of pairs in low density regions of the CfA and SSRS redshift catalogs. Bound pairs exist even at relatively large projected separations (up to 1 Mpc), indicating large amounts of dark matter around galaxies. The distribution of rp for bound pairs is flat, with significant numbers of pairs at projected separations less than 50 kpc. Pairs at small separations merge rapidly, but the distribution is replenished by those originally at larger separation that are being drawn toward merger by dynamical friction. As a result of this merger/replenishment process, at least 20% of galaxies in low density regions are the product of a merger; thus it seems that nearly all ellipicals and S0’s in these regions are created in this way.