Galaxy pairs in redshift catalogs as a probe of dark matter

Abstract

A combined northern and southern galaxy redshift catalog is analyzed. Examining galaxy pairs in terms of two observables: the line-of-sight velocity difference delta(v) and the projected spatial separation. (1) In low-density regions there is an excess of pairs with radial velocity differences of less than 150 km/s, even for pairs with projected separations r(p) as large as 1 Mpc; thus, any initial correlations would have been disrupted before now, and these pairs must be bound. (2) No Kepler relationship is seen between the pair properties, even for pairs that are isolated. (3) The distribution of r(p) for low-density pairs with delta(v) less than 150 km/s is flat and featureless from very small separations out to 2 Mpc. These three facts indicate that large amounts of dark matter surround a typical galaxy. The third point suggests a model where pairs at small r(p) merge rapidly by dynamical friction but are replenished by pairs that begin at larger separation and are heading toward merger, and it allows an estimate of the galactic merger rate. 34 refs.