Galaxy pairs in redshift catalogs as a probe of dark matter

Pairs of galaxies extracted from the CfA m_B = 14.5 and SSRS redshift catalogs are studied, separately in high- and low-density regions, focusing mostly on the low-density pairs. Our main conclusions are the follow-ing: (1) In low-density regions there is an excess of pairs with radial velocity differences Δv < 150 km s^-1, even for pairs with projected separations r_p as large as 1 Mpc. The transit time r_p/Δv is less than the Hubble time for at least those pairs with r_p < 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 (Δv, r_p, and L, the pair luminosity), even for pairs that are isolated. (3) The distribution of r_p for low-density pairs with Δv < 150 km s^-1 is flat and featureless from very small separations out to 2 Mpc. These three facts indicate that large amounts of dark matter (possibly as much as 10¹³ M_⊙ out to 1 or 2 Mpc) surround a typical galaxy. (4) 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. We suggest that many of the galaxies in low-density regions that are classified as elliptical or S0 are in fact merger remnants. (5) In low-density regions, very isolated pairs tend to have smaller values of Δv than those in small groups, but Δv depends little on the projected separation and luminosity of the pair. This suggests that galaxy groupings in low-density regions appear in common halos of dark matter (with the number of galaxies increasing with the total halo mass) rather than each galaxy having its own large spherical isothermal halo.