Pairwise Velocities of Galaxies in the CfA and SSRS2 Redshift Surveys

(compressed version) We combine the CfA Redshift Survey (CfA2) and the Southern Sky Redshift Survey (SSRS2) to estimate the pairwise velocity dispersion of galaxies $sig12$ on a scale of $sim 1 hmpc$. Both surveys are complete to an apparent magnitude limit $B(0)=15.5$. Our sample includes 12,812 galaxies distributed in a volume $1.8 imes 10^6 hmpc3$. We conclude: 1) The pairwise velocity dispersion of galaxies in the combined CfA2+SSRS2 redshift survey is $sig12=540 kms pm 180 kms$. Both the estimate and the variance of $sig12$ significantly exceed the canonical values $sig12=340 pm40$ measured by Davis & Peebles (1983) using CfA1. 2) We derive the uncertainty in $sig12$ from the variation among subsamples with volumes on the order of $7 imes 10^5$ hmpc3. This variation is nearly an order of magnitude larger than the formal error, 36 $kms$, derived using least-squares fits to the CfA2+SSRS2 correlation function. This variation among samples is consistent with the conclusions of Mo etal (1993) for a number of smaller surveys and with the analysis of CfA1 by Zurek etal (1994). 3) When we remove Abell clusters with $Rge1$ from our sample, the pairwise velocity dispersion of the remaining galaxies drops to $295 pm 99 kms$. Thus the dominant source of variance in $sig12$ is the shot noise contributed by dense virialized systems. 4) The distribution of pairwise velocities is consistent with an isotropic exponential with velocity dispersion independent of scale.