2.5 Galaxy Pairwise Velocities

Using the cosmic virial theorem, the relative velocity dispersion of galaxy pairs can be used to estimate the matter density (e.g. Davis & Peebles 1983). The Las Campanas Redshift Survey (Shectman et al. 1996) contains about 26,000 redshifts out to ~ 30,000 km/sec and provides an excellent sample of galaxies not dominated by clusters. Davis (this conference) presented results based on this sample, concluding that relative galaxy pairs have a one-dimensional velocity dispersion of only 260 km/sec, implying _m ~ 0.25.

This method is very clean and conceptually simple; however, it again is limited by the assumption that bias is independent of scale. Moreover, Frenk (1997) argues that bulk velocity flows are not sensitive to _m, and that the peculiar velocities are quite similar for a number of models with a range of values of _m.