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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 Omegam ~ 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 Omegam, and that the peculiar velocities are quite similar for a number of models with a range of values of Omegam.