2.4 Peculiar Velocities: Density and Velocity Comparisons
On scales of ~ 100 Mpc, the motions of field galaxies can be used to infer the mass density given independent distance information. These methods do not yield a measure of m directly, but rather yield the ratio = 0.6 / b where b is the bias parameter (describing the relation between mass and light) over a scale of a few hundred km/sec. These methods are again insensitive to both H0 and . Several different approaches have been investigated. For more details, the reader is referred to Dekel (1994), Willick et al. (1997) and Dekel, Burstein and White (1997).
All methods make use of radial velocity catalogs and distances based on the Tully-Fisher relation. The analyses differ in detail and there are advantages and disadvantages to each type of approach. At the present time, the results from this type of technique have not yet yielded a consistent picture. Earlier analyses (e.g. Dekel et al. 1993) suggested large values of ~ 1.3, and correspondingly rather high values of (subject to assumptions about the value of b). More recently, the estimates of have decreased somewhat (Dekel, Burstein & White 1997). At present, the results from different groups (e.g., Dekel, Willick, Davis and collaborators) appear to differ from the results of Giovanelli, Haynes, Da Costa and collaborators (see the contribution by Da Costa to this volume). Understanding the sources of the differences is clearly an important goal.