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.