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**Session 87 - Large Scale Structure.**

*Display session, Friday, January 09*

*Exhibit Hall, *

## [87.11] The RMS Peculiar Velocity of Clusters

*R. Watkins (Dartmouth College)*
We study the rms peculiar velocity of clusters as a convenient
statistic to put constraints on cosmological models. This
statistic is easy to compute theoretically given a model for the
power spectrum; we show that with some assumptions it can be
directly related to the quantity Ømega_0^0.6\sigma_8. We
develop a likelihood method for estimating the rms peculiar
velocity of clusters from line-of-sight velocity measurements
and their associated errors. We apply our method to two samples
of cluster peculiar velocities; a new sample known as the SCI
sample and a subsample of the Mark III catalog. Although these
two samples initially give results which are inconsistent, we
show that they can be put into good agreement by the removal,
particularly from the Mark III sample, of a set of clusters with
multiple mass concentrations, cluster membership ambiguities, or
excessive obscuration, properties which may have introduced
unaccounted for errors into measurements of their peculiar velocities.
Once these clusters are removed from the samples, they both
favor a relatively low value for the 1-D rms peculiar velocity
of clusters \sigma_v= 265^+106_-75\rm\ km\ s^-1 (at
90% confidence), leading to the constraint
Ømega_0^0.6\sigma_8= 0.44^+0.19_-0.13, consistent with
cluster abundance studies but inconsistent
with Ømega_0=1 CDM normalized to COBE at the
99.7% confidence level.

**Program
listing for Friday**