The dynamical state of brightest cluster galaxies and the formation of
  clusters
  A large sample of Abell clusters of galaxies, selected for the likely
presence of a dominant galaxy, is used to study the dynamical properties of
brightest cluster members (BCMs). From visual inspection of Digitized Sky
Survey images combined with redshift data we identify 1426 candidate BCMs in
1221 redshift components in 1169 different Abell clusters, the largest such
sample published so far. By our own morphological classification we find ~92%
of these BCMs to be early-type galaxies, and 48% of cD type. We confirm
previous findings based on much smaller samples, namely that a large fraction
of BCMs have significant peculiar velocities. For a subsample of 452 clusters
with at least 10 measured radial velocities, we find a median BCM peculiar
velocity of 32% of their host clusters' radial velocity dispersion. This
suggests that most BCMs are not at rest in the potential well of their
clusters, and that the phenomenon is thus not a special trait of clusters
hosting cD galaxies. We show that the peculiar velocity of the BCM is
independent of cluster richness and only slightly dependent on the Bautz-Morgan
type. We also find a weak trend for the peculiar velocity to rise with the
cluster velocity dispersion. The strongest dependence is with the morphological
type of the BCM: cD galaxies tend to have lower relative peculiar velocities
than elliptical galaxies. This result points to a connection between the
formation of the BCMs and that of their clusters. Our data are qualitatively
consistent with the merging-groups scenario, where BCMs in clusters formed
first in smaller subsystems comparable to compact groups of galaxies. In this
scenario, clusters would have formed recently from the mergers of many such
groups and would still be in a dynamically unrelaxed state.
