The dynamical state of brightest cluster galaxies and the formation of clusters

A large sample of Abell cluster of galaxies, selected for its likely presence of a dominant galaxy, is used to study the dynamical properties of their brightest cluster members (BCMs). From visual inspection of Digitized Sky Survey images combined with redshift information we identify 1430 candidate BCMs located in 1223 different redshift components associated with 1171 different Abell clusters. This is the largest sample published so far of such galaxies. From our own morphological classification we find that ~ 92% of the BCMs in our sample are early-type galaxies, and 48% are of cD type.

We confirm what was previously observed based on much smaller samples, namely that a large fraction of BCMs have significant peculiar velocities. From a subsample of 452 clusters having at least 10 measured radial velocities, we estimate 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. This phenomenon is common to galaxy clusters in our sample, and 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 observations seem difficult to reconcile with a model where BCMs form by mergers at the center of a dynamically relaxed and spherically symmetric potential well composed of non-baryonic cold dark matter. On the other hand, the data seem qualitatively consistent with the merging-groups scenario, where BCMs formed first in smaller subsystems comparable to groups of galaxies. In this scenario, clusters would have formed from the mergers of many such groups. Our observation suggests that most clusters harboring a dominant galaxy today are in an unrelaxed dynamical state.