Substructure in clusters and cD peculiar velocities: A2670

Abstract

An analysis of the Sharples et al. 1988 velocity and position data for A2670 provides evidence for the merger of at least two distinct subsystems. Statistical diagnostics of the velocity distribution and the combined velocity and position data indicate the presence of a strong correlation between the two quantities, as expected for a cluster which has not yet reached dynamical equilibrium. The high velocity offset reported for the central dominant galaxy is often cited as evidence that such galaxies are not preferentially located in the minimum of the gravitational potential of the host cluster. This poses a major problem for cD formation models, most of which require this preferential location for any formation process to be efficient (Merritt 1985; Tremaine 1990). Mixture modeling of the galaxy velocity and position data indicates the presence of two or more subclusters, which may be the remnants of the groups which formed the rich cluster. The velocity and position of the cD galaxy suggest that it is moving into the potential minimum of the coalescing system and that it was originally (i.e., at the time of its formation) at rest within one of the subsystems.