Dynamics of the galactic component of Abell S1063 and MACS J1206.2−0847

Abstract

Context. The galactic component in clusters is commonly thought to be generally nonrotating and in a dynamical state different from that of a collisionally relaxed system. In practice, a test for such conditions is often not available. Aims. We consider the member galaxies of the two clusters Abell S1063 and MACS J1206.2−0847 and study the possible presence of mean rotation and some properties of their distribution in phase space. We look for empirical evidence of factors normally found in collisionally relaxed systems and other characteristics of violently relaxed collisionless systems. Methods. Starting from the CLASH-VLT data, we obtained positions, stellar masses, and individual line-of-sight velocities for a large number of galaxies (NAS1063 ≈ 1200 and NM1206 ≈ 650) extending out to ≈1.6 (Abell) and ≈2.5 (MACS) times the radius r200. We studied the spatial distribution of the galaxy velocities and the properties of the available galaxy sets when divided into stellar mass bins. To test the presence of velocity dispersion anisotropy, we compared the results based on the Jeans equations with those obtained by assuming a specific form of the galaxy distribution function incorporating the picture of violent relaxation, where the total gravitational potential is imposed as set by the available gravitational lensing observations. Results. We find evidence of systematic rotation in both clusters, with significant rotation in each core (within 0.5′ from the center) and no signatures of rotation at large radii. While no signs of energy equipartition were found, there is a clear indication of (stellar) mass segregation. Velocity dispersion anisotropy is present and qualitatively similar to what has been found in violently relaxed collisionless systems. This last conclusion is strengthened by the overall success in matching the observations with the predictions of the physically justified distribution function.