7 The dynamical status of Abell 970

Several lines of evidence indicate that Abell 970 is not in an overall state of virialized equilibrium. Indeed, the presence of a substructure NW of the main galaxy concentration may indicate that the cluster has been capturing groups of galaxies in its neighbourhood. The large-scale gradient in the mean velocity of the galaxies, shown in Fig. 7, is also not expected in virialized systems. Other evidence for non-equilibrium include the discrepancy in the peaks of the X-ray emission and of the galaxy projected density, and the observed differences between optical and X-ray masses.

According to Allen (1998), there is a good agreement between X-ray and strong gravitational lensing mass measurements only in clusters with strong cooling flows; in clusters with modest or absent cooling flows, the masses determined from the X-ray data are 2 to 4 times smaller than those estimated from strong gravitational lensing. The reason for the mass discrepancy is the dynamical status of the central regions of the clusters: those with strong cooling flows are relaxed and virialized, while those with small cooling flows are out of equilibrium, and the assumption of hydrostatic equilibrium that underlies the X-ray mass estimates is not valid. Indeed, the presence or absence of cooling flows can be used as a diagnostic to verify whether galaxy clusters are in dynamical equilibrium in their central regions. Abell 970 has at most a weak cooling flow, of $\dot{M} = 20^{+32}_{-20}~M_\odot$ yr^-1 (Ebeling et al. 1996), and our results, indicating that this system is not relaxed, are consistent with the findings of Allen (1998).

It is interesting to note that the offset between the X-ray and galaxy distribution centres, although significant, is not very large. It is possible that Abell 970 had a much stronger cooling flow until recently, that was interrupted by dynamical perturbations induced by the arrival of a galaxy group (now observed as a substructure) in the central regions of the cluster. Given that in the cluster densest regions the gas relaxes very quickly, compared with the time the galaxy distribution takes to achieve equilibrium, it is natural to think of cooling flows as an intermittent process that is disrupted by dynamical perturbations and that resumes activity after relaxation is achieved. The time scale of intermittence, in this scenario, depends strongly on the accretion rate of groups by the cluster.