8 Summary and conclusion

In this paper, we have presented an analysis of the galaxy cluster Abell 970, based on a new set of radial velocities and on X-ray observations. The study of the galaxy projected positions reveals a relatively regular distribution, centered on an E/D dominant galaxy. The analysis with the adaptive kernel density map indicates the presence of a statistically significant substructure NW of the cluster main galaxy concentration, centered on a S0/S galaxy that is the brightest cluster member. The X-ray emission distribution does not reveal any emission excess due to that substructure but, interestingly, the peak of the X-ray emission is not coincident with the cluster centre (at the position of the dominant galaxy), being displaced towards the direction of the substructure. These results suggest that this substructure is real and that the cluster may not be in an overall state of dynamical equilibrium.

Further evidence that the cluster is in a state of non-equilibrium comes from the analysis of the radial velocity distribution. For instance, the cluster velocity dispersion, 845 kms^-1 (increasing to $\sim$1000 kms^-1 at the cluster centre), is significantly larger than the value expected from the $\sigma{-}T_{\rm X}$ relation, that is, $\sim$700 kms^-1. Also, the substructure detected in the galaxy projected distribution has a much smaller velocity dispersion, 381 kms^-1, that is typical of loose groups. Together, these results suggest that this substructure may be a group that recently arrived in the central regions of the cluster. The presence of large scale velocity gradients is further evidence that Abell 970 is out of equilibrium. The virial mass of this cluster is much larger than the mass inferred from the X-ray emission. This discrepancy is indeed expected if the underlying hypothesis of these mass estimators, namely that galaxies and gas inside the cluster are in hydrostatic equilibrium, is not actually fulfilled.

The fact that Abell 970 has a dim cooling flow also fits nicely in the above scenario if, as suggested by Allen (1998), only clusters in equilibrium exhibit massive cooling flows. Indeed, cooling flows may have an intermittent behaviour: phases of massive cooling flows may be followed by phases without significant cooling flows after the accretion of a galaxy group massive enough to disrupt the dynamical equilibrium in the centre of the clusters. After a new equilibrium is achieved, a massive cooling flow will be established again. Hence, in hierarchical scenarios for structure formation, intermittent cooling-flows should be a common phenomenon.

Acknowledgements

We thank the OHP, Pic du Midi and ESO staff for their assistance during the observations, and especially Gilles Charvin, student at the École Normale Supérieure de Lyon for his valuable scientific collaboration. We also thank Bill Forman and Christine Jones for valuable comments on the spectroscopic capability of the Einstein IPC detector, and Sergio Dos Santos for help with the wavelet package. LSJ, HVC, GBLN and HC thank the financial support provided by FAPESP, CNPq and PRONEX. DP and HQ acknowledge support from ECOS/CONICYT project C96U04. HQ was partly supported by the award of a Presidential Chair in Science (Chile). This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center, and of the ROE/NRL COSMOS UKST Southern Sky Object Catalog.