Abell 521: dynamical analysis of a young cluster ?

We present the results of a dynamical analysis of the rich X-Ray luminous galaxy cluster Abell 521, and discuss the nature of the arc-like structure first noted by Maurogordato et al. (1996). Our study is based on radial velocities for 41 cluster members, measured from spectra obtained at the Eu- ropean Southern Observatory and the Canada-France-Hawaii Telescope. Based on statistical analyses performed with the RO- STAT package, we find that Abell 521 is an intermediate-redshift cluster (CBI = 74132 +202 250 km s 1 ) with a rather high apparent value of the velocity dispersion SBI = 1386 +206 139 km s 1 . There are many indications that this cluster is presently un- dergoing strong dynamical evolution: a) the high value of the velocity dispersion, which cannot be explained by trivial pro- jection effects, b) significant clumping in the two-dimensional projected positions of the galaxies in the cluster, as quanti- fied by a mixture-model three-group partition significant at the 99 % level, c) the extreme value of the velocity dispersion ( 2000 km s 1 ) in a central high density NE/SW structure, d) a strong increase of the velocity dispersion as determined from the reddest and bluest galaxies, suggesting that cluster spi- rals are not yet virialized, e) the presence of multiple nuclei in the core of the brightest cluster galaxy as well as clear signatures of interaction effects, and f) an apparently different stellar pop- ulation for the various knots of the arc candidate which changes along the structure. The two brightest knots of the giant arc candidate are shown to be at the velocity of the cluster. The bright curved structure is thus probably due to interaction processes between the knots rather than due to strong gravitational lensing. However, gravi- tational lensing might be present in this cluster, as suggested by the colors of two fainter arclet-like structures.