Optical and X-ray analysis of the cluster of galaxies Abell 496

Abstract

We present a detailed analysis of the cluster of galaxies \a4. The optical data include a redshift catalogue of 466 galaxies, out of which 274 belong to the main cluster and a CCD photometric catalogue in a much smaller region, with 239 and 610 galaxies in the V and R bands respectively. The X-ray analysis is based on an image obtained with the ROSAT PSPC. Besides Abell 496 itself, the velocity distribution along the line of sight shows the existence of at least four structures at different redshifts, one of them seeming to be a poor cluster at a velocity of 30083 km/s. The other of these structures have a too large spatial extent to be clusters but may be filaments along the line of sight or other young structures. Various independent methods show that Abell 496 appears to be a quite relaxed cluster, except perhaps for the distribution of emission line galaxies. These appear to be distributed in two samples falling on to the main cluster, one from the back (the ELGs concentrated towards the west) and one from the front (the high velocity ELGs). The bright part of the galaxy luminosity function, built from the redshift catalogue, shows a flattening at R$\sim 16$ (M$_{\rm R}\sim -20.5$), and can be accounted for by a gaussian distribution of bright galaxies and a power law or Schechter function for faint galaxies. The deeper galaxy counts derived from CCD imaging show a dip at R$\sim 19.5$ (M$_{\rm R} \sim -17$) which can be modelled assuming a cut-off in the luminosity function such as that observed in Coma. We propose a model for the X-ray gas and derive the galaxy, X-ray gas and total dynamical masses, as well as the baryon fraction in the cluster. \a4 appears as a relaxed cluster which can be used as a prototype for further studies.