ar X iv : a st ro - p h / 03 11 11 8 v 1 5 N ov 2 00 3 Faint galaxy population in clusters : X - ray emission , cD halos and projection effects

We analyze samples of nearby clusters taken from the Abell catalog and the Xray Sample of Bright Clusters (De Grandi et al 1999) including a wide range of Xray luminosities. Using the usually adopted background subtraction procedures, we find that galaxies in clusters selected by means of their X-ray emission show a flat luminosity function (faint end slope α ≃ −1.1) consistent with that derived for galaxies in the field and groups. By contrast, the sample of Abell clusters that do not have an X-ray counterpart shows a galaxy luminosity function with a steep faint end (α ≃ −1.6). We investigate the possibility that cD halos could be formed by the disruption of galaxies in rich relaxed clusters that show an apparently flat faint end galaxy luminosity function (Lopez-Cruz et al 1997). We find that clusters dominated by a central cD galaxy (Bautz-Morgan classes I and II) show the same systematic trend: X-ray selected clusters have flatter faint end slopes than those clusters with no detected X-ray emission. Thus, it is likely the X-ray selection and not the cluster domination by central galaxies what correlates with background decontamination estimates of the galaxy luminosity function. Moreover, no significant correlation between X-ray luminosity and the galaxy LF faint end slope is found. These results do not support a scenario where flat faint end slopes are a consequence of cD formation via the disruption of faint galaxies. We argue that the clusters without X-ray emission are strongly affected by projection effects which give rise to spurious faint end slopes estimated using background subtraction procedures (Valotto et al 2001). Subject headings: galaxies: luminosity function; galaxies: clusters; galaxies: XRay IATE, Observatorio Astronómico, Laprida 854, Córdoba and CONICET, Argentina. Institute for Theoretical Physics, University of Zrich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland