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 X-Ray Sample of Bright Clusters of De Grandi and coworkers, including a wide range of X-ray 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 alpha similar or equal to -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 (alpha similar or equal to -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. 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 is what correlates with background decontamination estimates of the galaxy luminosity function. Moreover, no significant correlation between X-ray luminosity and the galaxy luminosity function faint-end slope is found. These results do not support a scenario in which 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.