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 theX-ray Sample of Bright Clusters(De Grandi et al 1999) including a wide range ofX-ray luminosities.Using the usually adopted background subtraction procedures,we find that galaxies in clusters selected by means of their X-ray emissionshow a flat luminosity function (faint end slope $\alpha \simeq -1.1$)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 agalaxy luminosity function with a steep faint end ($\alpha \simeq -1.6$). Weinvestigate the possibility that cD halos could be formed by the disruption ofgalaxies in rich relaxed clusters that show an apparently flat faint end galaxyluminosity function (Lopez-Cruz et al 1997). We find that clusters dominated by a central cD galaxy (Bautz-Morgan classesI and II) show the same systematic trend: X-ray selected clusters have flatterfaint end slopes than those clusters with no detected X-ray emission. Thus, itis likely the X-ray selection and not the cluster domination by centralgalaxies what correlates with background decontamination estimates of thegalaxy luminosity function. Moreover, no significant correlation between X-rayluminosity and the galaxy LF faint end slope is found. These results do notsupport a scenario where flat faint end slopes are a consequence of cDformation via the disruption of faint galaxies. We argue that the clusterswithout X-ray emission are strongly affected by projection effects which giverise to spurious faint end slopes estimated using background subtractionprocedures (Valotto et al 2001).Comment: Accepted for publication in ApJ (vol. 601