RASS-SDSS Galaxy cluster survey -: IV.: A ubiquitous dwarf galaxy population in clusters

We analyze the Luminosity Functions (LFs) of a subsample of 69 clusters from the RASS-SDSS galaxy cluster catalog. When calculated within the cluster physical sizes, given by r(200) or r(500), all the cluster LFs appear to have the same shape, well fitted by a composite of two Schechter functions with a marked upturn and a steepening at the faint-end. Previously reported cluster-to-cluster variations of the LF faint-end slope are due to the use of a metric cluster aperture for computing the LF of clusters of different masses. We determine the composite LF for early- and late-type galaxies, where the typing is based on the galaxy u - r colors. The late- type LF is well fitted by a single Schechter function with a steep slope (alpha = -2.0 in the r band, within r(200)). The early-type LF instead cannot be fitted by a single Schechter function, and a composite of two Schechter functions is needed. The faint-end upturn of the global cluster LF is due to the early- type cluster galaxies. The shape of the bright-end tail of the early- type LF does not seem to depend upon the local galaxy density or the distance from the cluster center. The late- type LF shows a significant variation only very near the cluster center. On the other hand, the faint-end tail of the early- type LF shows a significant and continuous variation with the environment. We provide evidence that the process responsible for creating the excess population of dwarf early type galaxies in clusters is a threshold process that occurs when the density exceeds similar to 500 times the critical density of the Universe. We interpret our results in the context of the "harassment" scenario, where faint early- type cluster galaxies are predicted to be the descendants of tidally-stripped late-type galaxies.