RASS-SDSS Galaxy cluster survey

IV. A ubiquitous dwarf galaxy population in clusters

¹ Max-Planck-Institut fur extraterrestrische Physik, 85748 Garching, Germany e-mail: ppopesso@eso.org
² INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131, Trieste, Italy
³ European Southern Observatory, Karl Scharzschild Strasse 2, 85748, Germany

Received: 1 March 2005
Accepted: 8 June 2005

Abstract

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 r₂₀₀ or r₅₀₀, 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  colors. The late-type LF is well fitted by a single Schechter function with a steep slope ( in the r band, within r₂₀₀). 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 ∼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.