EDP Sciences
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Volume 445, Number 1, January I 2006
Page(s) 29 - 42
Section Cosmology (including clusters of galaxies)
DOI http://dx.doi.org/10.1051/0004-6361:20052954

A&A 445, 29-42 (2006)
DOI: 10.1051/0004-6361:20052954

RASS-SDSS Galaxy cluster survey

IV. A ubiquitous dwarf galaxy population in clusters
P. Popesso1, A. Biviano2, H. Böhringer1 and M. Romaniello3

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 )

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 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 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 $\sim $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.

Key words: galaxies: clusters: general -- galaxies: luminosity function, mass function

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© ESO 2005