Kinematics and stellar populations of low-luminosity early-type galaxies in the Abell 496 cluster ^*,**,***

¹ Observatoire de Paris-Meudon, LERMA, UMR 8112, 61 Av. de l'Observatoire, 75014 Paris, France e-mail: igor.chilingarian@obspm.fr
² Sternberg Astronomical Institute, Moscow State University, 13 Universitetski prospect, 119992 Moscow, Russia
³ Observatoire de Paris-Meudon, LUTH, UMR 8102, 5 pl. Jules Janssen, 92195 Meudon, France
⁴ Institut d'Astrophysique de Paris, CNRS, UMR 7095, Université Pierre et Marie Curie, 98bis Bd Arago, 75014 Paris, France
⁵ LAM, Pôle de l'Etoile Site de Château-Gombert, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁶ Observatoire de Paris-Meudon, GEPI, UMR 8111, 5 pl. Jules Janssen, 92195 Meudon, France
⁷ Instituto de Astronomia, Geofísica e C. Atmosf./USP, R. do Matão 1226, 05508-090 São Paulo/SP, Brazil
⁸ Université de Lyon, 69000 Lyon; Université Lyon 1, 69622 Villeurbanne; Centre de Recherche Astronomique de Lyon, Observatoire de Lyon, 9 Av. Charles André, 69561 St. Genis Laval; CNRS, UMR 5574, École Normale Supérieure de Lyon, Lyon, France

Received: 19 September 2007
Accepted: 12 March 2008

Abstract

Context. The morphology and stellar populations of low-luminosity early-type galaxies in clusters have until now been limited to a few relatively nearby clusters such as Virgo or Fornax. Scenarii for the formation and evolution of dwarf galaxies in clusters are therefore not well constrained.

Aims. We investigate here the morphology and stellar populations of low-luminosity galaxies in the relaxed richness class 1 cluster Abell 496 ().

Methods. Deep multiband imaging obtained with the CFHT Megacam allowed us to select a sample of faint galaxies, defined here as objects with magnitudes  mag within a 1.2 arcsec fibre ( mag). We observed 118 galaxies spectroscopically with the ESO VLT FLAMES/Giraffe spectrograph with a resolving power . We present structural analysis and colour maps for the 48 galaxies belonging to the cluster. We fit the spectra of 46 objects with PEGASE.HR synthetic spectra to estimate the ages, metallicities, and velocity dispersions. We estimated possible biases by similarly analysing spectra of ~1200 early-type galaxies from the Sloan Digital Sky Survey Data Release 6 (SDSS DR6). We computed values of α/Fe abundance ratios from the measurements of Lick indices. We briefly discuss effects of the fixed aperture size on the measurements.

Results. For the first time, high-precision estimates of stellar population properties have been obtained for a large sample of faint galaxies in a cluster, allowing for the extension of relations between stellar populations and internal kinematics to the low-velocity dispersion regime. We have revealed a peculiar population of elliptical galaxies in the core of the cluster, resembling massive early-type galaxies by their stellar population properties and velocity dispersions, but having luminosities of about 2 mag fainter.

Conclusions. External mechanisms of gas removal (ram pressure stripping and gravitational harassment) are more likely to have occurred than internal mechanisms such as supernova-driven winds. The violent tidal stripping of intermediate-luminosity, early-type galaxies in the cluster core can explain the properties of the peculiar elliptical galaxies surrounding the cD galaxy.