The origin of the -M_B and Kormendy relations in dwarf elliptical galaxies

¹ Laboratoire d'Astrophysique de Marseille, UMR 6110 CNRS, 38 rue F. Joliot-Curie, 13388 Marseille France e-mail: [Alessandro.Boselli;Samuel.Boissier]@oamp.fr
² School of Physics and Astronomy, Cardiff University, 5, The Parade, Cardiff CF24 3YB, UK e-mail: Luca.Cortese@astro.cf.ac.uk
³ Universita degli Studi di Milano-Bicocca, Piazza delle Scienze 3, 20126 Milano, Italy e-mail: Giuseppe.Gavazzi@mib.infn.it

Received: 8 February 2008
Accepted: 8 July 2008

Abstract

Aims. The present work is aimed at studying the distribution of galaxies of different types and luminosities along different structural scaling relations to see whether massive and dwarf ellipticals have been shaped by the same formation process.

Methods. This exercise is done by comparing the distribution of Virgo cluster massive and dwarf ellipticals and star forming galaxies along the B band effective surface brightness and effective radius vs. absolute magnitude relations and the Kormendy relation to the predictions of models tracing the effects of ram-pressure stripping on disc galaxies entering the cluster environment and galaxy harassment.

Results. Dwarf ellipticals might have been formed from low luminosity, late-type spirals that recently entered into the cluster and lost their gas because of a ram-pressure stripping event, stopping their star formation. The perturbations induced by the abrupt decrease of the star formation activity are sufficient to modify the structural properties of disc galaxies into those of dwarf ellipticals. Galaxy harassment induces a truncation of the disc and generally an increase of the effective surface brightness of the perturbed galaxies. The lack of dynamical simulations of perturbed galaxies spanning a wide range in luminosity prevents us from deriving any firm conclusion on a possible harassment-induced origin of the low surface brightness dwarf elliptical galaxy population inhabiting the Virgo cluster.

Conclusions. Although the observed scaling relations are consistent with the idea that the distribution of elliptical galaxies along the mentioned scaling relation is due to a gradual variation with luminosity of the Sersic index n, the comparison with models indicates that dwarf ellipticals might have been formed by a totally different process to giant ellipticals.