Formation and evolution of dwarf elliptical galaxies

Free access article

Issue		A&A Volume 438, Number 2, August I 2005


Page(s)		491 - 505
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20042213

A&A 438, 491-505 (2005)
DOI: 10.1051/0004-6361:20042213

I. Structural and kinematical properties

S. De Rijcke¹, D. Michielsen¹, H. Dejonghe¹, W. W. Zeilinger² and G. K. T. Hau³

¹ Sterrenkundig Observatorium, Ghent University, Krijgslaan 281, S9, 9000 Gent, Belgium
e-mail: sven.derijcke@UGent.be
² Institut für Astronomie, Universität Wien, Türkenschanzstraße 17, 1180 Wien, Austria
³ Department of Physics, South Road, Durham, DH1 3LE, UK

(Received 19 October 2004 / Accepted 15 February 2005 )

Abstract
This paper is the first in a series in which we present the results of an ESO Large Program on the kinematics and internal dynamics of dwarf elliptical galaxies (dEs). We obtained deep major and minor axis spectra of 15 dEs and broad-band imaging of 22 dEs. Here, we investigate the relations between the parameters that quantify the structure (B-band luminosity L_B, half-light radius $R_{\rm e}$ , and mean surface brightness within the half-light radius $I_{\rm e} = L_B / 2 \pi R_{\rm e}^2$ ) and internal dynamics (velocity dispersion $\sigma$ ) of dEs. We confront predictions of the currently popular theories for dE formation and evolution with the observed position of dEs in $\log L_B$ vs. $\log \sigma$ , $\log L_B$ vs. $\log R_{\rm e}$ , $\log L_B$ vs. $\log I_{\rm e}$ , and $\log R_{\rm e}$ vs. $\log I_{\rm e}$ diagrams and in the ( $\log \sigma,\log R_{\rm e},\log I_{\rm e}$ ) parameter space in which bright and intermediate-luminosity elliptical galaxies and bulges of spirals define a Fundamental Plane (FP). In order to achieve statistical significance and to cover a parameter interval that is large enough for reliable inferences to be made, we merge the data set presented in this paper with two other recently published, equally large data sets.

We show that the dE sequences in the various univariate diagrams are disjunct from those traced by bright and intermediate-luminosity elliptical galaxies and bulges of spirals. It appears that semi-analytical models (SAMs) that incorporate quiescent star formation with an essentially z-independent star-formation efficiency, combined with post-merger starbursts and the dynamical response after supernova-driven gas-loss, are able to reproduce the position of the dEs in the various univariate diagrams. SAMs with star-formation efficiencies that rise as a function of redshift are excluded since they leave the observed sequences traced by dEs virtually unpopulated. dEs tend to lie above the FP and the FP residual declines as a function of luminosity. Again, models that take into account the response after supernova-driven mass-loss correctly predict the position of dEs in the ( $\log \sigma,\log R_{\rm e},\log I_{\rm e}$ ) parameter space as well as the trend of the FP residual as a function of luminosity.

While these findings are clearly a success for the hierarchical-merging picture of galaxy formation, they do not necessarily invalidate the alternative "harassment" scenario, which posits that dEs stem from perturbed and stripped late-type disk galaxies that entered clusters and groups of galaxies about 5 Gyr ago.

Key words: galaxies: dwarf -- galaxies: fundamental parameters -- galaxies: kinematics and dynamics -- galaxies: evolution -- galaxies: formation

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