Evolution of dwarf-elliptical galaxies

¹ Institut für Theoretische Physik und Astrophysik, Universität Kiel, Olshausenstr. 40, 24098 Kiel, Germany
² Institut für Astronomie, Universitäts-Sternwarte Wien, Türkenschanzstr. 17, 1180 Vienna, Austria e-mail: hensler@astro.univie.ac.at
³ Department of Astronomy, University of Wisconsin-Madison, 5534 Sterling Hall, 475 N. Charter St., Madison, WI 53706-1582, USA

Received: 14 March 2000
Accepted: 4 May 2004

Abstract

Here we present as an extension to already published models of massive spherical galaxies one-dimensional chemo-dynamical models of dwarf elliptical galaxies in the mass range between 10⁹ and 10¹⁰  and initial density fluctuation of 1σ and 3σ. Because of their vanishing angular momentum the models are restricted to spherical symmetry. Due to this limitation the dynamics of the different components consider only radial motions and also their interaction processes have only one degree of freedom. Therefore, the galaxy evolution is preferably determined by radial oscillatory phenomena caused by heating and cooling of the interstellar gas, which reenforce effects like starbursts. Nevertheless, the low gravitational binding energy of dwarf ellipticals can easily be exceeded by thermal and turbulent energy production in the interstellar medium, leading to gas expansion and even to galactic winds. Furthermore, gas phases as well as gas and stars can decouple dynamically during the galactic evolution. For comparison with observational signatures like stellar kinematics, radial densities and metallicity distributions of the different components, the chemo-dynamical treatment of galaxy evolution must take the multi-phase character of the interstellar medium and self-regulation of star formation at low gravitation into account. Since non-rotating low-mass galaxies with stochastic star-formation episodes are seen as dwarf ellipticals, the aim of this paper is to compare the 1d chemo-dynamical models with observed characteristics of this type of dwarf galaxies. Several features like stellar populations from separate formation episodes, low metallicities, significant mass loss, etc. can be reproduced by the models in a global manner. Quantitative disagreements between model predictions and observations provide insight into necessary improvements of subsequent chemo-dynamical models implying rotation, dark matter halos, and external effects by an intergalactic gas, like its pressure, gas infall and stripping of galactic gas. The results can be summarized as follows: 10⁹  models are characterised by a single star-formation event during the initial collapse with an active self-regulation leading to reexpansion and massive gas loss as well as remaining at very low metallicities; 10¹⁰  dwarf elliptical models become almost gas-free due to large initial star formation and a strong galactic wind. These as well as a   and 3σ model experience oscillatory but non-negligible later star-formation histories that can account for the observed intermediate-age stellar populations.