Early-type dwarf (dE) galaxies play a key role in understanding galaxy cluster evolution. Their low mass and large number make them ideal probes of the mechanisms that can alter the appearance of galaxies: internal processes as well as environmental influence through the high-density cluster region. The popular belief that dEs were formed at late epochs by environmentally-caused transformations of spiral and irregular galaxies is contrasted with the formation of dEs in models of a Lambda-CDM universe, as the descendants of cosmological building blocks and in that sense close relatives to their giant counterparts. Both options appear plausible in a parameter space of basic galaxy characteristics (see above figure), illustrating the complex task of tracing back the origin of dEs.
Our group has been active in studying most observational aspects of dEs, proving their heterogeneity: We introduced subclasses of dEs that differ systematically by their shape, rotational support, stellar population characteristics, and clustering properties. We found that ages and metallicities correlate with subclass type, galaxy mass, environmental density, and probably even orbital charateristics. Due to the moderate sample sizes so far, it remains uncertain which of these properties are directly caused by the local environment, and which are signatures of different formation processes.
The Virgo cluster is an ideal laboratory to study dEs: it contains hundreds of them, is close enough to resolve their detailed structure, and is a dynamically young cluster that is still evolving today. It thus allows us to conduct a comprehensive investigation of how the heterogeneous present-day population of quiescent dwarfs formed. To achieve a systematic spectro-photometric and kinematical characterization of Virgo dEs, we have embarked on an ambitious campaign (SMAKCED) to obtain spectra and near-infrared (NIR) images for a sample that is complete within -16 > Mr > -19 mag. This brightness range is ideal, since all dE subclasses coexist, allowing us to investigate in detail the complexity of this most abundant galaxy population in high-density environments.
The NIR images of SMAKCED characterize stellar mass and the old stellar population's morphology, providing the missing cornerstone to a comprehensive multi-wavelength coverage of Virgo galaxies. Ongoing or completed surveys are NGVS (optical), HeViCS (far-IR), ALFALFA+AGES (HI), G(alex)UVics (PI Boselli), and ACSVCS (high-res. optical). The spectra of SMAKCED will allow an analysis of the internal kinematics and the stellar population characteristics. Beyond analyzing the dEs themselves, this unique combination of datasets allows us to use the ubiquitous and susceptible dwarf galaxies as local laboratories for environmental influences on galaxy evolution.