Contributions of our group to the JENAM 2008 conference
(Vienna, Sep 08-12)

J. Janz, T. Lisker
A continuum of structure and stellar content from Virgo cluster early-type dwarfs to giants?
Based on the wealth of multiwavelength imaging data from the SDSS, we investigate whether dwarf and giant early-type galaxies in the Virgo cluster follow a continuum in their structural parameters and their stellar population characteristics. Despite the apparent continuity of their intensity profiles, dwarfs and giants do not seem to form a homogeneous family with respect to galaxy size. Moreover, while some of their colors (g-r, g-i) appear consistent with following a common color-magnitude relation, such a continuity is not observed in the age-sensitive u-r and the metallicity-sensitive i-z color. We discuss these findings in the light of the different formation mechanisms commonly assumed for dwarf and giant early types, thereby taking into account the existence of several distinct early-type dwarf subclasses.

T. Lisker, E.K. Grebel, B. Binggeli
Early-type dwarf galaxies: a mixed bag with various origins
The formation of early-type dwarf (dE) galaxies, the most numerous objects in clusters, is believed to be closely connected to the physical processes that drive galaxy cluster evolution, like galaxy harassment and ram-pressure stripping. However, the actual significance of each mechanism for building the observed cluster dE population is yet unknown. We have thus embarked on a comprehensive study of more than 400 dEs in the Virgo cluster, based primarily on SDSS multicolor data. Several distinct early-type dwarf subclasses can be identified, which show significant differences in their shape, stellar content, and distribution within the cluster. If this diversity of early-type dwarfs implies that they originate from various formation channels, can we tell which ones went through which formation process? Is it necessary, and even feasible, to explain the formation of all subclasses with recent infall and structural transformation of progenitor galaxies - or were many dEs already born within the (proto-)cluster environment?

S. Paudel, T. Lisker
Stellar populations of early-type dwarf galaxies and their nuclei
Dwarf galaxies are the numerically dominating population in the dense regions of the universe. Although they seem to be simple systems at first view, the stellar populations of dwarf elliptical galaxy might be fairly complex. Nucleated dEs are of particular interest, since a number of objects exhibit different stellar populations in their nuclei and host galaxy. We present optical spectra of five nucleated dE galaxies and their nuclei. After subtracting the scaled galaxy spectra from the flux calibrated nucleus spectra, we compared them with one another and try to explore their stellar population age using Lick indices. As a preliminary result we find that the stellar population age of the nuclei are more scattered than the galaxies. Finally, the nature of spectra and their scatter in age will be explained.

H.T. Meyer, R. Kotulla, P. Papaderos, Y. Izotov, K. Fricke
Revisiting the Luminosity–Metallicity relation for star-forming dwarf galaxies with SDSS
Understanding the chemical enrichment of galaxies is one of the central goals of extragalactic astronomy. This is especially true for dwarf galaxies as candidates for building-blocks of today's normal galaxies like the Milky Way. A close investigation of the luminosity-metallicity (L-Z) relation for dwarf galaxies is undoubtedly of paramount importance.
Here, we report on an ongoing investigation of the L-Z relation in star-forming dwarf galaxies (SFDGs), on the basis of a photometric analysis of a large sample of dwarf galaxies compiled from the Sloan Digital Sky Survey (SDSS). Our sample consists of 300 nearby (distance < 50 Mpc) SFDGs spanning a wide range in their absolute g magnitude, between -13 and -18 mag. In all sample SFDGs the [OIII]4363 Å, in connection with the [OII]7319,7330 Å emission lines were detected, allowing for a precise determination of the oxygen abundance. Our sample spans a metallicity between 1/3 and 1/50 solar, thus it is well suited for the study of the L-Z relation over a large baseline in metallicity.
An important and novel aspect of this ongoing programme is that we model and subtract the emission from the star-forming (SF) component in order to isolate the light from the old low-surface brightness (LSB) host of the sample galaxies. For this purpose we use surface photometry techniques and profile decomposition in the g and i SDSS bands. The SDSS with its large and homogenous imaging data opens up a new route to explore the much debated L-Z relation for SFDGs.
Currently, the derivation and decomposition of the surface brightness profiles are completed and the full details of our study will be published in Meyer (2008) and Meyer et al. (2008), in prep.

T. Lisker, E.K. Grebel, B. Binggeli
On the nature and origin of early-type dwarf galaxies with spiral structure
A significant fraction of Virgo cluster early-type dwarf (dE) galaxies exhibit disk substructure, like spiral arms and bars. Are these disks the still visible remnants of late-type progenitor galaxies that harassment transformed into dEs? Simulations of harassment predict tidal debris arcs around the remnants, as well as a distorted outer structure. We aimed at detecting such signatures by obtaining extremely deep images of dEs with spiral structure, in order to confirm or reject the significance of harassment for their formation. Here, we present several of these images, along with a preliminary analysis.

T. Lisker
On the stability of the Gini coefficient as a measure of galaxy structure
The Gini coefficient, a non-parametric measure of galaxy morphology, is frequently used for automated galaxy classification, and in particular for the identification of major galaxy mergers. From a comparison of HST/ACS imaging data from the GOODS and UDF surveys, I find that the Gini coefficient depends strongly on the signal-to-noise ratio, as well as on the choice of aperture within which it is measured. Therefore, selections of merger candidates, as well as other quantitative selection criteria involving the Gini coefficient, cannot be blindly transferred from one dataset to another. I show how signal-to-noise limits should be chosen to guarantee the reliability of measured Gini values in high-redshift galaxy samples.