We present and analyse the photometric properties of a nearly complete sample of blue compact dwarf (BCD) and irregular galaxies in the Virgo cluster from multi-band SDSS images. Our study intends to shed light on the ongoing debate of whether a stru
ctural evolution from present-day star-forming dwarf galaxies in a cluster environment into ordinary early-type dwarf galaxies is possible based on the structural properties. For this purpose, we decompose the surface brightness profiles of the BCDs into the luminosity contribution of the starburst component and that of their underlying low surface brightness (LSB) host. The latter dominates the stellar mass of the BCD. We find that the LSB-components of the Virgo BCDs are structurally compatible with the more compact half of the Virgo early-type dwarfs, except for a few extreme BCDs. Thus, after termination of starburst activity, the BCDs will presumably fade into galaxies that are structurally similar to ordinary early-type dwarfs. In contrast, the irregulars are more diffuse than the BCDs and are structurally similar to the more diffuse half of the Virgo early-type dwarfs. Therefore, the present-day Virgo irregulars are not simply non-starbursting BCDs. If starbursts in cluster BCDs are transient phenomena with a duration of ~100 Myr or less, during which the galaxies could not travel more than ~100 kpc, then a substantial number of non-starbursting counterparts of these systems must populate the same spatial volume, namely the Virgo cluster outskirts. The majority of them would have to be early-type dwarfs, based on the abundance of different galaxy types with similar colours and structural parameters to the LSB-components of the BCDs. However, most Virgo BCDs have redder LSB-host colours and a less prominent starburst than typical field BCDs, preventing a robust conclusion on possible oscillations between BCDs and early-type dwarfs.
How did the dwarf galaxy population of present-day galaxy clusters form and grow over time? We address this question by analysing the history of dark matter subhaloes in the Millennium-II cosmological simulation. A semi-analytic model serves as the l
ink to observations. We argue that a reasonable analogue to early morphological types or red-sequence dwarf galaxies are those subhaloes that experienced strong mass loss, or alternatively those that have spent a long time in massive haloes. This approach reproduces well the observed morphology-distance relation of dwarf galaxies in the Virgo and Coma clusters, and thus provides insight into their history. Over their lifetime, present-day late types have experienced an amount of environmental influence similar to what the progenitors of dwarf ellipticals had already experienced at redshifts above two. Therefore, dwarf ellipticals are more likely to be a result of early and continuous environmental influence in group- and cluster-size haloes, rather than a recent transformation product. The observed morphological sequences of late-type and early-type galaxies have developed in parallel, not consecutively. Consequently, the characteristics of todays late-type galaxies are not necessarily representative for the progenitors of todays dwarf ellipticals. Studies aiming to reproduce the present-day dwarf population thus need to start at early epochs, model the influence of various environments, and also take into account the evolution of the environments themselves.
We test how well available stellar population models can reproduce observed u,g,r,i,z-band photometry of the local galaxy population (0.02<=z<=0.03) as probed by the SDSS. Our study is conducted from the perspective of a user of the models, who has o
bservational data in hand and seeks to convert them into physical quantities. Stellar population models for galaxies are created by synthesizing star formations histories and chemical enrichments using single stellar populations from several groups (Starburst99, GALAXEV, Maraston2005, GALEV). The role of dust is addressed through a simplistic, but observationally motivated, dust model that couples the amplitude of the extinction to the star formation history, metallicity and the viewing angle. Moreover, the influence of emission lines is considered (for the subset of models for which this component is included). The performance of the models is investigated by: 1) comparing their prediction with the observed galaxy population in the SDSS using the (u-g)-(r-i) and (g-r)-(i-z) color planes, 2) comparing predicted stellar mass and luminosity weighted ages and metallicities, specific star formation rates, mass to light ratios and total extinctions with literature values from studies based on spectroscopy. Strong differences between the various models are seen, with several models occupying regions in the color-color diagrams where no galaxies are observed. We would therefore like to emphasize the importance of the choice of model. Using our preferred model we find that the star formation history, metallicity and also dust content can be constrained over a large part of the parameter space through the use of u,g,r,i,z-band photometry. However, strong local degeneracies are present due to overlap of models with high and low extinction in certain parts of color space.
In the light of the question whether most early-type dwarf (dE) galaxies in clusters formed through infall and transformation of late-type progenitors, we search for an imprint of such an infall history in the oldest, most centrally concentrated dE s
ubclass of the Virgo cluster: the nucleated dEs that show no signatures of disks or central residual star formation. We select dEs in a (projected) region around the central elliptical galaxies, and subdivide them by their line-of-sight velocity into fast-moving and slow-moving ones. These subsamples turn out to have significantly different shapes: while the fast dEs are relatively flat objects, the slow dEs are nearly round. Likewise, when subdividing the central dEs by their projected axial ratio into flat and round ones, their distributions of line-of-sight velocities differ significantly: the flat dEs have a broad, possibly two-peaked distribution, whereas the round dEs show a narrow single peak. We conclude that the round dEs probably are on circularized orbits, while the flat dEs are still on more eccentric or radial orbits typical for an infalling population. In this picture, the round dEs would have resided in the cluster already for a long time, or would even be a cluster-born species, explaining their nearly circular orbits. They would thus be the first generation of Virgo cluster dEs. Their shape could be caused by dynamical heating through repeated tidal interactions. Further investigations through stellar population measurements and studies of simulated galaxy clusters would be desirable to obtain definite conclusions on their origin.
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. Several distinct dE subclasses were identified, which show significant differences in their shape, stellar content, and distribution within the cluster. Does this diversity imply that dEs originate from various formation channels? Does cosmological formation play a role as well? I try to touch on these questions in this brief overview of dEs in galaxy clusters.
Various early-type dwarf galaxies with disk features were identified in the Virgo cluster, including objects that display weak grand-design spiral arms despite being devoid of gas. Are these still related to the classical dEs, or are they a continuat
ion of ordinary spiral galaxies? Kinematical information of acceptable quality is available for one of these galaxies, IC 3328. We perform an investigation of its dynamical configuration, taking into account the effect of asymmetric drift, and using the Toomre parameter as well as density wave considerations. The derived mass-to-light ratios and rotational velocities indicate the presence of a significant dynamically hot component in addition to the disk. However, unambiguous conclusions need to await the availability of further data for this and other early-type dwarfs with spiral structure.
The Gini coefficient, a non-parametric measure of galaxy morphology, has recently taken up an important role in the automated identification of galaxy mergers. I present a critical assessment of its stability, based on a comparison of HST/ACS imaging
data from the GOODS and UDF surveys. Below a certain signal-to-noise level, the Gini coefficient depends strongly on the signal-to-noise ratio, and thus becomes useless for distinguishing different galaxy morphologies. Moreover, at all signal-to-noise levels the Gini coefficient shows a strong dependence on the choice of aperture within which it is measured. Consequently, quantitative selection criteria involving the Gini coefficient, such as a selection of merger candidates, cannot always be straightforwardly applied to different datasets. I discuss whether these effects could have affected previous studies that were based on the Gini coefficient, and establish signal-to-noise limits above which measured Gini values can be considered reliable.
The flux excess of elliptical galaxies in the far-ultraviolet can be reproduced by population synthesis models when accounting for the population of old hot helium-burning subdwarf stars. This has been achieved by Han and coworkers through a quantita
tive model of binary stellar evolution. Here, we compare the resulting evolutionary population synthesis model to the GALEX far-near ultraviolet colors (FUV-NUV) of Virgo cluster early-type galaxies that were published by Boselli and coworkers. FUV-NUV is reddest at about the dividing luminosity of dwarf and giant galaxies, and becomes increasingly blue for both brighter and fainter luminosities. This behavior can be easily explained by the binary model with a continuous sequence of longer duration and later truncation of star formation at lower galaxy masses. Thus, in contrast to previous conclusions, the GALEX data do not require a dichotomy between the stellar population properties of dwarfs and giants. Their apparently opposite behavior in FUV-NUV occurs naturally when the formation of hot subdwarfs through binary evolution is taken into account.
We present an analysis of the optical colors of 413 Virgo cluster early-type dwarf galaxies (dEs), based on Sloan Digital Sky Survey imaging data. Our study comprises (1) a comparison of the color-magnitude relation (CMR) of the different dE subclass
es that we identified in Paper III of this series, (2) a comparison of the shape of the CMR in low and high-density regions, (3) an analysis of the scatter of the CMR, and (4) an interpretation of the observed colors with ages and metallicities from population synthesis models. We find that the CMRs of nucleated (dE(N)) and non-nucleated dEs (dE(nN)) are significantly different from each other, with similar colors at fainter magnitudes (r > 17 mag), but increasingly redder colors of the dE(N)s at brighter magnitudes. We interpret this with older ages and/or higher metallicities of the brighter dE(N)s. The dEs with disk features have similar colors as the dE(N)s and seem to be only slightly younger and/or less metal-rich on average. Furthermore, we find a small but significant dependence of the CMR on local projected galaxy number density, consistently seen in all of u-r, g-r, and g-i, and weakly i-z. We deduce that a significant intrinsic color scatter of the CMR is present, even when allowing for a distance spread of our galaxies. No increase of the CMR scatter at fainter magnitudes is observed down to r = 17 mag (Mr = -14 mag). The color residuals, i.e., the offsets of the data points from the linear fit to the CMR, are clearly correlated with each other in all colors for the dE(N)s and for the full dE sample. We conclude that there must be at least two different formation channels for early-type dwarfs in order to explain the heterogeneity of this class of galaxy. (Abridged)
