Evolution of Field Dwarf Galaxies with GEMS

We present a study of the evolution of late-type field dwarfs over the last ~ 1.9 Gyr, based on HST ACS observations carried out as part of the GEMS survey. This study is amongst the first to probe the evolution of dwarfs over such a large timescale. The comparison of structural properties, particularly size and scale length, indicates that the dwarfs in the redshift range z ~ 0.01 to 0.15 (look-back times up to 1.9 Gyr) are more extended than local dwarfs. We argue that this difference is due to the star formation activity becoming more centrally concentrated in late-type dwarfs over the last ~ 1.9 Gyr. We discuss several possible causes for this evolution. We also find a lack of blue compact dwarfs in the GEMS sample and interpret this as indicative of the fact that strong, centrally concentrated star formation is a feature of evolved dwarfs that are entering their final stages of evolution.Comment: 4 pages, to appear in the proceedings of IAUC198: Near-Field Cosmology with Dwarf Elliptical Galaxies, Helmut Jerjen and Bruno Binggeli, ed

Constraints on Bars in the Local Universe from 5000 SDSS Galaxies

We present the first study of bars in the local Universe, based on the Sloan Digitized Sky Survey (SDSS). The large sample of ~5000 local galaxies provides the largest study to date of local bars and minimizes the effect of cosmic variance. The sample galaxies have M_g<=-18.5 mag and cover the redshift range 0.01<=z<0.04. We use a color cut in the color-magnitude diagram and the Sersic index n to identify disk galaxies. We characterize bars and disks using r-band images and the method of iterative ellipse fits and quantitative criteria developed in Jogee at al. (2004, ApJL, 615, L105). After excluding highly inclined (i>60 degrees) systems our results are: (1) the optical (r-band) fraction of barred galaxies among local disk galaxies is 43%, which confirms the ubiquity of local bars, in agreement with other optical studies based on smaller samples (e.g.Eskridge et al. 2000, AJ, 119, 536, Marinova & Jogee 2006, astro-ph/0608039); (2) the optical bar fraction rises for bluer galaxies, suggesting a relation between bars and star formation; (3) preliminary analyzes suggest that the optical bar fraction increases steeply with the galaxy effective radius; (4) the optical bar fraction at z~0 is ~35% for bright disks (M_g0.4), large-scale (bar semi-major axis >1.5 kpc) bars, which is comparable to the value of 30+/-6% reported earlier (Jogee et al. 2004) for similar disks and bars at z~0.2-1.0.Comment: 1 page, Proceedings of the IAU Symposium No. 235, 2006, "Galaxy Evolution across the Hubble Time", F. Combes & J. Palous, ed

Bars in Disk-Dominated and Bulge-Dominated Galaxies at z~0: New Insights from ~3600 SDSS Galaxies

We present a study of large-scale bars in the local Universe, based on a large sample of ~3692 galaxies, with -18.5 <= M_g < -22.0 mag and redshift 0.01 <= z < 0.03, drawn from the Sloan Digitized Sky Survey. Our sample includes many galaxies that are disk-dominated and of late Hubble types. Both color cuts and S\'ersic cuts yield a similar sample of ~2000 disk galaxies. We characterize bars and disks by ellipse-fitting r-band images and applying quantitative criteria. After excluding highly inclined ($>60^{\circ}$) systems, we find the following results. (1) The optical r-band fraction (f_opt-r) of barred galaxies, when averaged over the whole sample, is ~48%-52%. (2) When galaxies are separated according to half light radius (r_e), or normalized r_e/R_24, which is a measure of the bulge-to-disk (B/D) ratio, a remarkable result is seen: f_opt-r rises sharply, from ~40% in galaxies that have small r_e/R_24 and visually appear to host prominent bulges, to ~70% for galaxies that have large r_e/R_24 and appear disk-dominated. (3) $f_{\rm opt-r}$ rises for galaxies with bluer colors (by ~30%) and lower masses (by ~15%-20%). (4) While hierarchical $\Lambda$CDM models of galaxy evolution models fail to produce galaxies without classical bulges, our study finds that ~20% of disk galaxies appear to be ``quasi-bulgeless''. (5) After applying the same cutoffs in magnitude (M_V= 1.5 kpc), and bar ellipticity (e_bar >=~0.4) that studies out to z~1 apply to ensure a complete sample, adequate spatial resolution, and reliable bar identification, we obtain an optical r-band bar fraction of 34%. This is comparable to the value reported at z~0.2-1.0, implying that the optical bar fraction does not decline dramatically by an order of magnitude in bright galaxies out to z~1. (abridged)Comment: accepted for publication in ApJ, 50 pages, 20 figure

VLT surface photometry and isophotal analysis of early-type dwarf galaxies in the Virgo cluster

We have carried out surface photometry and an isophotal analysis for a sample of 25 early-type dwarf (dE and dS0) galaxies in the Virgo cluster based on CCD images taken at the VLT with FORS1 and FORS2. For each galaxy we present $B$ and $R$-band surface brightness profiles, as well as the radial colour ($B-R$) profile. We give total apparent $BR$ magnitudes, effective radii, effective surface brightnesses and total colour indices. The light profiles have been fitted with S\'ersic models and the corresponding parameters are compared to the ones for other classes of objects. The observed profiles of the brightest cluster dwarfs show significant deviations from a simple S\'ersic model, indicating that there is more inner structure than just a nucleus. In addition, we find a relation between the effective surface brightness, at a given luminosity, and the strength of the offset of the galaxy's nucleus with respect to the center of the isophotes. Dwarfs with large nuclear offsets also tend to have stronger isophotal twists. In sum, our findings suggest the presence of substructure in most, and preferentially in the less compact, bright early-type dwarfs. The physical (dynamical) meaning of this has yet to be explored. (abridged)Comment: 29 pages, accepted for publication in A&

Optically-passive spirals: The missing link in gradual star formation suppression upon cluster infall

Galaxies migrate from the blue cloud to the red sequence when their star formation is quenched. Here, we report on galaxies quenched by environmental effects and not by mergers or strong AGN as often invoked: They form stars at a reduced rate which is optically even less conspicuous, and manifest a transition population of blue spirals evolving into S0 galaxies. These 'optically passive' or 'red spirals' are found in large numbers in the STAGES project (and by Galaxy Zoo) in the infall region of clusters and groups.Comment: Proceedings of "The Starburst-AGN connection" conference held in Shanghai, Oct 27-31, 200