118 research outputs found
More evidence for hidden spiral and bar features in bright early-type dwarf galaxies
Following the discovery of spiral structure in IC3328 (Jerjen et al.~2000),
we present further evidence that a sizable fraction of bright early-type dwarfs
in the Virgo cluster are genuine disk galaxies, or are hosting a disk
component. Among a sample of 23 nucleated dwarf ellipticals and dS0s observed
with the Very Large Telescope in and , we found another four systems
exhibiting non-axisymmetric structures, such as a bar and/or spiral arms,
indicative of a disk (IC0783, IC3349, NGC4431, IC3468). Particularly remarkable
are the two-armed spiral pattern in IC0783 and the bar and trailing arms in
NGC4431. For both galaxies the disk nature has recently been confirmed by a
rotation velocity measurement (Simien & Prugniel 2002). Our photometric search
is based on a Fourier decomposition method and a specific version of unsharp
masking. Some ``early-type'' dwarfs in the Virgo cluster seem to be former
late-type galaxies which were transformed to early-type morphology, e.g. by
``harassment'', during their infall to the cluster, while maintaining part of
their disk structure.Comment: A&A accepte
Relating basic properties of bright early-type dwarf galaxies to their location in Abell 901/902
We present a study of the population of bright early-type dwarf galaxies in
the multiple-cluster system Abell 901/902. We use data from the STAGES survey
and COMBO-17 to investigate the relation between the color and structural
properties of the dwarfs and their location in the cluster. The definition of
the dwarf sample is based on the central surface brightness and includes
galaxies in the luminosity range -16 >= M_B >~-19 mag. Using a fit to the color
magnitude relation of the dwarfs, our sample is divided into a red and blue
subsample. We find a color-density relation in the projected radial
distribution of the dwarf sample: at the same luminosity dwarfs with redder
colors are located closer to the cluster centers than their bluer counterparts.
Furthermore, the redder dwarfs are on average more compact and rounder than the
bluer dwarfs. These findings are consistent with theoretical expectations
assuming that bright early-type dwarfs are the remnants of transformed
late-type disk galaxies involving processes such as ram pressure stripping and
galaxy harassment. This indicates that a considerable fraction of dwarf
elliptical galaxies in clusters are the results of transformation processes
related to interactions with their host cluster.Comment: 12 pages, 8 figures, accepted for publication in A&A, typo corrected
in abstrac
Morphological Transformations of Galaxies in the A901/02 Supercluster from STAGES
We present a study of galaxies in the Abell 901/902 Supercluster at z~0.165,
based on HST ACS F606W, COMBO-17, Spitzer 24um, XMM-Newton X-ray, and
gravitational lensing maps, as part of the STAGES survey. We characterize
galaxies with strong externally-triggered morphological distortions and normal
relatively undisturbed galaxies, using visual classification and quantitative
CAS parameters. We compare normal and distorted galaxies in terms of their
frequency, distribution within the cluster, star formation properties, and
relationship to dark matter (DM) or surface mass density, and intra-cluster
medium (ICM) density. We revisit the morphology density relation, which
postulates a higher fraction of early type galaxies in dense environments, by
considering separately galaxies with a low bulge-to-disk (B/D) ratio and a low
gas content as these two parameters may not be correlated in clusters. We
report here on our preliminary analysis.Comment: To appear in the ASP conference proceedings of the "Frank N. Bash
Symposium 2007: New Horizons in Astronomy", Eds. A. Frebel, J. Maund, J.
Shen, M. Siegel. 4 pages, 4 figure
Evolution and Impact of Bars over the Last Eight Billion Years: Early Results from GEMS
Bars drive the dynamical evolution of disk galaxies by redistributing mass
and angular momentum, and they are ubiquitous in present-day spirals. Early
studies of the Hubble Deep Field reported a dramatic decline in the rest-frame
optical bar fraction f_opt to below 5% at redshifts z>0.7, implying that disks
at these epochs are fundamentally different from present-day spirals. The GEMS
bar project, based on ~8300 galaxies with HST-based morphologies and accurate
redshifts over the range 0.2-1.1, aims at constraining the evolution and impact
of bars over the last 8 Gyr. We present early results indicating that f_opt
remains nearly constant at ~30% over the range z=0.2-1.1,corresponding to
lookback times of ~2.5-8 Gyr. The bars detected at z>0.6 are primarily strong
with ellipticities of 0.4-0.8. Remarkably, the bar fraction and range of bar
sizes observed at z>0.6 appear to be comparable to the values measured in the
local Universe for bars of corresponding strengths. Implications for bar
evolution models are discussed.Comment: Submitted June 25, 2004. 10 pages 5 figures. To appear in Penetrating
Bars through Masks of Cosmic Dust: The Hubble Tuning Fork Strikes a New Note,
eds. D. Block, K. Freeman, R. Groess, I. Puerari, & E.K. Block (Dordrecht:
Kluwer), in pres
Exploring the Impact of Galaxy Interactions over Seven Billion Years with CAS
We explore galaxy assembly over the last seven billion years by
characterizing "normal" galaxies along the Hubble sequence, against strongly
disturbed merging/interacting galaxies with the widely used CAS system of
concentration (C), asymmetry (A), and 'clumpiness' (S) parameters, as well as
visual classification. We analyze Hubble Space Telescope (HST) ACS images of
~4000 intermediate and high mass (> 10^9 solar masses) galaxies from the GEMS
survey, one of the largest HST surveys conducted to date in two filters. We
explore the effectiveness of the CAS criteria [A>S and A>~0.35] in separating
normal and strongly disturbed galaxies at different redshifts, and quantify the
recovery and contamination rate. We also compare the average star formation
rate and the cosmic star formation rate density as a function of redshift
between normal and interacting systems identified by CAS.Comment: ASP conference proceedings of 2007 Bash Symposium. Latex with
asp2006.sty. 4 pages, 4 figure
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
Obscured star formation in intermediate-density environments:A Spitzer study of the Abell 901/902 supercluster
We explore the amount of obscured star formation as a function of environment in the Abell 901/902 (A901/902) supercluster at z = 0.165 in conjunction with a field sample drawn from the A901 and CDFS fields, imaged with the Hubble Space Telescope as part of the Space Telescope A901/902 Galaxy Evolution Survey and Galaxy Evolution from Morphology and Spectral Energy Distributions (SEDs) Survey. We combine the combo-17 near-UV/optical SED with Spitzer 24 mu m photometry to estimate both the unobscured and obscured star formation in galaxies with M-* > 10(10) M-circle dot. We find that the star formation activity in massive galaxies is suppressed in dense environments, in agreement with previous studies. Yet, nearly 40% of the star-forming (SF) galaxies have red optical colors at intermediate and high densities. These red systems are not starbursting; they have star formation rates (SFRs) per unit stellar mass similar to or lower than blue SF galaxies. More than half of the red SF galaxies have low infrared-to-ultraviolet (IR-to-UV) luminosity ratios, relatively high Sersicindices, and they are equally abundant at all densities. They might be gradually quenching their star formation, possibly but not necessarily under the influence of gas-removing environmental processes. The other greater than or similar to 40% of the red SF galaxies have high IR-to-UV luminosity ratios, indicative of high dust obscuration. They have relatively high specific SFRs and are more abundant at intermediate densities. Our results indicate that while there is an overall suppression in the SF galaxy fraction with density, the small amount of star formation surviving the cluster environment is to a large extent obscured, suggesting that environmental interactions trigger a phase of obscured star formation, before complete quenching
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