161 research outputs found
Observations of Stripped Edge-on Virgo Cluster Galaxies
We present observations of highly inclined, HI deficient, Virgo cluster
spiral galaxies. Our high-resolution VLA HI observations of edge-on galaxies
allow us to distinguish extraplanar gas from disk gas. All of our galaxies have
truncated H-alpha disks, with little or no disk gas beyond a truncation radius.
While all the gas disks are truncated, the observations show evidence for a
continuum of stripping states: symmetric, undisturbed truncated gas disks
indicate galaxies that were stripped long ago, while more asymmetric disks
suggest ongoing or more recent stripping. We compare these timescale estimates
with results obtained from two-dimensional stellar spectroscopy of the outer
disks of galaxies in our sample. One of the galaxies in our sample, NGC 4522 is
a clear example of active ram-pressure stripping, with 40% of its detected HI
being extraplanar. As expected, the outer disk stellar populations of this
galaxy show clear signs of recent (and, in fact, ongoing) stripping. Somewhat
less expected, however, is the fact that the spectrum of the outer disk of this
galaxy, with very strong Balmer absorption and no observable emission, would be
classified as ``k+a'' if observed at higher redshift. Our observations of NGC
4522 and other galaxies at a range of cluster radii allow us to better
understand the role that clusters play in the structure and evolution of disk
galaxies.Comment: 4 pages, 2 figures, to appear in the proceedings of the Island
Universes conference held in Terschelling, Netherlands, July 2005, ed. R. de
Jong, version with high resolution figures can be downloaded from
ftp://ftp.astro.yale.edu/pub/hugh/papers/iu_crowl_h.ps.g
Demonstrating Diversity in Star Formation Histories with the CSI Survey
We present coarse but robust star formation histories (SFHs) derived from
spectro-photometric data of the Carnegie-Spitzer-IMACS Survey, for 22,494
galaxies at 0.3<z<0.9 with stellar masses of 10^9 Msun to 10^12 Msun. Our study
moves beyond "average" SFHs and distribution functions of specific star
formation rates (sSFRs) to individually measured SFHs for tens of thousands of
galaxies. By comparing star formation rates (SFRs) with timescales of 10^10,
10^9, and 10^8 years, we find a wide diversity of SFHs: 'old galaxies' that
formed most or all of their stars early; galaxies that formed stars with
declining or constant SFRs over a Hubble time, and genuinely 'young galaxies'
that formed most of their stars since z=1. This sequence is one of decreasing
stellar mass, but, remarkably, each type is found over a mass range of a factor
of 10. Conversely, galaxies at any given mass follow a wide range of SFHs,
leading us to conclude that: (1) halo mass does not uniquely determine SFHs;
(2) there is no 'typical' evolutionary track; and (3) "abundance matching" has
limitations as a tool for inferring physics. Our observations imply that SFHs
are set at an early epoch, and that--for most galaxies--the decline and
cessation of star formation occurs over a Hubble-time, without distinct
"quenching" events. SFH diversity is inconsistent with models where galaxy
mass, at any given epoch, grows simply along relations between SFR and stellar
mass, but is consistent with a 2-parameter lognormal form, lending credence to
this model from a new and independent perspective.Comment: 17 pages, 10 figures; accepted by ApJ; version 2 - no substantive
changes; clarifications and correction
Evolution since z = 0.5 of the Morphology-Density relation for Clusters of Galaxies
Using traditional morphological classifications of galaxies in 10
intermediate-redshift (z~0.5) clusters observed with WFPC-2 on the Hubble Space
Telescope, we derive relations between morphology and local galaxy density
similar to that found by Dressler for low-redshift clusters. Taken
collectively, the `morphology-density' relationship, M-D, for these more
distant, presumably younger clusters is qualitatively similar to that found for
the local sample, but a detailed comparison shows two substantial differences:
(1) For the clusters in our sample, the M-D relation is strong in centrally
concentrated ``regular'' clusters, those with a strong correlation of radius
and surface density, but nearly absent for clusters that are less concentrated
and irregular, in contrast to the situation for low redshift clusters where a
strong relation has been found for both. (2) In every cluster the fraction of
elliptical galaxies is as large or larger than in low-redshift clusters, but
the S0 fraction is 2-3 times smaller, with a proportional increase of the
spiral fraction. Straightforward, though probably not unique, interpretations
of these observations are (1) morphological segregation proceeds
hierarchically, affecting richer, denser groups of galaxies earlier, and (2)
the formation of elliptical galaxies predates the formation of rich clusters,
and occurs instead in the loose-group phase or even earlier, but S0's are
generated in large numbers only after cluster virialization.Comment: 35 pages, 19 figures, uses psfig. Accepted for publication in Ap
The bright galaxy population of five medium redshift clusters. I. Color-Magnitude Relation, Blue Fractions and Visual Morphology
Using data of five clusters of galaxies within the redshift range 0.15
z 0.25, imaged with the Nordic Optical Telescope (NOT) in the central
1 Mpc in very good seeing conditions, we have performed an
exhaustive inspection of their bright galaxy population. That range of
redshift, where only a small amount of data with the required resolution and
quality is available, is particularly important for the understanding of the
formation and evolution of clusters of galaxies. We have inspected the
color-magnitude relation (CMR) for those clusters and measured the blue
fraction of galaxies in their cores to check for evidence of evolution as found
in other works. Moreover, the visual classification of the galaxy morphology
has been performed and the morphology-radius relation has been examined We have
not found signs of evolution neither in the slope of the CMR nor in the blue
fraction of galaxies. A diversity of situations regarding those parameters and
in the morphological mixing has been noticed, with two out of five clusters
containing a dominant late-type core population. The cluster A1878 stands out
as some of its properties differ from those of the other clusters in the
sample.
No clear signs of evolution appear in our analysis. The data support the view
that the morphology and the stellar content of the galaxies in our clusters
have been already settled at z 0.2. Only the fraction of interacting
galaxies in the clusters appear to be larger than in clusters like Coma
although the number of clusters in the sample is small to give a definitive
conclusion.Comment: 16 pages, 8 figures, 4 tables, 1 longtable The paper has been already
accepted but still not publishe
The Influence of Environment on the Star Formation Rates of Galaxies
We have used a sample of 15749 galaxies taken from the Las Campanas Redshift
Survey to investigate the effects of environment on the rate of star formation
(SFR) in galaxies. The size and homogeneity of this data set allows us to
sample, for the first time, the entire range of galactic environment, from the
voids to the clusters, in a uniform manner, thus, we could decouple the local
galaxy density from the membership in associations. This decoupling is very
crucial for constraining the physical processes responsible for the
environmental dependencies of SFR. On the other hand, the use of an
automatically-measured concentration index (C), rather than Hubble type, allows
us to cleanly separate the morphological component from the SFR vs. environment
relationship. We find that cluster galaxies exhibit lower SFR for the same C
than field galaxies, while a further division of clusters by `richness' reveals
a new possible excitation of `starbursts' in poor clusters. Meanwhile, a more
general environmental investigation reveals that the SFR of a given C shows a
continuous correlation with the local density. Interestingly, this trend is
also observed both inside and outside of clusters, implying that physical
processes responsible for this correlation might not be intrinsic to the
cluster environment. On the other hand, galaxies with differing levels of SFR
appear to respond differently to the local density. Low levels of SFR are more
sensitive to environment inside than outside of clusters. In contrast, high
levels of SFR, identified as ``starbursts'', are as sensitive to local density
in the field as in clusters. We conclude that at least two separate processes
are responsible for the environmental sensitivity of the SFR.Comment: 25 pages, 10 figures, submitted to Ap
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