304 research outputs found
Cosmic-Ray Rejection by Laplacian Edge Detection
Conventional algorithms for rejecting cosmic-rays in single CCD exposures
rely on the contrast between cosmic-rays and their surroundings, and may
produce erroneous results if the Point Spread Function (PSF) is smaller than
the largest cosmic-rays. This paper describes a robust algorithm for cosmic-ray
rejection, based on a variation of Laplacian edge detection. The algorithm
identifies cosmic-rays of arbitrary shapes and sizes by the sharpness of their
edges, and reliably discriminates between poorly sampled point sources and
cosmic-rays. Examples of its performance are given for spectroscopic and
imaging data, including HST WFPC2 images.Comment: Accepted for publication in the PASP (November 2001 issue). The
algorithm is implemented in the program L.A.Cosmic, which can be obtained
from http://www.astro.caltech.edu/~pgd/lacosmic
Keck Spectroscopy of distant GOODS Spheroidal Galaxies: Downsizing in a Hierarchical Universe
We analyze the evolution of the Fundamental Plane for 141 field spheroidal
galaxies in the redshift range 0.2<z<1.2, selected morphologically to a
magnitude limit F850LP=22.43 in the northern field of the Great Observatories
Origin Survey. For massive galaxies we find that the bulk of the star formation
was completed prior to z=2. However, for the lower mass galaxies, the
luminosity-weighted ages are significantly younger. The differential change in
mass-to-light ratio correlates closely with rest-frame color, consistent with
recent star formation and associated growth. Our data are consistent with mass
rather than environment governing the overall growth, contrary to the
expectations of hierarchical assembly. We discuss how feedback, conduction, and
galaxy interactions may explain the downsizing trends seen within our large
sample.Comment: ApJ Letters, in press. 4 figure
Revealing velocity dispersion as the best indicator of a galaxy's color, compared to stellar mass, surface mass density or morphology
Using data of nearby galaxies from the Sloan Digital Sky Survey we
investigate whether stellar mass, central velocity dispersion, surface mass
density, or the Sersic n parameter is best correlated with a galaxy's
rest-frame color. Specifically, we determine how the mean color of galaxies
varies with one parameter when another is fixed. When the stellar mass is fixed
we see that strong trends remain with all other parameters, whereas residual
trends are weaker when surface mass density, n, or velocity dispersion are
fixed. Overall velocity dispersion is the best indicator of a galaxy's typical
color, showing the largest residual color dependence when any of the other
three parameters are fixed, and stellar mass is the poorest. Other studies have
indicated that both the halo and black hole properties are better correlated
with velocity dispersion than with stellar mass, surface mass density or Sersic
n. Therefore, our results are consistent with a picture where a galaxy's star
formation history and present star formation rate are determined to some
significant degree by the current properties and assembly history of its dark
matter halo and/or the feedback from its central super massive black hole.Comment: 7 pages, 5 figures, submitted to ApJ Letter
Infall, the Butcher-Oemler Effect, and the Descendants of Blue Cluster Galaxies at z~0.6
Using wide-field HST/WFPC2 imaging and extensive Keck/LRIS spectroscopy, we
present a detailed study of the galaxy populations in MS2053--04, a massive,
X-ray luminous cluster at z=0.5866. Analysis of 149 confirmed cluster members
shows that MS2053 is composed of two structures that are gravitationally bound
to each other; their respective velocity dispersions are 865 km/s (113 members)
and 282 km/s (36 members). MS2053's total dynamical mass is 1.2x10^15 Msun.
MS2053 is a classic Butcher-Oemler cluster with a high fraction of blue members
(24%) and an even higher fraction of star-forming members (44%), as determined
from their [OII] emission. The number fraction of blue/star-forming galaxies is
much higher in the infalling structure than in the main cluster. This result is
the most direct evidence to date that the Butcher-Oemler effect is linked to
galaxy infall. In terms of their colors, luminosities, estimated internal
velocity dispersions, and [OII] equivalent widths, the infalling galaxies are
indistinguishable from the field population. MS2053's deficit of S0 galaxies
combined with its overabundance of blue spirals implies that many of these
late-types will evolve into S0 members. The properties of the blue cluster
members in both the main cluster and infalling structure indicate they will
evolve into low mass, L<L* galaxies with extended star formation histories like
that of low mass S0's in Coma. Our observations show that most of MS2053's blue
cluster members, and ultimately most of its low mass S0's, originate in the
field. Finally, we measure the redshift of the giant arc in MS2053 to be
z=3.1462; this object is one in only a small set of known strongly lensed
galaxies at z>3.Comment: Accepted by ApJ. Version with full resolution figures available at
http://www.exp-astro.phys.ethz.ch/tran/outgoing/ms2053.ps.g
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