8,303 research outputs found
Detection of Weak Gravitational Lensing by Large-scale Structure
We report a detection of the coherent distortion of faint galaxies arising
from gravitational lensing by foreground structures. This ``cosmic shear'' is
potentially the most direct measure of the mass power spectrum, as it is
unaffected by poorly-justified assumptions made concerning the biasing of the
distribution. Our detection is based on an initial imaging study of 14
separated 8' x 16' fields observed in good, homogeneous conditions with the
prime focus EEV CCD camera of the 4.2m William Herschel Telescope. We detect an
rms shear of 1.6% in 8' x 8' cells, with a significance of 3.4 sigma. We
carefully justify this detection by quantifying various systematic effects and
carrying out extensive simulations of the recovery of the shear signal from
artificial images defined according to measured instrument characteristics. We
also verify our detection by computing the cross-correlation between the shear
in adjacent cells. Including (gaussian) cosmic variance, we measure the shear
variance to be (0.016)^2 plus/minus (0.012)^2 plus/minus (0.006)^2, where these
1 sigma errors correspond to statistical and systematic uncertainties,
respectively. Our measurements are consistent with the predictions of
cluster-normalised CDM models (within 1 sigma) but a COBE-normalised SCDM model
is ruled out at the 3.0 sigma level. For the currently-favoured Lambda-CDM
model (with Omega_m = 0.3), our measurement provides a normalisation of the
mass power spectrum of sigma_8 = 1.5 plus/minus 0.5, fully consistent with that
derived from cluster abundances. Our result demonstrates that ground-based
telescopes can, with adequate care, be used to constrain the mass power
spectrum on various scales. The present results are limited mainly by cosmic
variance, which can be overcome in the near future with more observations.Comment: 17 LaTex pages, including 13 figures and 3 tables. Accepted for
publication in MNRAS, minor revisio
Cosmic Ray Rejection by Linear Filtering of Single Images
We present a convolution-based algorithm for finding cosmic rays in single
well-sampled astronomical images. The spatial filter used is the point spread
function (approximated by a Gaussian) minus a scaled delta function, and cosmic
rays are identified by thresholding the filtered image. This filter searches
for features with significant power at spatial frequencies too high for
legitimate objects. Noise properties of the filtered image are readily
calculated, which allows us to compute the probability of rejecting a pixel not
contaminated by a cosmic ray (the false alarm probability). We demonstrate that
the false alarm probability for a pixel containing object flux will never
exceed the corresponding probability for a blank sky pixel, provided we choose
the convolution kernel appropriately. This allows confident rejection of cosmic
rays superposed on real objects. Identification of multiple-pixel cosmic ray
hits can be enhanced by running the algorithm iteratively, replacing flagged
pixels with the background level at each iteration.Comment: Accepted for publication in PASP (May 2000 issue). An iraf script
implementing the algorithm is available from the author, or from
http://sol.stsci.edu/~rhoads/ . 16 pages including 3 figures. Uses AASTeX
aaspp4 styl
A Cepheid Distance to NGC 4603 in Centaurus
In an attempt to use Cepheid variables to determine the distance to the
Centaurus cluster, we have obtained images of NGC 4603 with the Hubble Space
Telescope on 9 epochs using WFPC2 and the F555W and F814W filters. This galaxy
has been suggested to lie within the ``Cen30'' portion of the cluster and is
the most distant object for which this method has been attempted. Previous
distance estimates for Cen30 have varied significantly and some have presented
disagreements with the peculiar velocity predicted from redshift surveys,
motivating this investigation. Using our observations, we have found 61
candidate Cepheid variable stars; however, a significant fraction of these
candidates are likely to be nonvariable stars whose magnitude measurement
errors happen to fit a Cepheid light curve of significant amplitude for some
choice of period and phase. Through a maximum likelihood technique, we
determine that we have observed 43 +/- 7 real Cepheids and that NGC 4603 has a
distance modulus of 32.61 +0.11/-0.10 (random, 1 sigma) +0.24/-0.25
(systematic, adding in quadrature), corresponding to a distance of 33.3 Mpc.
This is consistent with a number of recent estimates of the distance to NGC
4603 or Cen30 and implies a small peculiar velocity consistent with predictions
from the IRAS 1.2 Jy redshift survey if the galaxy lies in the foreground of
the cluster.Comment: Accepted for publication in the Astrophysical Journal. 17 pages with
17 embedded figures and 3 tables using emulateapj.sty. Additional figures and
images may be obtained from http://astro.berkeley.edu/~marc/n4603
Stellar mass functions of galaxies at 4<z<7 from an IRAC-selected sample in COSMOS/UltraVISTA: limits on the abundance of very massive galaxies
We build a Spitzer IRAC complete catalog of objects, obtained by
complementing the -band selected UltraVISTA catalog with objects
detected in IRAC only. With the aim of identifying massive (i.e.,
) galaxies at , we consider the systematic effects
on the measured photometric redshifts from the introduction of an old and dusty
SED template and from the introduction of a bayesian prior taking into account
the brightness of the objects, as well as the systematic effects from different
star formation histories (SFHs) and from nebular emission lines in the recovery
of stellar population parameters. We show that our results are most affected by
the bayesian luminosity prior, while nebular emission lines and SFHs only
introduce a small dispersion in the measurements. Specifically, the number of
galaxies ranges from 52 to 382 depending on the adopted configuration.
Using these results we investigate, for the first time, the evolution of the
massive end of the stellar mass functions (SMFs) at . Given the rarity
of very massive galaxies in the early universe, major contributions to the
total error budget come from cosmic variance and poisson noise. The SMF
obtained without the introduction of the bayesian luminosity prior does not
show any evolution from to , implying that massive
galaxies could already be present when the Universe was ~Gyr old.
However, the introduction of the bayesian luminosity prior reduces the number
of galaxies with best fit masses by 83%, implying
a rapid growth of very massive galaxies in the first 1.5 Gyr of cosmic history.
From the stellar-mass complete sample, we identify one candidate of a very
massive (), quiescent galaxy at , with
MIPS m detection suggesting the presence of a powerful obscured AGN.Comment: 23 pages, 18 figures. ApJ accepte
Improved background subtraction for the Sloan Digital Sky Survey images
We describe a procedure for background subtracting Sloan Digital Sky Survey
(SDSS) imaging that improves the resulting detection and photometry of large
galaxies on the sky. Within each SDSS drift scan run, we mask out detected
sources and then fit a smooth function to the variation of the sky background.
This procedure has been applied to all SDSS-III Data Release 8 images, and the
results are available as part of that data set. We have tested the effect of
our background subtraction on the photometry of large galaxies by inserting
fake galaxies into the raw pixels, reanalyzing the data, and measuring them
after background subtraction. Our technique results in no size-dependent bias
in galaxy fluxes up to half-light radii of 100 arcsec; in contrast, for
galaxies of that size the standard SDSS photometric catalog underestimates
fluxes by about 1.5 mag. Our results represent a substantial improvement over
the standard SDSS catalog results and should form the basis of any analysis of
nearby galaxies using the SDSS imaging data.Comment: accepted by the Astronomical Journa
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