244 research outputs found
A search for gravitational lensing in 38 X-ray selected clusters of galaxies
We present the results of a CCD imaging survey for gravitational lensing in a
sample of 38 X-ray-selected clusters of galaxies. Our sample consists of the
most X-ray luminous (Lx>= 2x10^{44} erg s^{-1}) clusters selected from the
Einstein Observatory Extended Medium Sensitivity Survey (EMSS) that are
observable from Mauna Kea (dec > -40deg). The sample spans a redshift range of
0.15 0.5. CCD images of the
clusters were obtained in excellent seeing. There is evidence of strong
gravitational lensing in the form of giant arcs (length l > 8'', axis ratio l/w
> 10) in 8 of the 38 clusters. Two additional clusters contain shorter arclets,
and 6 more clusters contain candidate arcs that require follow-up observations
to confirm their lensing origin. Since the survey does not have a uniform
surface brightness limit we do not draw any conclusion based on the statistics
of the arcs found. We note, however, that 60% (3 of 5) of the clusters with Lx
> 10^{45} erg s^{-1}, and none of the 15 clusters with Lx < 4x10^{44} erg
s^{-1} contain giant arcs, thereby confirming that high X-ray luminosity does
identify the most massive systems, and thus X-ray selection is the preferred
method for finding true, rich clusters at intermediate and high redshifts. The
observed geometry of the arcs, most of which are thin, have large axis ratios
(l/w > 10), and are aligned orthogonal to the optical major axes of the
clusters, indicate the cluster core mass density profiles must be compact
(steeper than isothermal). In several cases, however, there is also some
evidence, in the form of possible radial arcs, for density profiles with finite
core radii.Comment: Latex file, 17 pages, 7 jpeg figures, to be published in Astronomy
and Astrophysics Supplement
Mass and Light in the Universe
We present a weak lensing and photometric study of six half by half degree
fields observed at the CFHT using the UH8K CCD mosaic camera. The fields were
observed for a total of 2 hours each in I and V, resulting in catalogs
containing ~ 20 000 galaxies per passband per field. We use V-I color and I
magnitude to select bright early type galaxies at redshifts 0.1 < z < 0.9. We
measure the gravitational shear from faint galaxies in the range 21 < m_I < 25
from a composite catalog and find a strong correlation with that predicted from
the early types if they trace the mass with mass-to-light ratio 300\pm75 h (in
solar units) for a flat (Omega_m0 = 0.3, Omega_l0 = 0.7) lambda cosmology and
400\pm100 h for Einstein-de Sitter. We make two-dimensional reconstructions of
the mass surface density. Cross-correlation of the measured mass surface
density with that predicted from the early type galaxy distribution shows a
strong peak at zero lag (significant at the 5.2-sigma level). We azimuthally
average the cross- and auto-correlation functions. We conclude that the
profiles are consistent with early type galaxies tracing mass on scales of > 45
arcsec (> 200 kpc at z = 0.5). We sub-divide our bright early type galaxies by
redshift and obtain similar conclusions. These mass-to-light ratios imply
\Omega_m0 = 0.10\pm0.02 (\Omega_m0 = 0.13\pm0.03 for Einstein-de Sitter) of
closure density.Comment: 27 pages, 19 figs (4 ps, 15 gif), 4 tables, accepted for publication
in Ap.J. (email Gillian for better resolution ps versions of gif greyscale
plots
Code-Aligned Autoencoders for Unsupervised Change Detection in Multimodal Remote Sensing Images
Image translation with convolutional autoencoders has recently been used as an approach to multimodal change detection (CD) in bitemporal satellite images. A main challenge is the alignment of the code spaces by reducing the contribution of change pixels to the learning of the translation function. Many existing approaches train the networks by exploiting supervised information of the change areas, which, however, is not always available. We propose to extract relational pixel information captured by domain-specific affinity matrices at the input and use this to enforce alignment of the code spaces and reduce the impact of change pixels on the learning objective. A change prior is derived in an unsupervised fashion from pixel pair affinities that are comparable across domains. To achieve code space alignment, we enforce pixels with similar affinity relations in the input domains to be correlated also in code space. We demonstrate the utility of this procedure in combination with cycle consistency. The proposed approach is compared with the state-of-the-art machine learning and deep learning algorithms. Experiments conducted on four real and representative datasets show the effectiveness of our methodology
A New Strategy for Deep Wide-Field High Resolution Optical Imaging
We propose a new strategy for obtaining enhanced resolution (FWHM = 0.12
arcsec) deep optical images over a wide field of view. As is well known, this
type of image quality can be obtained in principle simply by fast guiding on a
small (D = 1.5m) telescope at a good site, but only for target objects which
lie within a limited angular distance of a suitably bright guide star. For high
altitude turbulence this 'isokinetic angle' is approximately 1 arcminute. With
a 1 degree field say one would need to track and correct the motions of
thousands of isokinetic patches, yet there are typically too few sufficiently
bright guide stars to provide the necessary guiding information. Our proposed
solution to these problems has two novel features. The first is to use
orthogonal transfer charge-coupled device (OTCCD) technology to effectively
implement a wide field 'rubber focal plane' detector composed of an array of
cells which can be guided independently. The second is to combine measured
motions of a set of guide stars made with an array of telescopes to provide the
extra information needed to fully determine the deflection field. We discuss
the performance, feasibility and design constraints on a system which would
provide the collecting area equivalent to a single 9m telescope, a 1 degree
square field and 0.12 arcsec FWHM image quality.Comment: 46 pages, 22 figures, submitted to PASP, a version with higher
resolution images and other supplementary material can be found at
http://www.ifa.hawaii.edu/~kaiser/wfhr
Dark energy constraints from cosmic shear power spectra: impact of intrinsic alignments on photometric redshift requirements
Cosmic shear constrains cosmology by exploiting the apparent alignments of
pairs of galaxies due to gravitational lensing by intervening mass clumps.
However galaxies may become (intrinsically) aligned with each other, and with
nearby mass clumps, during their formation. This effect needs to be
disentangled from the cosmic shear signal to place constraints on cosmology. We
use the linear intrinsic alignment model as a base and compare it to an
alternative model and data. If intrinsic alignments are ignored then the dark
energy equation of state is biased by ~50 per cent. We examine how the number
of tomographic redshift bins affects uncertainties on cosmological parameters
and find that when intrinsic alignments are included two or more times as many
bins are required to obtain 80 per cent of the available information. We
investigate how the degradation in the dark energy figure of merit depends on
the photometric redshift scatter. Previous studies have shown that lensing does
not place stringent requirements on the photometric redshift uncertainty, so
long as the uncertainty is well known. However, if intrinsic alignments are
included the requirements become a factor of three tighter. These results are
quite insensitive to the fraction of catastrophic outliers, assuming that this
fraction is well known. We show the effect of uncertainties in photometric
redshift bias and scatter. Finally we quantify how priors on the intrinsic
alignment model would improve dark energy constraints.Comment: 14 pages and 9 figures. Replaced with final version accepted in
"Gravitational Lensing" Focus Issue of the New Journal of Physics at
http://www.iop.org/EJ/abstract/1367-2630/9/12/E0
Gravitational lensing due to dark matter modelled by vector field
The specified constant 4-vector field reproducing the spherically symmetric
stationary metric of cold dark matter halo in the region of flat rotation
curves results in a constant angle of light deflection at small impact
distances. The effective deflecting mass is factor greater than the
dark matter mass. The perturbation of deflection picture due to the halo edge
is evaluated.Comment: 17 pages, LaTeX iopart class, 10 eps figures; explanaitions and
discussion are extended and improved, reference added; version to appear in
Classical and Quantum Gravit
Transients from initial conditions based on Lagrangian perturbation theory in N-body simulations
We explore the initial conditions for cosmological N-body simulations
suitable for calculating the skewness and kurtosis of the density field. In
general, the initial conditions based on the perturbation theory (PT) provide
incorrect second-order and higher-order growth. These errors implied by the use
of the perturbation theory to set up the initial conditions in N-body
simulations are called transients. Unless these transients are completely
suppressed compared with the dominant growing mode, we can not reproduce the
correct evolution of cumulants with orders higher than two, even though there
is no problem with the numerical scheme. We investigate the impact of
transients on the observable statistical quantities by performing -body
simulations with initial conditions based on Lagrangian perturbation theory
(LPT). We show that the effects of transients on the kurtosis from the initial
conditions, based on second-order Lagrangian perturbation theory (2LPT) have
almost disappeared by , as long as the initial conditions are set at . This means that for practical purposes, the initial conditions based on
2LPT are accurate enough for numerical calculations of skewness and kurtosis.Comment: 21 pages, 5 figures; accepted for publication in JCA
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