8,310 research outputs found
Double lenses
The analysis of the shear induced by a single cluster on the images of a
large number of background galaxies is all centered around the curl-free
character of a well-known vector field that can be derived from the data. Such
basic property breaks down when the source galaxies happen to be observed
through two clusters at different redshifts, partially aligned along the line
of sight. In this paper we address the study of double lenses and obtain five
main results. (i) First we generalize the procedure to extract the available
information, contained in the observed shear field, from the case of a single
lens to that of a double lens. (ii) Then we evaluate the possibility of
detecting the signature of double lensing given the known properties of the
distribution of clusters of galaxies. (iii) As a different astrophysical
application, we demonstrate how the method can be used to detect the presence
of a dark cluster that might happen to be partially aligned with a bright
cluster studied in terms of statistical lensing. (iv) In addition, we show that
the redshift distribution of the source galaxies, which in principle might also
contribute to break the curl-free character of the shear field, actually
produces systematic effects typically two orders of magnitude smaller than the
double lensing effects we are focusing on. (v) Remarkably, a discussion of
relevant contributions to the noise of the shear measurement has brought up an
intrinsic limitation of weak lensing analyses, since one specific contribution,
associated with the presence of a non-vanishing two-galaxy correlation
function, turns out not to decrease with the density of source galaxies (and
thus with the depth of the observations).Comment: 40 pages, 15 figures. Accepted for publication in ApJ main journa
Quiet Planting in the Locked Constraint Satisfaction Problems
We study the planted ensemble of locked constraint satisfaction problems. We
describe the connection between the random and planted ensembles. The use of
the cavity method is combined with arguments from reconstruction on trees and
first and second moment considerations; in particular the connection with the
reconstruction on trees appears to be crucial. Our main result is the location
of the hard region in the planted ensemble. In a part of that hard region
instances have with high probability a single satisfying assignment.Comment: 21 pages, revised versio
The Effects of Massive Substructures on Image Multiplicities in Gravitati onal Lenses
Surveys for gravitational lens systems have typically found a significantly
larger fraction of lenses with four (or more) images than are predicted by
standard ellipsoidal lens models (50% versus 25-30%). We show that including
the effects of smaller satellite galaxies, with an abundance normalized by the
observations, significantly increases the expected number of systems with more
than two images and largely explains the discrepancy. The effect is dominated
by satellites with ~20% the luminosity of the primary lens, in rough agreement
with the typical luminosities of the observed satellites. We find that the lens
systems with satellites cannot, however, be dropped from estimates of the
cosmological model based on gravitational lens statistics without significantly
biasing the results.Comment: 23 pages, 7 figures, more discussion of sis vs sie and inclusion of
uncorrelated contribution
Effect of Peculiar Motion in Weak Lensing
We study the effect of peculiar motion in weak gravitational lensing. We
derive a fully relativistic formula for the cosmic shear and the convergence in
a perturbed Friedmann Universe. We find a new contribution related to galaxies
peculiar velocity. This contribution does not affect cosmic shear in a
measurable way, since it is of second order in the velocity. However, its
effect on the convergence (and consequently on the magnification, which is a
measurable quantity) is important, especially for redshifts z < 1. As a
consequence, peculiar motion modifies also the relation between the shear and
the convergence.Comment: 11 pages, 7 figures; v2: discussion on the reduced shear added (5.C),
additional references, version accepted in PRD; v3: mistakes corrected in
eqs. (26), (31), (33) and (44); results unchange
Distance-Redshift in Inhomogeneous Friedmann-Lemaitre-Robertson-Walker Cosmology
Distance--redshift relations are given in terms of associated Legendre
functions for partially filled beam observations inspatially flat
Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmologies. These models are
dynamically pressure-free, flat FLRW on large scales but, due to mass
inhomogeneities, differ in their optical properties. The partially filled beam
area-redshift equation is a Lame equation for arbitrary FLRW and is
shown to simplify to the associated Legendre equation for the spatially flat,
i.e. case. We fit these new analytic Hubble curves to recent
supernovae (SNe) data in an attempt to determine both the mass parameter
and the beam filling parameter . We find that current data are
inadequate to limit . However, we are able to estimate what limits are
possible when the number of observed SNe is increased by factor of 10 or 100,
sample sizes achievable in the near future with the proposed SuperNova
Acceleration Probe satellite.Comment: 9 pages, 3 figure
Weak Lensing Analysis of the z~0.8 cluster CL 0152-1357 with the Advanced Camera for Surveys
We present a weak lensing analysis of the X-ray luminous cluster CL 0152-1357
at z~0.84 using HST/ACS observations. The unparalleled resolution and
sensitivity of ACS enable us to measure weakly distorted, faint background
galaxies to the extent that the number density reaches ~175 arcmin^-2. The PSF
of ACS has a complicated shape that also varies across the field. We construct
a PSF model for ACS from an extensive investigation of 47 Tuc stars in a
modestly crowded region. We show that this model PSF excellently describes the
PSF variation pattern in the cluster observation when a slight adjustment of
ellipticity is applied. The high number density of source galaxies and the
accurate removal of the PSF effect through moment-based deconvolution allow us
to restore the dark matter distribution of the cluster in great detail. The
direct comparison of the mass map with the X-ray morphology from Chandra
observations shows that the two peaks of intracluster medium traced by X-ray
emission are lagging behind the corresponding dark matter clumps, indicative of
an on-going merger. The overall mass profile of the cluster can be well
described by an NFW profile with a scale radius of r_s =309+-45 kpc and a
concentration parameter of c=3.7+-0.5. The mass estimates from the lensing
analysis are consistent with those from X-ray and Sunyaev-Zeldovich analyses.
The predicted velocity dispersion is also in good agreement with the
spectroscopic measurement from VLT observations. In the adopted WMAP cosmology,
the total projected mass and the mass-to-light ratio within 1 Mpc are estimated
to be 4.92+-0.44 10^14 solar mass and 95+-8 solar mass/solar luminosity,
respectively.Comment: Accepted for publication in Astrophysical Journal. 58 pages, 26
figures. Figures have been degraded to meet size limit; a higher resolution
version available at http://acs.pha.jhu.edu/~mkjee/ms_cl0152.pd
The GALATEA Test-Facility for High Purity Germanium Detectors
GALATEA is a test facility designed to investigate bulk and surface effects
in high purity germanium detectors. A vacuum tank houses an infrared screened
volume with a cooled detector inside. A system of three stages allows an almost
complete scan of the detector. The main feature of GALATEA is that there is no
material between source and detector. This allows the usage of alpha and beta
sources as well as of a laser beam to study surface effects. A 19-fold
segmented true-coaxial germanium detector was used for commissioning
Large-Scale QSO-Galaxy Correlations and Weak Lensing
Several recent studies show that bright, intermediate and high redshift
optically and radio selected QSOs are positively correlated with nearby
galaxies on a range of angular scales up to a degree. Obscuration by unevenly
distributed Galactic dust can be ruled out as the cause, leaving weak
statistical lensing as the physical process responsible. However the amplitude
of correlations on < 1 degree scales is at least a factor of a few larger than
lensing model predictions. A possible way to reconcile the observations and
theory is to revise the weak lensing formalism. We extend the standard lensing
formulation to include the next higher order term (second order) in the
geodesic equation of motion for photons. We derive relevant equations
applicable in the weak lensing regime, and discuss qualitative properties of
the updated formulation. We then perform numerical integrations of the revised
equation and study the effect of the extra term using two different types of
cosmic mass density fluctuations. We find that nearby large-scale coherent
structures increase the amplitude of the predicted lensing-induced correlations
between QSOs and foreground galaxies by ~ 10% (not a factor of several required
by observations), while the redshift of the optimal, i.e. `most correlated'
structures is moved closer to the observer compared to what is predicted using
the standard lensing equation.Comment: extended Section 2; 20 pages, including 4 figures, accepted to Ap
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