1,225 research outputs found
Gravitational Lensing in Clusters of Galaxies
Gravitational lensing in clusters of galaxies is an efficient tool to probe
the mass distribution of galaxies and clusters, high redshift objects thanks to
the gravitational amplification, and the geometry of the universe. We review
some important aspects of cluster lensing and related issues in observational
cosmology.Comment: invited review of the journal: Progress of Theoretical Physics (in
press) 51 pages - 33 figure
A Test of the Collisional Dark Matter Hypothesis from Cluster Lensing
Spergel & Steinhardt proposed the possibility that the dark matter particles
are self-interacting, as a solution to two discrepancies between the
predictions of cold dark matter models and the observations: first, the
observed dark matter distribution in some dwarf galaxies has large,
constant-density cores, as opposed to the predicted central cusps; and second,
small satellites of normal galaxies are much less abundant than predicted. The
dark matter self-interaction would produce isothermal cores in halos, and would
also expel the dark matter particles from dwarfs orbiting within large halos.
However, another inevitable consequence of the model is that halos should
become spherical once most particles have interacted. Here, I rule out this
model by the fact that the innermost regions of dark matter halos in massive
clusters of galaxies are elliptical, as shown by gravitational lensing and
other observations. The absence of collisions in the lensing cores of massive
clusters implies that any dark matter self-interaction is too weak to have
affected the observed density profiles in the dark-matter dominated dwarf
galaxies, or to have eased the destruction of dwarf satellites in galactic
halos. If is the cross section and the mass of the dark matter
particle, then s_x/m_x < 10^{-25.5} \cm^2/\gev.Comment: to appear in ApJ, January 1 200
Galaxy Galaxy Lensing as a Probe of Galaxy Dark Matter Halos
Gravitational lensing has now become a popular tool to measure the mass
distribution of structures in the Universe on various scales. Here we focus on
the study of galaxy's scale dark matter halos with galaxy-galaxy lensing
techniques: observing the shapes of distant background galaxies which have been
lensed by foreground galaxies allows us to map the mass distribution of the
foreground galaxies. The lensing effect is small compared to the intrinsic
ellipticity distribution of galaxies, thus a statistical approach is needed to
derive some constraints on an average lens population. An advantage of this
method is that it provides a probe of the gravitational potential of the halos
of galaxies out to very large radii, where few classical methods are viable,
since dynamical and hydrodynamical tracers of the potential cannot be found at
this radii. We will begin by reviewing the detections of galaxy-galaxy lensing
obtained so far. Next we will present a maximum likelihood analysis of
simulated data we performed to evaluate the accuracy and robustness of
constraints that can be obtained on galaxy halo properties. Then we will apply
this method to study the properties of galaxies which stand in massive cluster
lenses at z~0.2. The main result of this work is to find dark matter halos of
cluster galaxies to be significantly more compact compared to dark matter halos
around field galaxies of equivalent luminosity, in agreement with early
galaxy-galaxy lensing studies and with theoretical expectations, in particular
with the tidal stripping scenario. We thus provide a strong confirmation of
tidal truncation from a homogeneous sample of galaxy clusters. Moreover, it is
the first time that cluster galaxies are probed successfully using
galaxy-galaxy lensing techniques from ground based data.Comment: 8 pages, 5 figures, to appear in Moriond Proceedings, From Dark Halos
to Ligh
Deep Convolutional Neural Networks as strong gravitational lens detectors
Future large-scale surveys with high resolution imaging will provide us with
a few new strong galaxy-scale lenses. These strong lensing systems
however will be contained in large data amounts which are beyond the capacity
of human experts to visually classify in a unbiased way. We present a new
strong gravitational lens finder based on convolutional neural networks (CNNs).
The method was applied to the Strong Lensing challenge organised by the Bologna
Lens Factory. It achieved first and third place respectively on the space-based
data-set and the ground-based data-set. The goal was to find a fully automated
lens finder for ground-based and space-based surveys which minimizes human
inspect. We compare the results of our CNN architecture and three new
variations ("invariant" "views" and "residual") on the simulated data of the
challenge. Each method has been trained separately 5 times on 17 000 simulated
images, cross-validated using 3 000 images and then applied to a 100 000 image
test set. We used two different metrics for evaluation, the area under the
receiver operating characteristic curve (AUC) score and the recall with no
false positive (). For ground based data our
best method achieved an AUC score of and a
of . For space-based data our best
method achieved an AUC score of and a
of . On space-based data adding dihedral invariance to the CNN
architecture diminished the overall score but achieved a higher no
contamination recall. We found that using committees of 5 CNNs produce the best
recall at zero contamination and consistenly score better AUC than a single
CNN. We found that for every variation of our CNN lensfinder, we achieve AUC
scores close to within .Comment: 9 pages, accepted to A&
Lensed galaxies in Abell 370 I. Modeling the number counts and redshift distribution of background sources
We present new observations of the cluster-lens Abell 370: a deep HST/WFPC2
F675W image and ESO 3.6m spectroscopy of faint galaxies. These observations
shade new lights on the statistical properties of faint lensed galaxies. In
particular, we spectroscopically confirm the multiple image nature of the
B2--B3 gravitational pair (Kneib et al. 1993), and determine a redshift of
z=0.806 which is in very good agreement with earlier predictions. A refined
mass model of the cluster core (that includes cluster galaxy halos) is
presented, based on a number of newly identified multiple images. Following
Bezecourt et al. (1998a), we combine the new cluster mass model with a
spectrophotometric prescription for galaxy evolution to predict the arclets
number counts and redshift distribution in the HST image. In particular, the
ellipticity distribution of background sources is taken into account, in order
to properly estimate the statistical number and redshift distribution of
arclets. We show that the redshift distribution of arclets, and particularly
its high redshift tail can be used as a strong constraint to disentangle
different galaxy evolution scenario. A hierarchical model which includes a
number density evolution is favored by our analysis. Finally, we compute the
depletion curves in the faint galaxies number counts and discuss its wavelength
dependence.Comment: 10 pages, Astronomy and Astrophysics in pres
HST Observations of the Gravitationally Lensed Cloverleaf Broad Absorption Line QSO H1413+1143: Modeling the Lens
We investigate gravitational lens models for the quadruply-lensed Cloverleaf
BAL QSO H1413+1143 based on the HST WFPC/WFPC2 astrometric and photometric data
of the system by Turnshek et al. and the HST NICMOS-2 data by Falco et al. The
accurate image positions and the dust-extinction-corrected relative
amplifications, along with a possible detection of the lensing galaxy in the
infrared, permit more accurate lens models than were previously possible. While
more recent models are qualitatively consistent with the HST data, none of the
previous models considered the dust-extinction-corrected relative
amplifications of the image components. We use the power-law elliptical mass
model to fit the HST data. We find that a single elliptical galaxy perturbed by
an external shear can fit the image positions within the observational
uncertainties; however, the predicted relative magnifications are only roughly
consistent with the observational relative amplifications. We find that a
primary galaxy combined with a secondary galaxy in the vicinity of the
Cloverleaf or a cluster centered (south-)west of the Cloverleaf can fit both
the image positions and relative amplifications within the observational
uncertainties. We discuss future observations which could be used to test
and/or further constrain lens models of the Cloverleaf.Comment: 23 pages (in aaspp.sty) including 5 tables and 3 figures, Accepted
for publication in the Astrophysical Journa
The Galaxy Octopole Moment as a Probe of Weak Lensing Shear Fields
In this paper, we introduce the octopole moment of the light distribution in
galaxies as a probe of the weak lensing shear field. While traditional
ellipticity estimates of the local shear derived from the quadrupole moment are
limited by the width of the intrinsic ellipticity distribution of background
galaxies, the dispersion in the intrinsic octopole distribution is expected to
be much smaller, implying that the signal from this higher order moment is
ultimately limited by measurement noise, and not by intrinsic scatter. We
present the computation of the octopole moment and show that current
observations are at the regime where the octopole estimates will soon be able
to contribute to the overall accuracy of the estimates of local shear fields.
Therefore, the prospects for this estimator from future datasets like the
Advanced Camera for Survey and the Next Generation Space Telescope are very
promising.Comment: 9 pages, 2 PostScript figures; Submitted to Astrophysical Journa
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