73 research outputs found
Extensive light profile fitting of galaxy-scale strong lenses
We investigate the merits of a massive forward modeling of ground-based
optical imaging as a diagnostic for the strong lensing nature of Early-Type
Galaxies, in the light of which blurred and faint Einstein rings can hide. We
simulate several thousand mock strong lenses under ground- and space-based
conditions as arising from the deflection of an exponential disk by a
foreground de Vaucouleurs light profile whose lensing potential is described by
a Singular Isothermal Ellipsoid. We then fit for the lensed light distribution
with sl_fit after having subtracted the foreground light emission off (ideal
case) and also after having fitted the deflector's light with galfit. By
setting thresholds in the output parameter space, we can decide the
lens/not-a-lens status of each system. We finally apply our strategy to a
sample of 517 lens candidates present in the CFHTLS data to test the
consistency of our selection approach. The efficiency of the fast modeling
method at recovering the main lens parameters like Einstein radius, total
magnification or total lensed flux, is quite comparable under CFHT and HST
conditions when the deflector is perfectly subtracted off (only possible in
simulations), fostering a sharp distinction between the good and the bad
candidates. Conversely, for a more realistic subtraction, a substantial
fraction of the lensed light is absorbed into the deflector's model, which
biases the subsequent fitting of the rings and then disturbs the selection
process. We quantify completeness and purity of the lens finding method in both
cases. This suggests that the main limitation currently resides in the
subtraction of the foreground light. Provided further enhancement of the
latter, the direct forward modeling of large numbers of galaxy-galaxy strong
lenses thus appears tractable and could constitute a competitive lens finder in
the next generation of wide-field imaging surveys.Comment: A&A accepted version, minor changes (13 pages, 10 figures
Constraints on MOND from the lensing cluster MS2137-23
The cluster of galaxies MS2137-23 presents the most constrained lensing configuration of gravitational images ever detected in a distant cluster of galaxies. With a set of two arc systems with known redshifts and many other arclets spread between ~30 h-1 kpc and 1 h-1 Mpc, MS2137-27 is well suited to probe its mass profile and to challenge theoretical expectations from CDM scenario with more speculative alternatives, like MOND models. In this note, I describe the recent lensing analysis I carried out on this cluster using together arcs and CHANDRA X-ray data. Even with this unique data set, I cannot reject neither an isothermal model nor an NFW model. The MOND model is not compatible with the observations, unless a significant fraction of the baryonic matter has not been detected by CHANDRA. The need for much more baryons in the MOND model than for the dark matter paradigm implies significant dynamical differences between these models which can be explored at very large radial distance. In particular, MOND lensing signal becomes isotropic beyond 1 h-1 Mpc, in contrast with collisionless DM halos which produce elliptical potentials. This difference can in principle be used to test MOND prescriptions, regardless the exact relation between matter and the gravitational deflection law assumed for MOND
RingFinder: automated detection of galaxy-scale gravitational lenses in ground-based multi-filter imaging data
We present RingFinder, a tool for finding galaxy-scale strong gravitational
lenses in multiband imaging data. By construction, the method is sensitive to
configurations involving a massive foreground early-type galaxy and a faint,
background, blue source. RingFinder detects the presence of blue residuals
embedded in an otherwise smooth red light distribution by difference imaging in
two bands. The method is automated for efficient application to current and
future surveys, having originally been designed for the 150-deg2 Canada France
Hawaii Telescope Legacy Survey (CFHTLS). We describe each of the steps of
RingFinder. We then carry out extensive simulations to assess completeness and
purity. For sources with magnification mu>4, RingFinder reaches 42% (resp. 25%)
completeness and 29% (resp. 86%) purity before (resp. after) visual inspection.
The completeness of RingFinder is substantially improved in the particular
range of Einstein radii 0.8 < REin < 2. and lensed images brighter than g =
22.5, where it can be as high as 70%. RingFinder does not introduce any
significant bias in the source or deflector population. We conclude by
presenting the final catalog of RingFinder CFHTLS galaxy-scale strong lens
candidates. Additional information obtained with Hubble Space Telescope and
Keck Adaptive Optics high resolution imaging, and with Keck and Very Large
Telescope spectroscopy, is used to assess the validity of our classification,
and measure the redshift of the foreground and the background objects. From an
initial sample of 640,000 early type galaxies, RingFinder returns 2500
candidates, which we further reduce by visual inspection to 330 candidates. We
confirm 33 new gravitational lenses from the main sample of candidates, plus an
additional 16 systems taken from earlier versions of RingFinder. First
applications are presented in the SL2S galaxy-scale Lens Sample paper series.Comment: 32 pages (aastex 2col format), 6 figs, ApJ Accepte
On the Complementarity of Different Cosmological Probes with SLACS, BELLS and SL2S Strong Gravitational Lensing Data
Accelerating expansion of the Universe is now an indisputable observational fact and became one of the most important issues of both physics and cosmology today, known as dark energy (DE) problem. The nature of
this phenomenon is still unknown and from observational point of view the only way to put some light on cosmic expansion history is to combine different methods which are alternative to each other. In this light, we explore
the idea that strong gravitational lensing systems offer new opportunity to constrain DE parameters in a way complementary to other cosmological probes. It turns out that the angle of the confidence contour major axis for
strong lensing measurements depends on the redshift of the sample what may help to break the degeneracy in the w0âwa parameters plane in the ChevalierâPolarskiâLinder parametrization of DE equation of state
Caustics in Dark Matter Haloes
Caustics are formally singular structures, with infinite density, that form
in collisionless media. The non-negligible velocity dispersion of dark matter
particles renders their density finite. We evaluate the maximum density of the
caustics within the framework of secondary infall model of formation of dark
matter haloes. The result is then used to demonstrate that caustics can be
probed by properly stacking the weak-lensing signal of about 600 haloes. CFHTLS
accompanied by X-ray observations and the space-based experiments like SNAP or
DUNE can provide us with the required statistics and hence a way of
distinguishing between the viable dark matter particle candidates. The
extension of our results to more realistic models including the effects of
mergers of haloes is briefly outlined.Comment: Minor changes, two references added, 6 pages, 3 figures, to appear in
Proc. 21st IAP Colloquium "Mass Profiles and Shapes of Cosmological
Structures", Paris 4-9 July 2005, [EAS Publications Series, eds: G. Mamon, F.
Combes, C. Deffayet, B. Fort
The SL2S Galaxy-scale Lens Sample. V. Dark Matter Halos and Stellar IMF of Massive Early-type Galaxies out to Redshift 0.8
We investigate the cosmic evolution of the internal structure of massive
early-type galaxies over half of the age of the Universe. We perform a joint
lensing and stellar dynamics analysis of a sample of 81 strong lenses from the
SL2S and SLACS surveys and combine the results with a hierarchical Bayesian
inference method to measure the distribution of dark matter mass and stellar
IMF across the population of massive early-type galaxies. Lensing selection
effects are taken into account. We find that the dark matter mass projected
within the inner 5 kpc increases for increasing redshift, decreases for
increasing stellar mass density, but is roughly constant along the evolutionary
tracks of early-type galaxies. The average dark matter slope is consistent with
that of an NFW profile, but is not well constrained. The stellar IMF
normalization is close to a Salpeter IMF at and scales
strongly with increasing stellar mass. No dependence of the IMF on redshift or
stellar mass density is detected. The anti-correlation between dark matter mass
and stellar mass density supports the idea of mergers being more frequent in
more massive dark matter halos.Comment: Accepted for publication on The Astrophysical Journal. Revised
version. (25 pages, 18 figures
A weak lensing study of the Coma cluster
Due to observational constraints, dark matter determinations in nearby
clusters based on weak lensing are still extremely rare, in spite of their
importance for the determination of cluster properties independent of other
methods. We present a weak lensing study of the Coma cluster (redshift 0.024)
based on deep images obtained at the CFHT. After obtaining photometric
redshifts for the galaxies in our field based on deep images in the u (1x1
deg2), and in the B, V, R and I bands (42'x52'), allowing us to eliminate
foreground galaxies, we apply weak lensing calculations on shape measurements
performed in the u image. We derive a map of the mass distribution in Coma, as
well as the radial shear profile, and the mass and concentration parameter at
various radii. We obtain M_200c = 5.1+4.3-2.1 x10^14 Msun and
c_200c=5.0+3.2-2.5, in good agreement with previous measurements. With deep
wide field images it is now possible to analyze nearby clusters with weak
lensing techniques, thus opening a broad new field of investigation
Intrinsic alignment of simulated galaxies in the cosmic web: implications for weak lensing surveys
The intrinsic alignment of galaxy shapes (by means of their angular momentum)
and their cross-correlation with the surrounding dark matter tidal field are
investigated using the 160 000, z=1.2 synthetic galaxies extracted from the
high-resolution cosmological hydrodynamical simulation Horizon-AGN. One- and
two-point statistics of the spin of the stellar component are measured as a
function of mass and colour. For the low-mass galaxies, this spin is locally
aligned with the tidal field `filamentary' direction while, for the high-mass
galaxies, it is perpendicular to both filaments and walls. The bluest galaxies
of our synthetic catalog are more strongly correlated with the surrounding
tidal field than the reddest galaxies, and this correlation extends up to 10
Mpc/h comoving distance. We also report a correlation of the projected
ellipticities of blue, intermediate mass galaxies on a similar scale at a level
of 10^(-4) which could be a concern for cosmic shear measurements. We do not
report any measurable intrinsic alignments of the reddest galaxies of our
sample. This work is a first step toward the use of very realistic catalog of
synthetic galaxies to evaluate the contamination of weak lensing measurement by
the intrinsic galactic alignments.Comment: 15 pages, accepted for publication in MNRA
Strong lensing selection effects
Context. Strong lenses are a biased subset of the general population of
galaxies.
Aims. The goal of this work is to quantify how lens galaxies and lensed
sources differ from their parent distribution, namely the strong lensing bias.
Methods. We first studied how the strong lensing cross-section varies as a
function of lens and source properties. Then, we simulated strong lensing
surveys with data similar to that expected for Euclid and measured the strong
lensing bias in different scenarios. We focused particularly on two quantities:
the stellar population synthesis mismatch parameter, , defined as
the ratio between the true stellar mass of a galaxy and the stellar mass
obtained from photometry, and the central dark matter mass at fixed stellar
mass and size.
Results. Strong lens galaxies are biased towards larger stellar masses,
smaller half-mass radii and larger dark matter masses. The amplitude of the
bias depends on the intrinsic scatter in the mass-related parameters of the
galaxy population and on the completeness in Einstein radius of the lens
sample. For values of the scatter that are consistent with observed scaling
relations and a minimum detectable Einstein radius of , the strong
lensing bias in is , while that in the central dark matter
mass is . The bias has little dependence on the properties of the source
population: samples of galaxy-galaxy lenses and galaxy-quasar lenses that probe
the same Einstein radius distribution are biased in a very similar way.
Conclusions. Given current uncertainties, strong lensing observations can be
used directly to improve our current knowledge of the inner structure of
galaxies, without the need to correct for selection effects. Time-delay
measurements of from lensed quasars can take advantage of prior
information obtained from galaxy-galaxy lenses with similar Einstein radii.Comment: Published on Astronomy & Astrophysics. A two-minute summary video of
this paper is available at https://youtu.be/UmS9jRHTmZ
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