45 research outputs found
Searching for galaxy clusters using the aperture mass statistics in 50 VLT fields
Application of the aperture mass (Map-) statistics provides a weak lensing
method for the detection of cluster-sized dark matter halos. We present a new
aperture filter function and maximise the effectiveness of the Map-statistics
to detect cluster-sized halos using analytical models. We then use weak lensing
mock catalogues generated from ray-tracing through N-body simulations, to
analyse the effect of image treatment on the expected number density of halos.
Using the Map-statistics, the aperture radius is typically several arcminutes,
hence the aperture often lies partly outside a data field, consequently the
signal-to-noise ratio of a halo detection decreases. We study these border
effects analytically and by using mock catalogues. We find that the expected
number density of halos decreases by a factor of two if the size of a field is
comparable to the diameter of the aperture used. We finally report on the
results of a weak lensing cluster search applying the Map-statistics to 50
randomly selected fields which were observed with FORS1 at the VLT. Altogether
the 50 VLT fields cover an area of 0.64 square degrees. The I-band images were
taken under excellent seeing conditions (average seeing 0.6 arcsec.) which
results in a high number density of galaxies used for the weak lensing analysis
(26/sq.arcmin). In five of the VLT fields, we detect a significant Map-signal
which coincides with an overdensity of the light distribution. These detections
are thus excellent candidates for shear-selected clusters.Comment: 23 pages, 5 tables, 24 figures, published in A&A, Sect. 3.5 and 7 are
changed or altered; Fig. 11 is change
The Garching-Bonn Deep Survey (GaBoDS) Wide-Field-Imaging Reduction Pipeline
We introduce our publicly available Wide-Field-Imaging reduction pipeline
THELI. The procedures applied for the efficient pre-reduction and astrometric
calibration are presented. A special emphasis is put on the methods applied to
the photometric calibration. As a test case the reduction of optical data from
the ESO Deep Public Survey including the WFI-GOODS data is described. The
end-products of this project are now available via the ESO archive Advanced
Data Products section.Comment: 6 pages, 3 figures, proceedings of ESO Calibration Workshop 200
GaBoDS: The Garching-Bonn Deep Survey -- II. Confirmation of EIS cluster candidates by weak gravitational lensing
We report the first confirmation of colour-selected galaxy cluster candidates
by means of weak gravitational lensing. Significant lensing signals were
identified in the course of the shear-selection programme of dark matter haloes
in the Garching-Bonn Deep Survey, which currently covers 20 square degrees of
deep, high-quality imaging data on the southern sky. The detection was made in
a field that was previously covered by the ESO Imaging Survey (EIS) in 1997. A
highly significant shear-selected mass-concentration perfectly coincides with
the richest EIS cluster candidate at z~0.2, thus confirming its cluster nature.
Several other shear patterns in the field can also be identified with cluster
candidates, one of which could possibly be part of a filament at z~0.45.Comment: 4 pages, 4 figures, submitted to A&A Letter
Cosmic shear analysis of archival HST/ACS data: I. Comparison of early ACS pure parallel data to the HST/GEMS Survey
This is the first paper of a series describing our measurement of weak
lensing by large-scale structure using archival observations from the Advanced
Camera for Surveys (ACS) on board the Hubble Space Telescope (HST).
In this work we present results from a pilot study testing the capabilities
of the ACS for cosmic shear measurements with early parallel observations and
presenting a re-analysis of HST/ACS data from the GEMS survey and the GOODS
observations of the Chandra Deep Field South (CDFS). We describe our new
correction scheme for the time-dependent ACS PSF based on observations of
stellar fields. This is currently the only technique which takes the full time
variation of the PSF between individual ACS exposures into account. We estimate
that our PSF correction scheme reduces the systematic contribution to the shear
correlation functions due to PSF distortions to < 2*10^{-6} for galaxy fields
containing at least 10 stars. We perform a number of diagnostic tests
indicating that the remaining level of systematics is consistent with zero for
the GEMS and GOODS data confirming the success of our PSF correction scheme.
For the parallel data we detect a low level of remaining systematics which we
interpret to be caused by a lack of sufficient dithering of the data.
Combining the shear estimate of the GEMS and GOODS observations using 96
galaxies arcmin^{-2} with the photometric redshift catalogue of the GOODS-MUSIC
sample, we determine a local single field estimate for the mass power spectrum
normalisation sigma_{8,CDFS}=0.52^{+0.11}_{-0.15} (stat) +/- 0.07 (sys) (68%
confidence assuming Gaussian cosmic variance) at fixed Omega_m=0.3 for a
LambdaCDM cosmology. We interpret this exceptionally low estimate to be due to
a local under-density of the foreground structures in the CDFS.Comment: Version accepted for publication in Astronomy & Astrophysics with 28
pages, 25 figures. A version with full resolution figures can be downloaded
from http://www.astro.uni-bonn.de/~schrabba/papers/cosmic_shear_acs1_v2.pd
GaBoDS: The Garching-Bonn Deep Survey - III. Lyman-Break Galaxies in the Chandra Deep Field South
We present first results of our search for high-redshift galaxies in deep CCD
mosaic images. As a pilot study for a larger survey, very deep images of the
Chandra Deep Field South (CDFS), taken withWFI@MPG/ESO2.2m, are used to select
large samples of 1070 U-band and 565 B-band dropouts with the Lyman-break
method. The data of these Lyman-break galaxies are made public as an electronic
table. These objects are good candidates for galaxies at z~3 and z~4 which is
supported by their photometric redshifts. The distributions of apparent
magnitudes and the clustering properties of the two populations are analysed,
and they show good agreement to earlier studies. We see no evolution in the
comoving clustering scale length from z~3 to z~4. The techniques presented here
will be applied to a much larger sample of U-dropouts from the whole survey in
near future.Comment: 11 pages, 11 figures, replaced with version accepted by A&A. Minor
changes and tabular appendix with LBG catalogues. Version with full
resolution figures available at
http://www.astro.uni-bonn.de/~hendrik/2544.pd
An analytic approach to number counts of weak-lensing peak detections
We develop and apply an analytic method to predict peak counts in
weak-lensing surveys. It is based on the theory of Gaussian random fields and
suitable to quantify the level of spurious detections caused by chance
projections of large-scale structures as well as the shape and shot noise
contributed by the background galaxies. We compare our method to peak counts
obtained from numerical ray-tracing simulations and find good agreement at the
expected level. The number of peak detections depends substantially on the
shape and size of the filter applied to the gravitational shear field. Our main
results are that weak-lensing peak counts are dominated by spurious detections
up to signal-to-noise ratios of 3--5 and that most filters yield only a few
detections per square degree above this level, while a filter optimised for
suppressing large-scale structure noise returns up to an order of magnitude
more.Comment: 9 pages, 5 figures, submitted to A&
Testing the reliability of weak lensing cluster detections
We study the reliability of dark-matter halo detections with three different
linear filters applied to weak-lensing data. We use ray-tracing in the multiple
lens-plane approximation through a large cosmological simulation to construct
realizations of cosmic lensing by large-scale structures between redshifts zero
and two. We apply the filters mentioned above to detect peaks in the
weak-lensing signal and compare them with the true population of dark matter
halos present in the simulation. We confirm the stability and performance of a
filter optimized for suppressing the contamination by large-scale structure. It
allows the reliable detection of dark-matter halos with masses above a few
times 1e13 M_sun/h with a fraction of spurious detections below ~10%. For
sources at redshift two, 50% of the halos more massive than ~7e13 M_sun/h are
detected, and completeness is reached at ~2e14 M_sun/h.Comment: 14 pages, 13 figures, accepted on A&
Cosmological constraints from COMBO-17 using 3D weak lensing
We present the first application of the 3D cosmic shear method developed in
Heavens et al. (2006) and the geometric shear-ratio analysis developed in
Taylor et al. (2006), to the COMBO-17 data set. 3D cosmic shear has been used
to analyse galaxies with redshift estimates from two random COMBO-17 fields
covering 0.52 square degrees in total, providing a conditional constraint in
the (sigma_8, Omega_m) plane as well as a conditional constraint on the
equation of state of dark energy, parameterised by a constant w= p/rho c^2. The
(sigma_8, Omega_m) plane analysis constrained the relation between sigma_8 and
Omega_m to be sigma_8(Omega_m/0.3)^{0.57 +- 0.19}=1.06 +0.17 -0.16, in
agreement with a 2D cosmic shear analysis of COMBO-17. The 3D cosmic shear
conditional constraint on w using the two random fields is w=-1.27 +0.64 -0.70.
The geometric shear-ratio analysis has been applied to the A901/2 field, which
contains three small galaxy clusters. Combining the analysis from the A901/2
field, using the geometric shear-ratio analysis, and the two random fields,
using 3D cosmic shear, w is conditionally constrained to w=-1.08 +0.63 -0.58.
The errors presented in this paper are shown to agree with Fisher matrix
predictions made in Heavens et al. (2006) and Taylor et al. (2006). When these
methods are applied to large datasets, as expected soon from surveys such as
Pan-STARRS and VST-KIDS, the dark energy equation of state could be constrained
to an unprecedented degree of accuracy.Comment: 10 pages, 4 figures. Accepted to MNRA
The Shear TEsting Programme 1: Weak lensing analysis of simulated ground-based observations
The Shear TEsting Programme, STEP, is a collaborative project to improve the accuracy and reliability of all weak lensing measurements in preparation for the next generation of wide-field surveys. In this first STEP paper we present the results of a blind analysis of simulated ground-based observations of relatively simple galaxy morphologies. The most successful methods are shown to achieve percent level accuracy. From the cosmic shear pipelines that have been used to constrain cosmology, we find weak lensing shear measured to an accuracy that is within the statistical errors of current weak lensing analyses, with shear measurements accurate to better than 7%. The dominant source of measurement error is shown to arise from calibration uncertainties where the measured shear is over or under-estimated by a constant multiplicative factor. This is of concern as calibration errors cannot be detected through standard diagnostic tests. The measured calibration errors appear to result from stellar contamination, false object detection, the shear measurement method itself, selection bias and/or the use of biased weights. Additive systematics (false detections of shear) resulting from residual point-spread function anisotropy are, in most cases, reduced to below an equivalent shear of 0.001, an order of magnitude below cosmic shear distortions on the scales probed by current surveys. Our results provide a snapshot view of the accuracy of current ground-based weak lensing methods and a benchmark upon which we can improve. To this end we provide descriptions of each method tested and include details of the eight different implementations of the commonly used Kaiser, Squires and Broadhurst (1995) method (KSB+) to aid the improvement of future KSB+ analyses
A weak lensing analysis of the Abell 2163 cluster
Weak lensing analysis is applied to a deep, one square degree r-band
CFHT-Megacam image of the Abell 2163 field. The observed shear is fitted with
Single Isothermal Sphere and Navarro-Frenk-White models to obtain the velocity
dispersion and the mass, respectively; in addition, aperture densitometry is
also used to provide a mass estimate at different distances from the cluster
centre. The luminosity function is finally derived, allowing to estimate the
mass/luminosity ratio. Previous weak lensing analyses performed at smaller
scales produced somewhat contradictory results. The mass and velocity
dispersion obtained in the present paper are compared and found to be in good
agreement with the values computed by other authors from X-ray and
spectroscopic data.Comment: 8 pages, 9 figures, Accepted for publication on Astronomy &
Astrophysics, final versio