864 research outputs found
The dark clump near Abell 1942: dark matter halo or statistical fluke?
Weak lensing surveys provide the possibility of identifying dark matter halos
based on their total matter content rather than just the luminous matter
content. On the basis of two sets of observations carried out with the CFHT,
Erben et al. (2000) presented the first candidate dark clump, i.e. a dark
matter concentration identified by its significant weak lensing signal without
a corresponding galaxy overdensity or X-ray emission.
We present a set of HST mosaic observations which confirms the presence of an
alignment signal at the dark clump position. The signal strength, however, is
weaker than in the ground-based data. It is therefore still unclear whether the
signal is caused by a lensing mass or is just a chance alignment. We also
present Chandra observations of the dark clump, which fail to reveal any
significant extended emission.
A comparison of the ellipticity measurements from the space-based HST data
and the ground-based CFHT data shows a remarkable agreement on average,
demonstrating that weak lensing studies from high-quality ground-based
observations yield reliable results.Comment: 33 pages, 34 figures, submitted to A&A. Version with full resolution
figures available at http://www.mpa-garching.mpg.de/~anja/aaclump.pd
GaBoDS: The Garching-Bonn Deep Survey VIII. Lyman-break galaxies in the ESO Deep Public Survey
Aims. The clustering properties of a large sample of U-dropouts are
investigated and compared to very precise results for B-dropouts from other
studies to identify a possible evolution from z=4 to z=3. Methods. A population
of ~8800 candidates for star-forming galaxies at z=3 is selected via the
well-known Lyman-break technique from a large optical multicolour survey (the
ESO Deep Public Survey). The selection efficiency, contamination rate, and
redshift distribution of this population are investigated by means of extensive
simulations. Photometric redshifts are estimated for every Lyman-break galaxy
(LBG) candidate from its UBVRI photometry yielding an empirical redshift
distribution. The measured angular correlation function is deprojected and the
resulting spatial correlation lengths and slopes of the correlation function of
different subsamples are compared to previous studies. Results. By fitting a
simple power law to the correlation function we do not see an evolution in the
correlation length and the slope from other studies at z=4 to our study at z=3.
In particular, the dependence of the slope on UV-luminosity similar to that
recently detected for a sample of B-dropouts is confirmed also for our
U-dropouts. For the first time number statistics for U-dropouts are sufficient
to clearly detect a departure from a pure power law on small scales down to ~2"
reported by other groups for B-dropouts.Comment: 10 pages, 11 figures, accepted by A&A, full resolution version
available at http://www.astro.uni-bonn.de/~hendrik/5880.pd
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
CFHTLenS: the CanadaâFranceâHawaii Telescope Lensing Survey â imaging data and catalogue products
We present data products from the CanadaâFranceâHawaii Telescope Lensing Survey (CFHTLenS). CFHTLenS is based on the Wide component of the CanadaâFranceâHawaii Telescope Legacy Survey (CFHTLS). It encompasses 154âdeg^2 of deep, optical, high-quality, sub-arcsecond imaging data in the five optical filters u*gâČrâČiâČzâČ. The scientific aims of the CFHTLenS team are weak gravitational lensing studies supported by photometric redshift estimates for the galaxies. This paper presents our data processing of the complete CFHTLenS data set. We were able to obtain a data set with very good image quality and high-quality astrometric and photometric calibration. Our external astrometric accuracy is between 60 and 70 mas with respect to Sloan Digital Sky Survey (SDSS) data, and the internal alignment in all filters is around 30 mas. Our average photometric calibration shows a dispersion of the order of 0.01â0.03âmag for gâČrâČiâČzâČ and about 0.04âmag for u* with respect to SDSS sources down to i_(SDSS) †21. We demonstrate in accompanying papers that our data meet necessary requirements to fully exploit the survey for weak gravitational lensing analyses in connection with photometric redshift studies. In the spirit of the CFHTLS, all our data products are released to the astronomical community via the Canadian Astronomy Data Centre at http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/community/CFHTLens/query.html. We give a description and how-to manuals of the public products which include image pixel data, source catalogues with photometric redshift estimates and all relevant quantities to perform weak lensing studies
Detection of Dark Matter Concentrations in the Field of Cl 1604+4304 from Weak Lensing Analysis
We present a weak-lensing analysis of a region around the galaxy cluster Cl
1604+4304 (z=0.897) on the basis of the deep observations with the HST/WFPC2.
We apply a variant of Schneider's aperture mass technique to the observed WFPC2
field and obtain the distribution of weak-lensing signal-to-noise (S/N) ratio
within the field. The resulting S/N map reveals a clear pronounced peak located
about 1.7 arcmin (850h_{50}^{-1} kpc at z=0.897) southwest of the second peak
associated with the optical cluster center determined from the dynamical
analysis of Postman et al. A non-linear finite-field inversion method has been
used to reconstruct the projected mass distribution from the observed shear
field. The reconstructed mass map shows a super-critical feature at the
location of the S/N peak as well as in the cluster central region. Assuming the
redshift distribution of field galaxies, we obtain the total mass in the
observed field to be 1.0 h_{50}^{-1} 10^{15} M_sun for =1.0. The estimated
mass within a circular aperture of radius 280h_{50}^{-1} kpc centered on the
dark clump is 2.4h_{50}^{-1} 10^{14} M_sun. We have confirmed the existence of
the ` dark ' mass concentration from another deep HST observation with a
slightly different ~20 arcsec pointing.Comment: 7 pages, 3 figure
Cosmic shear surveys
Gravitational weak shear produced by large-scale structures of the universe
induces a correlated ellipticity distribution of distant galaxies. The
amplitude and evolution with angular scale of the signal depend on cosmological
models and can be inverted in order to constrain the power spectrum and the
cosmological parameters. We present our recent analysis of 50 uncorrelated VLT
fields and the very first constrains on () and the nature of
primordial fluctuations based on the join analysis of present-day cosmic shear
surveys.Comment: Latex, 7 pages. To appear in the ESO Proceedings ``Deep Fields'',
Garching Oct 9-12, 200
Mass-detection of a matter concentration projected near the cluster Abell 1942: Dark clump or high-redshift cluster?
A weak-lensing analysis of wide-field - and -band images centered on
the cluster Abell 1942 has uncovered a mass concentration arcminutes
South of the cluster center. A statistical analysis shows that the detections
are highly significant. No strong concentration of bright galaxies is seen at
the position of the mass concentration, though a slight galaxy number
overdensity and a weak extended X-ray source are present about 1' away from its
center.
From the spatial dependence of the tangential alignment around the center of
the mass concentration, we inferred a lower bound on the mass inside a sphere
of radius \ts Mpc of , much higher
than crude mass estimates based on X-ray data. No firm conclusion can be
inferred about the nature of the clump. If it were a high-redshift cluster, the
weak X-ray flux would indicate that it had an untypically low X-ray luminosity
for its mass; if the X-ray emission were physically unrelated to the mass
concentration, this conclusion would be even stronger.
The search for massive halos by weak lensing enables us for the first time to
select halos based on their mass properties only and to detect new types of
objects, e.g., dark halos. The mass concentration in the field of A1942 may be
the first example of such a halo.Comment: Sumitted to A&A Main Journal. 15 pages, 11 figures. 75 Kb gzipped tar
file. Figures with images not included, but available on ftp.iap.fr
/pub/from_users/mellier/A1942: a1942darkclump.ps.gz (2.1 Mb
Discovery of a Galaxy Cluster via Weak Lensing
We report the discovery of a cluster of galaxies via its weak gravitational
lensing effect on background galaxies, the first spectroscopically confirmed
cluster to be discovered through its gravitational effects rather than by its
electromagnetic radiation. This fundamentally different selection mechanism
promises to yield mass-selected, rather than baryon or photon-selected, samples
of these important cosmological probes. We have confirmed this cluster with
spectroscopic redshifts of fifteen members at z=0.276, with a velocity
dispersion of 615 km/s. We use the tangential shear as a function of source
photometric redshift to estimate the lens redshift independently and find z_l =
0.30 +- 0.08. The good agreement with the spectroscopy indicates that the
redshift evolution of the mass function may be measurable from the imaging data
alone in shear-selected surveys.Comment: revised version with minor changes, to appear in ApJ
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