435 research outputs found

### The effect of distant large scale structure on weak lensing mass estimates

We quantify the uncertainty in weak lensing mass estimates of clusters of
galaxies, caused by distant (uncorrelated) large scale structure along the line
of sight. We find that the effect is fairly small for deep observations
(20<R<26) of massive clusters (sigma=1000 km/s) at intermediate redshifts,
where the bulk of the sources are at high redshifts compared to the cluster
redshift. If the lensing signal is measured out to 1.5 h_{50}^{-1} Mpc the
typical 1sigma relative uncertainty in the mass is about 6%. However, in other
situations the induced uncertainty can be larger. For instance, in the case of
nearby clusters, such as the Coma cluster, background structures introduce a
considerable uncertainty in the mass, limiting the maximum achievable S/N-ratio
to \sim 7, even for deep observations. The noise in the cluster mass estimate
caused by the large scale structure increases with increasing aperture size,
which will also complicate attempts to constrain cluster mass profiles at large
distances from the cluster centre. However, the distant large scale structure
studied here can be considered an additional (statistical) source of error, and
by averaging the results of several clusters the noise is decreased.Comment: accepted for publication in Astronomy and Astrophysics 11 pages, 9
figure

### Weak lensing constraints on galaxy halos

Weak gravitational lensing has become an important tool to study the
properties of dark matter halos around galaxies, thanks to the advent of large
panoramic cameras on 4m class telescopes. This area of research has been
developing rapidly in the past few years, and in these proceedings we present
some results based on the Red-Sequence Cluster Survey, thus highlighting what
can be achieved with current data sets. We present results on the measurement
of virial masses as a function of luminosity and the extent of dark matter
halos. Much larger surveys are underway or planned, which will result in an
impressive improvement in the accuracy of the measurements. However, the
interpretation of future results will rely more and more on comparison with
numerical simulations, thus providing direct tests of galaxy formation models.Comment: 8 pages, to appear in Proc. XXIst IAP Colloquium "Mass Profiles and
Shapes of Cosmological Structures", Paris 4-9 July 2005 (EAS Publications
Series, G. Mamon, F. Combes, C. Deffayet, B. Fort eds.

### Implications of a wavelength dependent PSF for weak lensing measurements

The convolution of galaxy images by the point-spread function (PSF) is the
dominant source of bias for weak gravitational lensing studies, and an accurate
estimate of the PSF is required to obtain unbiased shape measurements. The PSF
estimate for a galaxy depends on its spectral energy distribution (SED),
because the instrumental PSF is generally a function of the wavelength. In this
paper we explore various approaches to determine the resulting `effective' PSF
using broad-band data. Considering the Euclid mission as a reference, we find
that standard SED template fitting methods result in biases that depend on
source redshift, although this may be remedied if the algorithms can be
optimised for this purpose. Using a machine-learning algorithm we show that, at
least in principle, the required accuracy can be achieved with the current
survey parameters. It is also possible to account for the correlations between
photometric redshift and PSF estimates that arise from the use of the same
photometry. We explore the impact of errors in photometric calibration, errors
in the assumed wavelength dependence of the PSF model and limitations of the
adopted template libraries. Our results indicate that the required accuracy for
Euclid can be achieved using the data that are planned to determine photometric
redshifts

### A study of the sensitivity of shape measurements to the input parameters of weak lensing image simulations

Improvements in the accuracy of shape measurements are essential to exploit
the statistical power of planned imaging surveys that aim to constrain
cosmological parameters using weak lensing by large-scale structure. Although a
range of tests can be performed using the measurements, the performance of the
algorithm can only be quantified using simulated images. This yields, however,
only meaningful results if the simulated images resemble the real observations
sufficiently well. In this paper we explore the sensitivity of the
multiplicative bias to the input parameters of Euclid-like image simulations.We
find that algorithms will need to account for the local density of sources. In
particular the impact of galaxies below the detection limit warrants further
study, because magnification changes their number density, resulting in
correlations between the lensing signal and multiplicative bias. Although
achieving sub-percent accuracy will require further study, we estimate that
sufficient archival Hubble Space Telescope data are available to create
realistic populations of galaxies.Comment: 18 pages, accepted for publications in MNRA

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