500 research outputs found
Effect of Sub-Structure in Clusters on the Local Weak-Shear Field
Weak shear maps of the outer regions of clusters have been successfully used
to map the distribution of mass at large radii. The effects of substructure in
clusters on such reconstructions of the total mass have not been systematically
studied. We propose a new method to study the effect of perturbers (bright
cluster galaxies or sub-groups within the cluster) on the weak shear field. We
present some analytic results below.Comment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
Constraining the mass distribution of galaxies using galaxy-galaxy lensing in clusters and in the field
We present a maximum-likelihood analysis of galaxy-galaxy lensing effects in
galaxy clusters and in the field. The aim is to determine the accuracy and
robustness of constraints that can be obtained on galaxy halo properties in
both environments - the high density cluster and the low density field. This
paper is theoretically motivated, therefore, we work exclusively with simulated
data (nevertheless defined to match observations) to study the accuracy with
which input parameters for mass distributions for galaxies can be extracted. We
model galaxies in the cluster and the field using a wide range of mass
profiles: the truncated pseudo isothermal elliptical mass distribution, the
Navarro, Frenk and White profile, and a Power Law model with a core radius. We
find that independent of the choice of profile the mean mass of galaxies (of
the order of 10^{12}Mo) can be estimated to within 15% from ground-based data
and with an error of less than 10% with space observations. Additionally robust
constraints can be obtained on the mean slope of the mass profile. The two
standard parameters that characterise galaxy halo models, the central velocity
dispersion and the truncation radius can also be retrieved reliably from the
maximum-likelihood analysis. Furthermore, going beyond the usual formulation,
we propose a re-parameterisation of the mass models that allows us to put yet
stronger constraints on the aperture mass of a galaxy halo (with less than 10%
error). The gain in signal to noise using space observations, expected for
instance with the proposed SNAP satellite compared to ground based data in
terms of accuracy of retrieving input parameters is highly significant.Comment: 15 pages, 48 figure
A new method to break the mass sheet degeneracy using aperture moments
Mass determinations from gravitational lensing shear and the higher order
estimator flexion are both subject to the mass sheet degeneracy. Mass sheet
degeneracy refers to a transformation that leaves the reduced shear and flexion
invariant. In general, this transformation can be approximated by the addition
of a constant surface mass density sheet. We propose a new technique to break
the mass sheet degeneracy. The method uses mass moments of the shear or flexion
fields in combination with convergence information derived from number counts
which exploit the magnification bias. The difference between the measured mass
moments provides an estimator for the magnitude of the additive constant that
is the mass-sheet. For demonstrating this, we derive relations that hold true
in general for n-th order moments and show how they can be employed effectively
to break the degeneracy. We investigate the detectability of this degeneracy
parameter from our method and find that the degeneracy parameter can be
feasibly determined from stacked galaxy-galaxy lensing data and cluster lensing
data. Furthermore, we compare the signal-to-noise ratios of convergence
information from number counts with shear and flexion for SIS and NFW models.
We find that the combination of shear and flexion performs best on galaxy and
cluster scales and the convergence information can therefore be used to break
the mass sheet degeneracy without quality loss in the mass reconstruction. In
summary, there is power in the combination of shear, flexion, convergence and
their higher order moments. With the anticipated wealth of lensing data from
upcoming and future satellite missions - EUCLID and WFIRST - this technique
will be feasible.Comment: 23 pages, 2 figures, 1 table, accepted for publication in MNRAS;
minor text improvement
Strong and Weak Lensing Constraints on Galaxy Mass Distribution
Gravitational Lensing is a UNIQUE tool to constrain the mass distribution of
collapsed structures, this is particularly true for galaxies, either on a case
by case basis using multiple images of background sources (such as quasars), or
statistically using the so called galaxy-galaxy lensing technique. First, I
will present the lensing theory, and then discuss the various methods applied
to current observations. Finally, I will review the bright future prospects of
galaxy lensing that will benefit of the development of high resolution, large,
wide and deep (lensing) surveys.Comment: to appear in the proceedings of the Yale 2001 Cosmology Workshop on
the Shapes of Galaxies and Their Halos (8 pages
A Faint Star-Forming System Viewed Through the Lensing Cluster Abell 2218: First Light at z~5.6?
We discuss the physical nature of a remarkably faint pair of Lyman
alpha-emitting images discovered close to the giant cD galaxy in the lensing
cluster Abell 2218 (z=0.18) during a systematic survey for highly-magnified
star-forming galaxies beyond z=5. A well-constrained mass model suggests the
pair arises via a gravitationally-lensed source viewed at high magnification.
Keck spectroscopy confirms the lensing hypothesis and implies the unlensed
source is a very faint (I~30) compact (<150 pc) and isolated object at z=5.576
whose optical emission is substantially contained within the Lyman alpha
emission line; no stellar continuum is detectable. The available data suggest
the source is a promising candidate for an isolated ~10^6 solar mass system
seen producing its first generation of stars close to the epoch of
reionization.Comment: 11 pages, 3 figures, to appear in Ap J Lett, minor revision following
referee's repor
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