1,544 research outputs found
Sersiclets - A Matched Filter Extension of Shapelets for Weak Lensing Studies
The precision study of dark matter using weak lensing by large scale
structure is strongly constrained by the accuracy with which one can measure
galaxy shapes. Several methods have been devised but none have demonstrated the
ability to reach the level of precision required by future weak lensing
surveys. In this Letter we explore new avenues to the existing Shapelets
approach, combining a priori knowledge of the galaxy profile with the power of
orthogonal basis function decomposition. This Letter discusses the new issues
raised by this matched filter approach and proposes promising alternatives to
shape measurement techniques. In particular it appears that the use of a
matched filter (e.g. Sersic profile) restricted to elliptical radial fitting
functions resolves several well known Shapelet issues.Comment: 6 pages, 6 figures. MNRAS Accepte
Redshift and Shear Calibration: Impact on Cosmic Shear Studies and Survey Design
The cosmological interpretation of weak lensing by large-scale structures
requires knowledge of the redshift distribution of the source galaxies. Current
lensing surveys are often calibrated using external redshift samples which span
a significantly smaller sky area in comparison to the lensing survey, and are
thus subject to sample variance. Some future lensing surveys are expected to be
calibrated in the same way, in particular the fainter galaxy populations where
the entire color coverage, and hence photometric redshift estimate, could be
challenging to obtain. Using N-body simulations, we study the impact of this
sample variance on cosmic shear analysis and show that, to first approximation,
it behaves like a shear calibration error 1+/-epsilon. Using the Hubble Deep
Field as a redshift calibration survey could therefore be a problem for current
lensing surveys. We discuss the impact of the redshift distribution sampling
error and a shear calibration error on the design of future lensing surveys,
and find that a lensing survey of area Theta square degrees and limiting
magnitude m_lim}, has a minimum shear and redshift calibration accuracy
requirements given by epsilon = epsilon_0 10^{beta(m_lim-24.5)} / sqrt(Theta/
200). Above that limit, lensing surveys would not reach their full potential.
Using the galaxy number counts from the Hubble Ultra-Deep Field, we find
(epsilon_0,beta)=(0.015,-0.18) and (epsilon_0,beta)=(0.011,-0.23) for ground
and space based surveys respectively. Lensing surveys with no or limited
redshift information and/or poor shear calibration accuracy will loose their
potential to analyse the cosmic shear signal in the sub-degree angular scales,
and therefore complete photometric redshift coverage should be a top priority
for future lensing surveys.Comment: Accepted version to Astroparticle Physic
Cluster Masses Accounting for Structure along the Line of Sight
Weak gravitational lensing of background galaxies by foreground clusters
offers an excellent opportunity to measure cluster masses directly without
using gas as a probe. One source of noise which seems difficult to avoid is
large scale structure along the line of sight. Here I show that, by using
standard map-making techniques, one can minimize the deleterious effects of
this noise. The resulting uncertainties on cluster masses are significantly
smaller than when large scale structure is not properly accounted for, although
still larger than if it was absent altogether.Comment: 5 pages, 5 figure
Very weak lensing in the CFHTLS Wide: Cosmology from cosmic shear in the linear regime
We present an exploration of weak lensing by large-scale structure in the
linear regime, using the third-year (T0003) CFHTLS Wide data release. Our
results place tight constraints on the scaling of the amplitude of the matter
power spectrum sigma_8 with the matter density Omega_m. Spanning 57 square
degrees to i'_AB = 24.5 over three independent fields, the unprecedented
contiguous area of this survey permits high signal-to-noise measurements of
two-point shear statistics from 1 arcmin to 4 degrees. Understanding systematic
errors in our analysis is vital in interpreting the results. We therefore
demonstrate the percent-level accuracy of our method using STEP simulations, an
E/B-mode decomposition of the data, and the star-galaxy cross correlation
function. We also present a thorough analysis of the galaxy redshift
distribution using redshift data from the CFHTLS T0003 Deep fields that probe
the same spatial regions as the Wide fields. We find sigma_8(Omega_m/0.25)^0.64
= 0.785+-0.043 using the aperture-mass statistic for the full range of angular
scales for an assumed flat cosmology, in excellent agreement with WMAP3
constraints. The largest physical scale probed by our analysis is 85 Mpc,
assuming a mean redshift of lenses of 0.5 and a LCDM cosmology. This allows for
the first time to constrain cosmology using only cosmic shear measurements in
the linear regime. Using only angular scales theta> 85 arcmin, we find
sigma_8(Omega_m/0.25)_lin^0.53 = 0.837+-0.084, which agree with the results
from our full analysis. Combining our results with data from WMAP3, we find
Omega_m=0.248+-0.019 and sigma_8 = 0.771+-0.029.Comment: 23 pages, 16 figures (A&A accepted
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
The vacuum energy with non-ideal boundary conditions via an approximate functional equation
We discuss the vacuum energy of a quantized scalar field in the presence of
classical surfaces, defining bounded domains ,
where the field satisfies ideal or non-ideal boundary conditions. For the
electromagnetic case, this situation describes the conductivity correction to
the zero-point energy. Using an analytic regularization procedure, we obtain
the vacuum energy for a massless scalar field at zero temperature in the
presence of a slab geometry with Dirichlet
boundary conditions. To discuss the case of non-ideal boundary conditions, we
employ an asymptotic expansion, based on an approximate functional equation for
the Riemann zeta-function, where finite sums outside their original domain of
convergence are defined. Finally, to obtain the Casimir energy for a massless
scalar field in the presence of a rectangular box, with lengths and
, i.e., with non-ideal boundary
conditions, we employ an approximate functional equation of the Epstein
zeta-function.Comment: 10 page
Photometric Redshift Calibration with Self Organising Maps
Accurate photometric redshift calibration is central to the robustness of all
cosmology constraints from cosmic shear surveys. Analyses of the KiDS
re-weighted training samples from all overlapping spectroscopic surveys to
provide a direct redshift calibration. Using self-organising maps (SOMs) we
demonstrate that this spectroscopic compilation is sufficiently complete for
KiDS, representing of the effective 2D cosmic shear sample. We use the
SOM to define a represented `gold' cosmic shear sample, per tomographic
bin. Using mock simulations of KiDS and the spectroscopic training set, we
estimate the uncertainty on the SOM redshift calibration, and find that
photometric noise, sample variance, and spectroscopic selection effects
(including redshift and magnitude incompleteness) induce a combined maximal
scatter on the bias of the redshift distribution reconstruction () of
in all tomographic bins. We show
that the SOM calibration is unbiased in the cases of noiseless photometry and
perfectly representative spectroscopic datasets, as expected from theory. The
inclusion of both photometric noise and spectroscopic selection effects in our
mock data introduces a maximal bias of , or at
confidence, once quality flags have been applied to the SOM. The method
presented here represents a significant improvement over the previously adopted
direct redshift calibration implementation for KiDS, owing to its diagnostic
and quality assurance capabilities. The implementation of this method in future
cosmic shear studies will allow better diagnosis, examination, and mitigation
of systematic biases in photometric redshift calibration.Comment: 22 pages, 10 figures, 4 appendices, accepted for publication in A&
A bias in cosmic shear from galaxy selection: results from ray-tracing simulations
We identify and study a previously unknown systematic effect on cosmic shear
measurements, caused by the selection of galaxies used for shape measurement,
in particular the rejection of close (blended) galaxy pairs. We use ray-tracing
simulations based on the Millennium Simulation and a semi-analytical model of
galaxy formation to create realistic galaxy catalogues. From these, we quantify
the bias in the shear correlation functions by comparing measurements made from
galaxy catalogues with and without removal of close pairs. A likelihood
analysis is used to quantify the resulting shift in estimates of cosmological
parameters. The filtering of objects with close neighbours (a) changes the
redshift distribution of the galaxies used for correlation function
measurements, and (b) correlates the number density of sources in the
background with the density field in the foreground. This leads to a
scale-dependent bias of the correlation function of several percent,
translating into biases of cosmological parameters of similar amplitude. This
makes this new systematic effect potentially harmful for upcoming and planned
cosmic shear surveys. As a remedy, we propose and test a weighting scheme that
can significantly reduce the bias.Comment: 9 pages, 9 figures, version accepted for publication in Astronomy &
Astrophysic
Siedlungsspuren aus Mittel- und Spätneolithikum, Bronzezeit und Eisenzeit in Geistingen, Huizerhof (Provinz Limburg)
status: publishe
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