1,274 research outputs found
A fast empirical method for galaxy shape measurements in weak lensing surveys
We describe a simple and fast method to correct ellipticity measurements of
galaxies from the distortion by the instrumental and atmospheric point spread
function (PSF), in view of weak lensing shear measurements. The method performs
a classification of galaxies and associated PSFs according to measured shape
parameters, and corrects the measured galaxy ellipticites by querying a large
lookup table (LUT), built by supervised learning. We have applied this new
method to the GREAT10 image analysis challenge, and present in this paper a
refined solution that obtains the competitive quality factor of Q = 104,
without any shear power spectrum denoising or training. Of particular interest
is the efficiency of the method, with a processing time below 3 ms per galaxy
on an ordinary CPU.Comment: 8 pages, 6 figures. Metric values updated according to the final
GREAT10 analysis software (Kitching et al. 2012, MNRAS 423, 3163-3208), no
qualitative changes. Associated code available at
http://lastro.epfl.ch/megalu
3D Photometric Cosmic Shear
Here we present a number of improvements to weak lensing 3D power spectrum
analysis, 3D cosmic shear, that uses the shape and redshift information of
every galaxy to constrain cosmological parameters. We show how photometric
redshift probability distributions for individual galaxies can be directly
included in this statistic with no averaging. We also include the Limber
approximation, considerably simplifying full 3D cosmic shear analysis, and we
investigate its range of applicability. Finally we show the relationship
between weak lensing tomography and the 3D cosmic shear field itself; the steps
connecting them being the Limber approximation, a harmonic-space transform and
a discretisation in wavenumber. Each method has its advantages: 3D cosmic shear
analysis allows straightforward inclusion of all relevant modes, thus ensuring
minimum error bars, and direct control of the range of physical wavenumbers
probed, to avoid the uncertain highly nonlinear regime. On the other hand,
tomography is more convenient for checking systematics through direct
investigation of the redshift dependence of the signal. Finally, for
tomography, we suggest that the angular modes probed should be
redshift-dependent, to recover some of the 3D advantages.Comment: Accepted to MNRAS. 15 pages, 7 figure
Propagating Residual Biases in Cosmic Shear Power Spectra
In this paper we derive a full expression for the propagation of
multiplicative and additive shape measurement biases into the cosmic shear
power spectrum. In doing so we identify several new terms that are associated
with selection effects, as well as cross-correlation terms between the
multiplicative and additive biases and the shear field. The computation of the
resulting bias in the shear power spectrum scales as the fifth power of the
maximum multipole considered. Consequently the calculation is unfeasible for
large l-modes, and the only tractable way to assess the full impact of shape
measurement biases on cosmic shear power spectrum is through forward modelling
of the effects. To linear order in bias parameters the shear power spectrum is
only affected by the mean of the multiplicative bias field over a survey and
the cross correlation between the additive bias field and the shear field. If
the mean multiplicative bias is zero then second order convolutive terms are
expected to be orders of magnitude smaller.Comment: 10 pages, accepted to the Open Journal of Astrophysic
Figures of Merit for Testing Standard Models: Application to Dark Energy Experiments in Cosmology
Given a standard model to test, an experiment can be designed to: (i) measure
the standard model parameters; (ii) extend the standard model; or (iii) look
for evidence of deviations from the standard model. To measure (or extend) the
standard model, the Fisher matrix is widely used in cosmology to predict
expected parameter errors for future surveys under Gaussian assumptions. In
this article, we present a frame- work that can be used to design experiments
such that it maximises the chance of finding a deviation from the standard
model. Using a simple illustrative example, discussed in the appendix, we show
that the optimal experimental configuration can depend dramatically on the
optimisation approach chosen. We also show some simple cosmology calculations,
where we study Baryonic Acoustic Oscillation and Supernove surveys. In doing
so, we also show how external data, such as the positions of the CMB peaks
measured by WMAP, and theory priors can be included in the analysis. In the
cosmological cases that we have studied (DETF Stage III), we find that the
three optimisation approaches yield similar results, which is reassuring and
indicates that the choice of optimal experiment is fairly robust at this level.
However, this may not be the case as we move to more ambitious future surveys.Comment: Submitted to MNRAS. 12 pages, 9 figure
Measuring dark energy properties with 3D cosmic shear
We present parameter estimation forecasts for present and future 3D cosmic
shear surveys. We demonstrate that, in conjunction with results from cosmic
microwave background (CMB) experiments, the properties of dark energy can be
estimated with very high precision with large-scale, fully 3D weak lensing
surveys. In particular, a 5-band, 10,000 square degree ground-based survey to a
median redshift of zm=0.7 could achieve 1- marginal statistical errors,
in combination with the constraints expected from the CMB Planck Surveyor, of
w0=0.108 and wa=0.099 where we parameterize w by
w(a)=w0+wa(1-a) where a is the scale factor. Such a survey is achievable with a
wide-field camera on a 4 metre class telescope. The error on the value of w at
an intermediate pivot redshift of z=0.368 is constrained to
w(z=0.368)=0.0175. We compare and combine the 3D weak lensing
constraints with the cosmological and dark energy parameters measured from
planned Baryon Acoustic Oscillation (BAO) and supernova Type Ia experiments,
and find that 3D weak lensing significantly improves the marginalized errors. A
combination of 3D weak lensing, CMB and BAO experiments could achieve
w0=0.037 and wa=0.099. Fully 3D weak shear analysis avoids the
loss of information inherent in tomographic binning, and we show that the
sensitivity to systematic errors is much less. In conjunction with the fact
that the physics of lensing is very soundly based, this analysis demonstrates
that deep, wide-angle 3D weak lensing surveys are extremely promising for
measuring dark energy properties.Comment: 18 pages, 16 figures. Accepted to MNRAS. Figures now in grayscale.
Further discussions on non-Gaussianity and photometric redshift errors. Some
references adde
Path Integral Marginalization for Cosmology: Scale Dependent Galaxy Bias & Intrinsic Alignments
We present a path-integral likelihood formalism that extends parameterized
likelihood analyses to include continuous functions. The method finds the
maximum likelihood point in function-space, and marginalizes over all possible
functions, under the assumption of a Gaussian-distributed function-space. We
apply our method to the problem of removing unknown systematic functions in two
topical problems for dark energy research : scale-dependent galaxy bias in
redshift surveys; and galaxy intrinsic alignments in cosmic shear surveys. We
find that scale-dependent galaxy bias will degrade information on cosmological
parameters unless the fractional variance in the bias function is known to 10%.
Measuring and removing intrinsic alignments from cosmic shear surveys with a
flat-prior can reduce the dark energy Figure-of-Merit by 20%, however provided
that the scale and redshift-dependence is known to better than 10% with a
Gaussian-prior, the dark energy Figure-of-Merit can be enhanced by a factor of
two with no extra assumptions.Comment: 11 pages, 4 figures, submitted to MNRA
Fisher matrix decomposition for dark energy prediction
Within the context of constraining an expansion of the dark energy equation of state w(z), we show that the eigendecomposition of Fisher matrices is sensitive to both the maximum order of the expansion and the basis set choice. We investigate the Fisher matrix formalism in the case that a particular function is expanded in some basis set. As an example we show results for an all-sky weak lensing tomographic experiment. We show that the set of eigenfunctions is not unique and that the best constrained functions are only reproduced accurately at very higher order N≳ 100, a top-hat basis set requires an even higher order. We show that the common approach used for finding the marginalized eigenfunction errors is sensitive to the choice of non-w(z) parameters and priors. The eigendecomposition of Fisher matrices is a potentially useful tool that can be used to determine the predicted accuracy with which an experiment could constrain w(z). It also allows for the reconstruction of the redshift sensitivity of the experiment to changes in w(z). However, the technique is sensitive to both the order and the basis set choice. Publicly available code is available as part of icosmo at http://www.icosmo.or
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