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Register a new userWide-field cosmic shear surveys
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Authors
Y. Mellier
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We present the current status of cosmic shear based on all surveys done so far. Taken together, they cover more about 70 deg$^2$ and concern more than 3 million galaxies with accurate shape measurement. Theoretical expectations, observational results and their cosmological interpretations are discussed in the framework of standard cosmology and CDM scenarios. The potentials of the next generation cosmic shear surveys are discussed.
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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 ($Omega_m,sigma_8$) and the nature of primordial fluctuations based on the join analysis of present-day cosmic shear surveys.
The recent detections of cosmic shear signal announced by several groups have demonstrated the feasibility of this challenging program and convinced astro- nomers of its potential for cosmology. Cosmic shear analysis demands to handle Gigabytes of data in order to probe several square degrees in subarcsecond deep imaging mode. The success of these surveys is sensitive to the designs of the observation strategy, the organization of the data reduction pipelines and the links of the data base with surveys like X-ray or spectroscopic follow up. We describe the cosmic shear surveys we have carried out at the VLT and at CFHT and the way we handle this huge data set in a more general context including the VIRMOS and the XMM-LSS surveys, and the future CMB surveys.
In the past few years, several independent collaborations have presented cosmological constraints from tomographic cosmic shear analyses. These analyses differ in many aspects: the datasets, the shear and photometric redshift estimation algorithms, the theory model assumptions, and the inference pipelines. To assess the robustness of the existing cosmic shear results, we present in this paper a unified analysis of four of the recent cosmic shear surveys: the Deep Lens Survey (DLS), the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), the Science Verification data from the Dark Energy Survey (DES-SV), and the 450 deg$^{2}$ release of the Kilo-Degree Survey (KiDS-450). By using a unified pipeline, we show how the cosmological constraints are sensitive to the various details of the pipeline. We identify several analysis choices that can shift the cosmological constraints by a significant fraction of the uncertainties. For our fiducial analysis choice, considering a Gaussian covariance, conservative scale cuts, assuming no baryonic feedback contamination, identical cosmological parameter priors and intrinsic alignment treatments, we find the constraints (mean, 16% and 84% confidence intervals) on the parameter $S_{8}equiv sigma_{8}(Omega_{rm m}/0.3)^{0.5}$ to be $S_{8}=0.94_{-0.045}^{+0.046}$ (DLS), $0.66_{-0.071}^{+0.070}$ (CFHTLenS), $0.84_{-0.061}^{+0.062}$ (DES-SV) and $0.76_{-0.049}^{+0.048}$ (KiDS-450). From the goodness-of-fit and the Bayesian evidence ratio, we determine that amongst the four surveys, the two more recent surveys, DES-SV and KiDS-450, have acceptable goodness-of-fit and are consistent with each other. The combined constraints are $S_{8}=0.79^{+0.042}_{-0.041}$, which is in good agreement with the first year of DES cosmic shear results and recent CMB constraints from the Planck satellite.
We present a semi-analytic model for the shear two-point correlation function of a cosmic shear survey with non-uniform depth. Ground-based surveys are subject to depth variations that primarily arise through varying atmospheric conditions. For a survey like the Kilo-Degree Survey (KiDS), we find that the measured depth variation increases the amplitude of the observed shear correlation function at the level of a few percent out to degree-scales, relative to the assumed uniform-depth case. The impact on the inferred cosmological parameters is shown to be insignificant for a KiDS-like survey. For next-generation cosmic shear experiments, however, we conclude that variable depth should be accounted for.
We show that it is possible to build effective matter density power spectra in tomographic cosmic shear observations that exhibit the Baryonic Acoustic Oscillations (BAO) features once a nulling transformation has been applied to the data. The precision with which the amplitude and position of these features can be reconstructed is quantified in terms of sky coverage, intrinsic shape noise, median source redshift and number density of sources. BAO detection in Euclid or LSST like wide surveys will be possible with a modest signal-to-noise ratio. It would improve dramatically for slightly deeper surveys.
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