arXiv subjects

Ludovic Van Waerbeke

Publications and source records attributed to Ludovic Van Waerbeke.

79 records · Page 5Linked to original sources

arXiv2006

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.

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arXiv2006

Potential sources of contamination to weak lensing measurements: constraints from N-body simulations

We investigate the expected correlation between the weak gravitational shear of distant galaxies and the orientation of foreground galaxies, through the use of numerical simulations. This shear-ellipticity correlation can mimic a cosmological weak lensing signal, and is potentially the limiting physical systematic effect for cosmology with future high-precision weak lensing surveys. We find that, if uncorrected, the shear-ellipticity correlation could contribute up to 10% of the weak lensing signal on scales up to 20 arcminutes, for lensing surveys with a median depth z=1. The most massive foreground galaxies are expected to cause the largest correlations, a result also seen in the Sloan Digital Sky Survey. We find that the redshift dependence of the effect is proportional to the lensing efficiency of the foreground, and this offers prospects for removal to high precision, although with some model dependence. The contamination is characterised by a weakly negative B-mode, which can be used as a diagnostic of systematic errors. We also provide more accurate predictions for a second potential source of error, the intrinsic alignment of nearby galaxies. This source of contamination is less important, however, as it can be easily removed with distance information.

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arXiv2006

The Shear TEsting Programme 1: Weak lensing analysis of simulated ground-based observations

The Shear TEsting Programme, STEP, is a collaborative project to improve the accuracy and reliability of all weak lensing measurements in preparation for the next generation of wide-field surveys. In this first STEP paper we present the results of a blind analysis of simulated ground-based observations of relatively simple galaxy morphologies. The most successful methods are shown to achieve percent level accuracy. From the cosmic shear pipelines that have been used to constrain cosmology, we find weak lensing shear measured to an accuracy that is within the statistical errors of current weak lensing analyses, with shear measurements accurate to better than 7%. The dominant source of measurement error is shown to arise from calibration uncertainties where the measured shear is over or under-estimated by a constant multiplicative factor. This is of concern as calibration errors cannot be detected through standard diagnostic tests. The measured calibration errors appear to result from stellar contamination, false object detection, the shear measurement method itself, selection bias and/or the use of biased weights. Additive systematics (false detections of shear) resulting from residual point-spread function anisotropy are, in most cases, reduced to below an equivalent shear of 0.001, an order of magnitude below cosmic shear distortions on the scales probed by current surveys. Our results provide a snapshot view of the accuracy of current ground-based weak lensing methods and a benchmark upon which we can improve. To this end we provide descriptions of each method tested and include details of the eight different implementations of the commonly used Kaiser, Squires and Broadhurst (1995) method (KSB+) to aid the improvement of future KSB+ analyses.

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arXiv2003

Constraining Dark Energy Evolution with Gravitational Lensing by Large Scale Structures

We study the sensitivity of weak lensing by large scale structures to the evolution of dark energy. We explore a 2-parameters model of dark energy evolution, inspired by tracking quintessence models. To this end, we compute the likelihood of a few representative models with varying and non varying equation of states. Based on an earlier work, we show that the evolution of dark energy has a much stronger impact on the non-linear structure growth than on the angular diameter distance, which makes large scale structure measurements a very efficient probe of this evolution. For the different models, we investigate the dark energy parameters degeneracies with the mass power spectrum shape Gamma, normalisation sigma_8, and with the matter mean density Omega_M. This result is a strong motivation for performing large scale structure simulations beyond the simple constant dark energy models, in order to calibrate the non-linear regime accurately. Prospective for the Canada France Hawaii Telescope Legacy Survey (CFHTLS) and Super-Novae Acceleration Probe (SNAP) are given. These results complement nicely the cosmic microwave background and Super-Novae constraints. Weak lensing is shown to be more sensitive to a variation of the equation of state, whereas CMB and SNIa give information on its constant part.

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arXiv1999

Statistics of Dark Matter Halos from Gravitational Lensing

We present a new approach to measure the mass function of dark matter halos and to discriminate models with differing values of Omega through weak gravitational lensing. We measure the distribution of peaks from simulated lensing surveys and show that the lensing signal due to dark matter halos can be detected for a wide range of peak heights. Even when the signal-to-noise is well below the limit for detection of individual halos, projected halo statistics can be constrained for halo masses spanning galactic to cluster halos. The use of peak statistics relies on an analytical model of the noise due to the intrinsic ellipticities of source galaxies. The noise model has been shown to accurately describe simulated data for a variety of input ellipticity distributions. We show that the measured peak distribution has distinct signatures of gravitational lensing, and its non-Gaussian shape can be used to distinguish models with different values of Omega. The use of peak statistics is complementary to the measurement of field statistics, such as the ellipticity correlation function, and possibly not susceptible to the same systematic errors.

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arXiv1998

The evolution rate of quasars at various redshifts

The evolution of optically selected quasars is usually supposed to be well described by a single constant parameter, either k_L or k_D, depending whether we refer to luminosity or density evolution. In this paper we present a study of the variations of the evolution parameters with redshift, for different cosmological models, in order to probe the differential evolution with redshift. Two different quasar samples have been analyzed, the AAT Boyle's et al. and the LBQS catalogues. Basically, these samples are divided in redshift intervals and in each of them k_L and k_D are estimated by forcing that =0.5. The dependence with respect to the cosmological parameters is small. Both AAT and LBQS show roughly the same tendencies. LBQS, however, shows strong fluctuations, whose origin is not statistical but rather due to the selection criteria. A discussion on selection techniques, biases and binning effects explains the differences between these results. We finally conclude that the evolution parameter is almost constant in the redshift range 0.7 < z < 1.7, at least within 2 sigma, while it decreases slightly afterwards. Results depend on the binning chosen (but not in a very significant way). The method has been tested with Monte-Carlo simulated catalogues in order to give a better understanding of the results coming from the real catalogues. A correlation between k_L (k_D) and is also derived and is used for the calculation of the error bars on the evolution parameter.

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arXiv1997

Scale dependence of the bias investigated by weak lensing

The statistical analysis of the lensing effects coupled with the statistical analysis of the number counts is a tool to probe directly the relation between the mass and the light. In particular, some properties of the bias parameter can be investigated. The correlation between the shear of a given population of galaxies, and the number counts of a different population of galaxies along the same line of sight is calculated for the linear and the non-linear power spectra of density fluctuations for different cosmologies. The estimator R defined as the ratio of this correlation and the variance of the number counts is inversely proportional to the bias parameter. Unfortunately, R is highly dependent on the cosmological model, which makes a direct measure of the bias quite difficult. However, it is showed that Rb is independent on the power spectrum and the smoothing scale, thus R is a direct measure of the inverse of the bias times a function of the cosmological parameters. From R, a new estimator is defined which is only sensible to the scale dependence of the bias. It is showed that with a sample of 25 square degrees, one can measure a scale variation of the bias larger than 20% in the 1' to 10' scale range, almost independently of the cosmological parameters, the redshift distribution of the galaxies, and the power spectrum.

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