An investigation of cosmological dark matter using weak gravitational lensing.

Abstract

We present a comprehensive description of all components of cosmic shear measurements via weak gravitational lensing. We begin with a review of the theoretical underpinnings of the field, focusing on the development of shear statistics which can be applied to observed shear fields to constrain cosmological parameters. We then discuss how to accurately estimate shear from galaxy ellipticities. For this purpose, we develop optimal measurements techniques for individual galaxies and an optimal weighting scheme for estimating shear from an ensemble of shapes. Anisotropic seeing will produce shape correlations which, if uncorrected, will mask a lensing signature. The effect can be up to 100 times larger than the shear one is trying to measure, so it is an especially difficult problem. To this end, we develop a novel correction technique which involves a convolution to remove the bias due to the shape of the PSF, followed by an analytic correction for the size of the PSF. We also present two other biases for which one must correct in lensing surveys. Finally, we present results of a 75 square degree survey of galaxy shapes, for which we measure seeing-corrected ellipticities of 2 million galaxies with magnitude <italic>R</italic> &le; 23 in 12 widely separated fields. We detect ellipticity correlations at high signal-to-noise at scales from 1--200<super> '</super>. The signal at scales &gsim; 30<super>'</super> exhibit the nearly pure E-mode behavior. We use this range of the data to find <display-math> <fd> s8Wm/0. 3</fen>0.57=0.69+0.16 -0.18 </fd> </display-math>At smaller scales, we find significant contamination in a B-mode of order half of our E-mode signal, which precludes us from being able to break the degeneracy between s8 and Wm . We conclude with a discussion of the implications of this work and some future directions.