Cosmological weak lensing with the HST GEMS survey

We present our cosmic shear analysis of {GEMS}, one of the largest wide-field surveys ever undertaken by the Hubble Space Telescope. Imaged with the Advanced Camera for Surveys ({ACS)}, {GEMS} spans 795 arcmin2 in the Chandra Deep Field South. We detect weak lensing by large-scale structure in high-resolution {F606W} {GEMS} data from {\textasciitilde}60 resolved galaxies per square arcminute. We measure the two-point shear correlation function, the top-hat shear variance and the shear power spectrum, performing an {E/B} mode decomposition for each statistic. We show that we are not limited by systematic errors and use our results to place joint constraints on the matter density parameter ??m and the amplitude of the matter power spectrum ??8. We find ??8(??m/0.3)0.65= 0.68 +/- 0.13 where the 1?? error includes both our uncertainty on the median redshift of the survey and sampling variance. Removing image and point spread function ({PSF)} distortions are crucial to all weak lensing analyses. We therefore include a thorough discussion on the degree of {ACS} {PSF} distortion and anisotropy which we characterize directly from {GEMS} data. Consecutively imaged over 20 d, {GEMS} data also allow us to investigate {PSF} instability over time. We find that, even in the relatively short {GEMS} observing period, the {ACS} {PSF} ellipticity varies at the level of a few per cent which we account for with a semi-time-dependent {PSF} model. Our correction for the temporal and spatial variability of the {PSF} is shown to be successful through a series of diagnostic tests.