Cosmological weak lensing with the HST GEMS survey

Catherine Heymans*, Michael L. Brown, Marco Barden, John A.R. Caldwell, Knud Jahnke, Chien Y. Peng, Hans Walter Rix, Andy Taylor, Steven V.W. Beckwith, Eric F. Bell, Andrea Borch, Boris Häußler, Shardha Jogee, Daniel H. McIntosh, Klaus Meisenheimer, Sebastian F. Sánchez, Rachel Somerville, Lutz Wisotzki, Christian Wolf

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


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 arcmin 2 in the Chandra Deep Field South. We detect weak lensing by large-scale structure in high-resolution F606W GEMS data from ∼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 Ω m and the amplitude of the matter power spectrum σ 8. We find σ 8m/0.3) 0.65 = 0.68 ± 0.13 where the 1σ 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.

Original languageEnglish
Pages (from-to)160-176
Number of pages17
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Publication statusPublished - 21 Jul 2005


  • Cosmology: observations
  • Gravitational lensing
  • Large-scale structure of Universe


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