Detection of Cosmic Shear with the HST Survey Strip

Weak lensing by large-scale structure provides a unique method to directly measure matter fluctuations in the universe, and has recently been detected from the ground. Here, we report the first detection of this `cosmic shear based on space-based images. The detection was derived from the Hubble Space Telescope (HST) Survey Strip (or Groth Strip), a 4 by 42 set of 28 contiguous WFPC2 pointings with I<27. The small size of the HST Point-Spread Function (PSF) affords both a lower statistical noise, and a much weaker sensitivity to systematic effects, a crucial limiting factor of cosmic shear measurements. Our method and treatment of systematic effects were discussed in an earlier paper (Rhodes, Refregier & Groth 2000). We measure an rms shear of 1.8% on the WFPC2 chip scale (1.27), in agreement with the predictions of cluster-normalized CDM models. Using a Maximum Likelihood (ML) analysis, we show that our detection is significant at the 99.5% confidence level (CL), and measure the normalization of the matter power spectrum to be sigma8*Omega_m^(0.48) = 0.51 (+0.14,-0.17), in a LambdaCDM universe. These 68% CL errors include (Gaussian) cosmic variance, systematic effects and the uncertainty in the redshift distribution of the background galaxies. Our result is consistent with earlier lensing measurements from the ground, and with the normalization derived from cluster abundance. We discuss how our measurement can be improved with the analysis of a large number of independent WFPC2 fields.