Detection of cosmic shear with the Hubble Space Telescope 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' x 42' set of 28 contiguous Wide Field Planetary Camera 2 (WFPC2) pointings with I < 27. The small size of the HST point-spread function 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. We measure an rms shear of 1.8% on the WFPC2 chip scale (1.'27), in agreement with the predictions of cluster-normalized cold dark matter (CDM) models. Using a maximum likelihood 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 <sigma>(8)Ohm (0.48)(m) = 0.51(-0.17)(+0.14), in a Lambda CDM universe. These 68% CL errors include (Gaussian) cosmic variance, systematic effects, and the uncertainty in the redshift distribution of the background galaxies. The signal comes primarily from the chip scale (1.'27) with gradually decreasing contributions up to roughly 10'. 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.