Measurement of Cosmic Shear with the Space Telescope Imaging Spectrograph

Weak lensing by large-scale structure allows a direct measure of the darkmatter distribution. We have used parallel images taken with the SpaceTelescope Imaging Spectrograph (STIS) on the Hubble Space Telescope to measureweak lensing, or cosmic shear. We measure the shapes of 26036 galaxies in 1292STIS fields and measure the shear variance at a scale of 0.51 arcminutes. Thecharge transfer efficiency (CTE) of STIS has degraded over time and introducesa spurious ellipticity into galaxy shapes during the readout process. Wecorrect for this effect as a function of signal to noise and CCD position. Wefurther show that the detected cosmic shear signal is nearly constant in timeover the approximately four years of observation. We detect cosmic shear at the5.1 sigma level, and our measurement of the shear variance is consistent withtheoretical predictions in a LambdaCDM universe. This provides a measure of thenormalization of the mass power spectrum sigma_8=(1.02 +- 0.16)(0.3/Omega_m)^{0.46} (0.21/Gamma)^{0.18}$. The one-sigma error includes noise,cosmic variance, systematics and the redshift uncertainty of the sourcegalaxies. This is consistent with previous cosmic shear measurements, but tendsto favor those with a high value of sigma_8. It is also consistent with therecent determination of sigma_8 from the Wilkinson Microwave Anisotropy Probe(WMAP) experiment.Comment: 9 pages, 5 figure, 1 table, Accepted to Ap