Cosmic shear from STIS pure parallels - II. Analysis

We report on the marginal detection of cosmic shear on sub-arcmin scales with archive data from the STIS camera on board HST. For the measurement 121 galaxy fields with a field of view of 51'' x 51'' are used to obtain an rms cosmic shear of ~ 4% with 1.5sigma significance. This value is consistent with groundbased results obtained by other groups on larger scales, and with theoretical predictions for a standard LambdaCDM cosmology. To show the suitability of STIS for weak shear measurements we carefully investigated the stability of the PSF. We demonstrate that small temporal changes do not affect the cosmic shear measurement by more than ~ 10%. We also discuss the influence of various weighting and selection schemes for the galaxy ellipticities.