Cosmic Shear and Power Spectrum Normalization with the Hubble Space Telescope

TL;DR

This paper delivers a high-significance space-based measurement of cosmic shear using 271 WFPC2 fields from the HST Medium Deep Survey to constrain the amplitude of matter fluctuations. A robust estimator for the shear variance is developed and applied, correcting for camera distortion, PSF anisotropy, noise, and systematics, with a depth-weighted combination across fields. The authors obtain a chip-scale shear variance that yields $\sigma_{8} = (0.94 \pm 0.10 \pm 0.14) (\tfrac{0.3}{\Omega_m})^{0.44} (\tfrac{0.21}{\Gamma})^{0.15}$, and find consistency with other cosmic-shear measurements while noting mild tension with the latest cluster-based normalizations. A combined analysis of four cosmic-shear studies gives $\sigma_{8} \approx 0.91 \pm 0.02$ for a $\Lambda$CDM model, underscoring the role of weak lensing in cross-checking cosmological parameter estimates and highlighting potential systematics in cluster-based inferences. These results support the viability of space-based weak lensing and motivate careful calibration for future surveys.

Abstract

Weak lensing by large-scale structure provides a direct measurement of matter fluctuations in the universe. We report a measurement of this `cosmic shear' based on 271 WFPC2 archival images from the Hubble Space Telescope Medium Deep Survey (MDS). Our measurement method and treatment of systematic effects were discussed in an earlier paper. We measure the shear variance on scales ranging from 0.7' to 1.4', with a detection significance greater than 3.8. This allows us to measure the normalization of the matter power spectrum to be sigma8 = (0.94 +/- 0.10 +/- 0.14) (0.3/Omega_m)^0.44 (0.21/Gamma)^0.15, in a LCDM universe. The first 1sigma error includes statistical errors only, while the latter also includes (gaussian) cosmic variance and the uncertainty in the galaxy redshift distribution. Our results are consistent with earlier cosmic shear measurements from the ground and from space. We compare our cosmic shear results and those from other groups to the normalization from cluster abundance and galaxy surveys. We find that the combination of four recent cosmic shear measurements are somewhat inconsistent with the recent normalization using these methods, and discuss possible explanations for the discrepancy.