Cosmic Shear And Power Spectrum Normalization With The Hubble Space Telescope

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 photographs from the Hubble Space Telescope Medium Deep Survey (MDS). Our measurement method and therapy of systematic results have been discussed in an earlier paper. Our outcomes are per earlier cosmic shear measurements from the ground and from space. We examine our cosmic shear outcomes and Wood Ranger Tools people from other teams to the normalization from cluster abundance and galaxy surveys. We find that the mix of 4 latest cosmic shear measurements are considerably inconsistent with the current normalization using these strategies, and talk about possible explanations for the discrepancy. Weak gravitational lensing by giant-scale structure has been shown to be a precious method of measuring mass fluctuations within the universe (see Mellier at al. This effect has been detected both from the ground (Wittman et al.



2000; van Waerbeke et al. 2000, 2001; Bacon et al. 2000, 2002; Kaiser et al. 2000; Hoekstra et al. 2002) and from area (Rhodes, Refregier, & Groth 2001, RRGII; Hämmerle et al. 2001). These results bode properly for the prospect of measuring cosmological parameters and the mass distribution of the universe using weak lensing. On this letter, we current the best significance detection of cosmic shear using area-primarily based photos. It is based on photographs from the Hubble Space Telescope (HST) Medium Deep Survey (MDS; Ratnatunga et al. 1999). We apply the strategies for the correction of systematic results and detection of shear we've beforehand Wood Ranger Power Shears coupon Ranger Power Shears manual developed (Rhodes, Wood Ranger Tools Refregier, and Groth 2000; RRGI) to 271 WFPC2 fields within the MDS. 0.8" from the ground). This affords us a better surface density of resolved galaxies in addition to a diminished sensitivity to PSF smearing when in comparison with ground-primarily based measurements. We develop an optimum depth-weighted common of chosen MDS fields to extract a weak lensing sign.



We then use this signal to derive constraints on the amplitude of the mass power spectrum and evaluate this to measurements from earlier cosmic shear surveys and from different methods. The MDS consists of major and parallel observations taken with the Wide Field Planetary Camera 2 (WFPC2) on HST. We chosen solely the I-band images in chips 2,3, and 4 to study weak lensing. To make sure random strains-of-sight, we discarded fields which were pointed at galaxy clusters, leaving us with 468 I-band fields. We used the MDS object catalogs (Ratnatunga et al. 1999) to find out the position, magnitude, and area of every object, in addition to to separate galaxies from stars. We used the chip-particular backgrounds listed within the MDS skysig information, which are in line with backgrounds calculated utilizing the IRAF activity imarith. Not using object-specific backgrounds necessitated the discarding of another 20 fields with a large sky gradient. Our remaining catalog thus consisted of 271 WFPC2 fields amounting to an area of about 0.36 deg2.



The procedure we used for measuring galaxy ellipticities and shear from the supply photos is described intimately in RRGI (1999) (see also RRGII and Rhodes 1999). It is based on the method launched by Kaiser, Squires, and Broadhurst (1995), however modified and examined for applications to HST pictures. The usefulness of our method was demonstrated by our detection of cosmic shear in the HST Groth Strip (RRGII). We correct for digicam distortion and convolution by the anisotropic PSF utilizing gaussian-weighted moments. Camera distortions were corrected utilizing a map derived from stellar astrometric shifts (Holtzman, et al., 1995). PSF corrections were decided from HST observations of 4 stellar fields These fields were chosen to span the main target vary of the HST as proven by Biretta et al. G𝐺G is the shear susceptibility factor given by equation (30) in RRGI. To restrict the affect of noise and systematics, we made a number of cuts to pick out our galaxy sample.