Cross-correlation cosmic shear with the SDSS and VLA FIRST surveys

Abstract

We measure the cosmic shear power spectrum on large angular scales by cross-correlating the shapes of ~ 9 million galaxies measured in the optical Sloan Digital Sky Survey (SDSS) with the shapes of ~ 2.7 × 105 radio galaxies measured by the overlapping Very Large Array FIRST (Faint Images of the Radio Sky at Twenty centimetres) survey. Our measurements span the multipole range 10 < ℓ < 130, corresponding to angular scales 2° < θ < 20°. On these scales, the shear maps from both surveys suffer from significant systematic effects that prohibit a measurement of the shear power spectrum from either survey alone. Conversely, we demonstrate that a power spectrum measured by cross-correlating the two surveys is unbiased, reducing the impact of the systematics by at least an order of magnitude. We measure anE-mode power spectrum from the data that is inconsistent with zero signal at the 99 per cent confidence (~ 2.7σ) level. The B-mode and EB cross-correlations are both found to be consistent with zero (within 1σ). These constraints are obtained after a careful error analysis that accounts for uncertainties due to cosmic variance, random galaxy shape noise and shape measurement errors, as well as additional errors associated with the observed large-scale systematic effects in the two surveys. Our constraints are consistent with the expected signal in the concordance cosmological model assuming recent estimates of the cosmological parameters from the Planck satellite, and literature values for the median redshifts of the SDSS and FIRST galaxy populations. The cross-correlation approach will be ideal for extracting robust results, with the exquisite control of systematics required, from future cosmic shear surveys with the Square Kilometre Array, Large Synoptic Survey Telescope, Euclid and WFIRST-AFTA.