Constraints on Ωmand σ8from Weak Lensing in Red‐Sequence Cluster Survey Fields

Abstract

We have analyzed 53 deg 2 of R C -band imaging data from the Red-Sequence Cluster Survey (RCS) and measured the excess correlations in the shapes of galaxies on scales out to ~1 5. We separate the signal into an "E" (lensing) and "B" (systematics) mode, which allows us to study the contribution from residual systematics and intrinsic alignments. On scales larger than 10', we find no B mode, suggesting that the signal at those scales is caused solely by gravitational lensing. On smaller scales we find a small but significant B mode. This signal is also present when we select a sample of bright (20 < R C < 22) galaxies. These galaxies are rather insensitive to observational distortions, and we therefore conclude that the observed B mode is likely to be caused by intrinsic alignments. To minimize the effect of intrinsic alignments, we limit the cosmic shear analysis to galaxies with 22 < R C < 24. We derive joint constraints on Ω m and σ 8 by marginalizing over Γ, Ω Λ , and the source redshift distribution, using different priors. Marginalizing over Γ and Ω Λ , and using a flat prior for the source redshift distribution, yields a conservative constraint of σ 8 = 0.45 Ω (95% confidence). A better constraint is derived when we use Gaussian priors for Γ (from the Two-Degree Field survey) and Ω m + Ω Λ (from cosmic microwave background [CMB]), and the source redshift distribution. For this choice of priors, we find σ 8 = (0.46 )Ω (95% confidence). We also investigated whether the RCS data can be used to constrain Γ. Using our set of Gaussian priors, we find that we can only place a lower bound on Γ for which we find Γ > 0.1 + 0.16Ω m (95% confidence). Comparison of the RCS results with three other recent cosmic shear measurements shows excellent agreement. The current weak-lensing results are also in good agreement with CMB measurements, when we allow the reionization optical depth τ and the spectral index n s to vary. We present a simple demonstration of how the weak-lensing results can be used as a prior in the parameter estimation from CMB measurements to derive constraints on the reionization optical depth τ.