Constraints on Omega_m and sigma_8 from weak lensing in RCS fields
Henk Hoekstra, Howard Yee, Mike Gladders
TL;DR
This paper uses ~53 deg$^2$ of $R_C$-band weak-lensing data from the Red-Sequence Cluster Survey to measure cosmic shear and separate E- and B-modes, finding negligible B-modes on scales $>10$ arcmin but a small B-mode at smaller scales likely from intrinsic alignments. By modeling the source redshift distribution and employing a likelihood analysis over $\Omega_m$, $\sigma_8$, and $\Gamma$, marginalizing over priors, they derive joint constraints such as $\sigma_8\approx 0.46\,\Omega_m^{-0.52}$ for Gaussian priors and a conservative $\sigma_8\approx 0.45\Omega_m^{-0.55}$ otherwise. The results are consistent with other cosmic-shear measurements and CMB data when allowing for variations in $\tau$ and $n_s$, and they illustrate how weak-lensing information can constrain the reionization optical depth and redshift. Overall, the work highlights intrinsic-alignments as a key systematic on small scales and demonstrates the value of combining weak lensing with CMB priors to improve cosmological parameter inferences.
Abstract
We have analysed 53 square degrees of 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 degrees. We separate the signal into an ``E''- (lensing) and ``B''-mode (systematics), which allows us to study residual systematics. On scales larger than 10 arcminutes, we find no ``B''-mode. On smaller scales we find a small, but significant ``B''-mode. This signal is also present when we select a sample of bright galaxies. These galaxies are rather insensitive to observational distortions, and we therefore conclude that the oberved ``B''-mode is likely to be caused by intrinsic alignments. We therefore limit the cosmic shear analysis to galaxies with 22<R_C<24. We derive joint constraints on Omega_m and sigma_8, by marginalizing over Gamma, Omega_Lambda and the source redshift distribution, using different priors. We obtain a conservative constraint of $σ_8=0.45^{+0.09}_{-0.12} Ω_m^{-0.55}$ (95% confidence). A better constraint is derived when we use Gaussian priors redshift distribution. For this choice of priors, we find $σ_8=(0.46^{+0.05}_{-0.07})Ω_m^{-0.52}$ (95% confidence). Using our setof Gaussian priors, we find that we can place a lower bound on Gamma: Gamma>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 tau 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 tau. (abridged)