ALBERT

Description: Numerical N -body simulations play a central role in the assessment of weak gravitational lensing statistics, residual systematics and error analysis. In this paper, we investigate and quantify the impact of finite simulation volume on weak lensing two- and four-point statistics. These finite support (FS) effects are modelled for several estimators, simulation box sizes and source redshifts, and validated against a new large suite of 500 N -body simulations. The comparison reveals that our theoretical model is accurate to better than 5 per cent for the shear correlation function + () and its error. We find that the most important quantities for FS modelling are the ratio between the measured angle and the angular size of the simulation box at the source redshift, box ( z s ), or the multipole equivalent / box ( z s ). When this ratio reaches 0.1, independently of the source redshift, the shear correlation function + is suppressed by 5, 10, 20 and 25 per cent for L box  = 1000, 500, 250 and 147 h –1 Mpc, respectively. The same effect is observed in – (), but at much larger angles. This has important consequences for cosmological analyses using N -body simulations and should not be overlooked. We propose simple semi-analytic correction strategies that account for shape noise and survey masks, generalizable to any weak lensing estimator. From the same simulation suite, we revisit the existing non-Gaussian covariance matrix calibration of the shear correlation function, and propose a new one based on the 9-year Wilkinson Microwave Anisotropy Probe )+baryon acoustic oscillations+supernova cosmology. Our calibration matrix is accurate at 20 per cent down to the arcminute scale, for source redshifts in the range 0 〈  z  〈 3, even for the far off-diagonal elements. We propose, for the first time, a parametrization for the full – covariance matrix, also 20 per cent accurate for most elements.

Description: The effect of baryonic feedback on the dark matter mass distribution is generally considered to be a nuisance to weak gravitational lensing. Measurements of cosmological parameters are affected as feedback alters the cosmic shear signal on angular scales smaller than a few arcminutes. Recent progress on the numerical modelling of baryon physics has shown that this effect could be so large that, rather than being a nuisance, the effect can be constrained with current weak lensing surveys, hence providing an alternative astrophysical insight on one of the most challenging questions of galaxy formation. In order to perform our analysis, we construct an analytic fitting formula that describes the effect of the baryons on the mass power spectrum. This fitting formula is based on three scenarios of the OverWhelmingly Large hydrodynamical simulations. It is specifically calibrated for z  〈 1.5, where it models the simulations to an accuracy that is better than 2 per cent for scales k  〈 10 h Mpc –1 and better than 5 per cent for 10 〈  k  〈 100 h Mpc –1 . Equipped with this precise tool, this paper presents the first constraint on baryonic feedback models using gravitational lensing data, from the Canada France Hawaii Telescope Lensing Survey (CFHTLenS). In this analysis, we show that the effect of neutrino mass on the mass power spectrum is degenerate with the baryonic feedback at small angular scales and cannot be ignored. Assuming a cosmology precision fixed by WMAP9 , we find that a universe with massless neutrinos is rejected by the CFHTLenS lensing data with 85–98 per cent confidence, depending on the baryon feedback model. Some combinations of feedback and non-zero neutrino masses are also disfavoured by the data, although it is not yet possible to isolate a unique neutrino mass and feedback model. Our study shows that ongoing weak gravitational lensing surveys (KiDS, HSC and DES) will offer a unique opportunity to probe the physics of baryons at galactic scales, in addition to the expected constraints on the total neutrino mass.