The potential of X-ray cluster surveys to constrain primordial non-Gaussianity
Abstract
We present forecasts for constraints on deviations from Gaussian distribution of primordial density perturbations from future X-ray surveys of galaxy clusters. Our analysis is based on computing the Fisher-Matrix for number counts and large-scale power spectrum of clusters. We consider a survey with high-sensitivity and wide-area to detect about 2.5 x 105 extended sources. Based on the self-calibration approach, and including Planck priors in our analysis, we constrain 9 cosmological parameters and 4 nuisance parameters, which define the relation between cluster mass and X-ray flux. Because of the scale dependence of large-scale bias induced by local-shape non-Gaussianity, we find that the power spectrum provides strong constraints on the non-Gaussianity fnl parameter, which complement the stringent constraints on the power spectrum normalization, σ8, from the number counts. To quantify the joint constraints on theese two parameters, that specify the timing of structure formation for a fixed background expansion, we define the figure-of-merit FoMSFT = (det[Cov(sigma8,fnl)])-1/2. We find that our surveys constrain deviations from Gaussianity with a precision of fnl~10 at 1 σ confidence level, with FoMSFT~39. We point out that constraints on fnl are weakly sensitive to the uncertainties in the knowledge of nuisance parameters. As an application of non-Gaussian constraints from available data, we analyse the impact of positive skewness on the occurrence of XMMU-J2235, a massive distant cluster discovered at z~1.4. We confirm that in a WMAP-7 Gaussian Lambda CDM cosmology, within the survey volume, ~5x10-3 objects like this are expected to be found: to increase this probability by a factor of at least 10, one needs to evade either the available constraints on fnl or sigma8.