Impact of survey geometry and super-sample covariance on future photometric galaxy surveys

Abstract

Photometric galaxy surveys probe the late-time Universe where the density field is highly non-Gaussian. A consequence is the emergence of the super-sample covariance (SSC), a non-Gaussian covariance term that is sensitive to fluctuations on scales larger than the survey window. In this work, we study the impact of the survey geometry on the SSC and, subsequently, on cosmological parameter inference. We devise a fast SSC approximation that accounts for the survey geometry and compare its performance to the common approximation of rescaling the results by the fraction of the sky covered by the survey, $f_{\mathrm{SKY}}$, dubbed 'full-sky approximation'. To gauge the impact of our new SSC recipe, dubbed 'partial-sky', we perform Fisher forecasts on the parameters of the $(w_0,w_a)$-CDM model in a 3x2 points analysis, varying the survey area, the geometry of the mask and the galaxy distribution inside our redshift bins. The differences in the marginalised forecast errors, with the full-sky approximation performing poorly for small survey areas but excellently for stage-IV-like areas, are found to be absorbed by the marginalisation on galaxy bias nuisance parameters. For large survey areas, the unmarginalised errors are underestimated by about 10% for all probes considered. This is a hint that, even for stage-IV-like surveys, the partial-sky method introduced in this work will be necessary if tight priors are applied on these nuisance parameters.

12 pages, 11 figures, 2 tables, 2 appendices. Published version in A&A

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• Euclid: Covariance of weak lensing pseudo-$C_{\ell }$ estimates. Calculation, comparison to simulations, and dependence on survey geometry
• Testing the accuracy of likelihoods for cluster abundance cosmology
• Euclid preparation. LII. Forecast impact of super-sample covariance on 3x2pt analysis with Euclid
• Efficient computation of the super-sample covariance for stage IV galaxy surveys
• What is the super-sample covariance? A fresh perspective for second-order shear statistics
• Cosmological inference including massive neutrinos from the matter power spectrum: biases induced by uncertainties in the covariance matrix
• Improving CMB constraints on early Universe physics with LSS: A multi-probe forecast including cross-covariance
• Super sample covariance and the volume scaling of galaxy survey covariance matrices
• Towards including super-sample covariance in the unbinned likelihood for cluster abundance cosmology