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

TL;DR

This paper investigates how survey geometry affects super-sample covariance in photometric galaxy surveys and introduces a new approximation method to improve cosmological parameter forecasts, especially for small survey areas.

Contribution

The authors develop a fast partial-sky SSC approximation that accounts for survey geometry, improving upon the common full-sky rescaling method for future galaxy surveys.

Findings

Full-sky approximation underestimates errors for small surveys.

Partial-sky method accurately captures covariance effects across survey sizes.

Errors are absorbed by nuisance parameter marginalization for large 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.