Comparing approximate methods for mock catalogues and covariance matrices I: correlation function

TL;DR

This study compares seven approximate methods for estimating covariance matrices of the anisotropic two-point correlation function in galaxy clustering, assessing their accuracy against full N-body simulations for cosmological parameter inference.

Contribution

It provides a comprehensive comparison of various approximate methods for covariance estimation, highlighting their accuracy and impact on cosmological analyses.

Findings

All methods recover mean parameters within 5-10% of N-body results.

Parameter uncertainties agree within 5-10% with N-body covariances.

Most methods yield similar results, with no clear best approach.

Abstract

This paper is the first in a set that analyses the covariance matrices of clustering statistics obtained from several approximate methods for gravitational structure formation. We focus here on the covariance matrices of anisotropic two-point correlation function measurements. Our comparison includes seven approximate methods, which can be divided into three categories: predictive methods that follow the evolution of the linear density field deterministically (ICE-COLA, Peak Patch, and Pinocchio), methods that require a calibration with N-body simulations (Patchy and Halogen), and simpler recipes based on assumptions regarding the shape of the probability distribution function (PDF) of density fluctuations (log-normal and Gaussian density fields). We analyse the impact of using covariance estimates obtained from these approximate methods on cosmological analyses of galaxy clustering measurements, using as a reference the covariances inferred from a set of full N-body simulations. We find that all approximate methods can accurately recover the mean parameter values inferred using the N-body covariances. The obtained parameter uncertainties typically agree with the corresponding N-body results within 5% for our lower mass threshold, and 10% for our higher mass threshold. Furthermore, we find that the constraints for some methods can differ by up to 20% depending on whether the halo samples used to define the covariance matrices are defined by matching the mass, number density, or clustering amplitude of the parent N-body samples. The results of our configuration-space analysis indicate that most approximate methods provide similar results, with no single method clearly outperforming the others.