Cosmological Simulations of Number Counts

TL;DR

This paper presents the first relativistic N-body simulation results for angular power spectra of number counts, comparing them with theoretical models and analyzing the contributions of various effects, highlighting the accuracy of certain approximations.

Contribution

It introduces non-perturbative relativistic simulation results for number counts and evaluates the accuracy of existing theoretical approximations and models.

Findings

HMCODE approximation matches non-perturbative results well for density and convergence.

Current redshift-space distortion models in Boltzmann codes are inaccurate.

Cross-correlation of density and lensing dominates unequal-redshift spectra.

Abstract

In this paper we present for the first time the angular power spectra $C_\ell(z,z')$ for number counts from relativistic N-body simulations. We use the relativistic N-body code gevolution with its exact integration of lightlike geodesics which include all relativistic scalar contributions to the number counts. We compare our non-perturbative numerical results with the results from CLASS using the HMCODE approximation for the non-linear matter power spectrum. We find that this simple description is excellent for both, the density and the convergence. On the other hand, the current implementation of redshift-space distortions in Boltzmann codes is not accurate. We also find that the largest contribution to the unequal-redshift power spectra is the cross-correlation of the density and the lensing contribution to the number counts, especially for redshift bins that are far apart. Correlating the number counts with the convergence map we find that the signal is dominated by the lensing-lensing term when the convergence field redshift is not higher than the number counts one, while it is dominated by the density-lensing term in the opposite case. In the present study, the issue of galaxy bias is deliberately left aside by considering only unbiased samples of matter particles from the simulations.