# Dark matter statistics for large galaxy catalogs: power spectra and covariance matrices

## TL;DR

This paper introduces a highly efficient simulation method for dark matter clustering statistics, achieving 1% accuracy in power spectra and revealing complex covariance matrix structures that depend on cosmology.

## Contribution

We developed PPM-GLAM, a new parallel N-body code enabling large-scale simulations with low computational cost, providing detailed insights into the covariance matrix of dark matter power spectra.

## Key findings

- Power spectra accuracy within 1% up to k~1h/Mpc.
- Covariance matrix exhibits non-diagonal complex structure.
- Covariance matrix depends on redshift and cosmological parameters.

## Abstract

Upcoming and existing large-scale surveys of galaxies require accurate theoretical predictions of the dark matter clustering statistics for thousands of mock galaxy catalogs. We demonstrate that this goal can be achieve with our new Parallel Particle-Mesh (PM) Nbody code (PPM-GLAM) at a very low computational cost. We run about 15,000 simulations with ~2 billion particles that provide ~1% accuracy of the dark matter power spectra P(k) for wave-numbers up to k~ 1h/Mpc. Using this large data-set we study the power spectrum covariance matrix, the stepping stone for producing mock catalogs. In contrast to many previous analytical and numerical results, we find that the covariance matrix normalised to the power spectrum C(k,k')/P(k)P(k') has a complex structure of non-diagonal components. It has an upturn at small k, followed by a minimum at k=0.1-0.2h/Mpc. It also has a maximum at k=0.5-0.6h/Mpc. The normalised covariance matrix strongly evolves with redshift: C(k,k')~delta(t)^alpha P(k)P(k'), where delta is the linear growth factor and alpha ~ 1-1.25, which indicates that the covariance matrix depends on cosmological parameters. We also show that waves longer than 1Gpc have very little impact on the power spectrum and covariance matrix. This significantly reduces the computational costs and complexity of theoretical predictions: relatively small volume ~ (1Gpc)^3 simulations capture the necessary properties of dark matter clustering statistics. All the power spectra obtained from many thousands of our simulations are publicly available.