A Comparison of Cosmological Codes: Properties of Thermal Gas and Shock Waves in Large Scale Structures

TL;DR

This study compares three cosmological simulation codes to analyze thermal gas properties and shock waves in large-scale structures, revealing both agreements and significant differences especially between grid and SPH methods.

Contribution

It provides a detailed comparison of Eulerian and Lagrangian codes in simulating thermal gas and shock properties, highlighting key differences and uncertainties.

Findings

Significant differences in gas density, temperature, and entropy between codes.

Evidence of an entropy core inside galaxy clusters in grid simulations.

Distinct phase diagrams and shock morphologies between methods.

Abstract

[...] We present results for the statistics of thermal gas and the shock wave properties for a large volume simulated with three different cosmological numerical codes: the Eulerian total variations diminishing code TVD, the Eulerian piecewise parabolic method-based code ENZO, and the Lagrangian smoothed-particle hydrodynamics code GADGET. Starting from a shared set of initial conditions, we present convergence tests for a cosmological volume of side-length 100 Mpc/h, studying in detail the morphological and statistical properties of the thermal gas as a function of mass and spatial resolution in all codes. By applying shock finding methods to each code, we measure the statistics of shock waves and the related cosmic ray acceleration efficiencies, within the sample of simulations and for the results of the different approaches. We discuss the regimes of uncertainties and disagreement among codes, with a particular focus on the results at the scale of galaxy clusters. Even if the bulk of thermal and shock properties are reasonably in agreement among the three codes, yet some significant differences exist (especially between Eulerian methods and smoothed particle hydrodynamics). In particular, we report: a) differences of huge factors (10-100) in the values of average gas density, temperature, entropy, Mach number and shock thermal energy flux in the most rarefied regions of the simulations between grid and SPH methods; b) the hint of an entropy core inside clusters simulated in grid codes; c) significantly different phase diagrams of shocked cells in grid codes compared to SPH; d) sizable differences in the morphologies of accretion shocks between grid and SPH methods.