A Hydrodynamic Simulation of Cluster Formation

Greg L. Bryan
Michael L. Norman
National Center for Supercomputing Applications
5600 Beckman Institute, D-25, 405 N. Mathews Ave., Urbana, IL 61801; and Department of Astronomy, University of Illinois at Urbana-Champaign

Abstract

We have developed a hybrid cosmological code to probe the baryonic gas dynamics of large cosmological structures. An adiabatic simulation of large scale structure formation has been performed, using initial conditions specified by the cold dark matter (CDM) power spectrum normalized to the COBE detection. A baryon fraction of :06
is adopted and the simulation follows an 85h?1 Mpc box with 25 million zones and 3 million particles. Analysis of the resulting data provides a powerful diagnostic of this particular model. The method shows promise in general by removing another layer of uncertainty between theory and observation. To demonstrate this, we compute an X-ray luminosity function that is straightforward to compare with observations. We also investigate some astrophysics of cluster formation.

1. Introduction

Although relatively rare, the high X-ray brightness of the large clusters allows them to be seen for great distances and complete samples of relatively nearby clusters have been developed9. Previously, others have used the cold dark matter (CDM) prescription to make a quantitative prediction of cluster statistics expected from such a model8;10. These comparisons evolved only the dark, collisionless component, identifying galaxies with the centers of dark halos and comparing the derived clusters against optical compilations, such as Abell's catalog. There are, however, several steps in the analysis, such as identifying cluster characteristics (due to the absence of baryonic matter in the simulations) and comparison against optical observations (projection effects) which increase the uncertainty of the results. Here we model both the hot gas as well as the dark matter and compare the result against X-ray observations which are largely insensitive to projection effects.

The numerical method combines a particle-mesh code which models the dark matter as collisionless particles with a Eulerian grid-based solver for the hydrodynamics. This is based on the third order-accurate piecewise parabolic method4. We have implemented this scheme for cosmological systems and have developed a comprehensive test suite3.

The results described below were computed with initial conditions described by a CDM power spectrum, normalized to COBE6. This implies that the variance of the mass overdensity within spheres of radius 8h?1 Mpc is oe28 = 1:1. A periodic box with 2703 zones and