Baryon fractions in clusters of galaxies: Evidence against a pre-heating model for entropy generation

Abstract

The Millennium Gas Project aims to undertake smoothed particle hydrodynamic resimulations of the millennium simulation, providing many hundred massive galaxy clusters for comparison with X-ray surveys (170 clusters with kT sl > 3 keV). This paper looks at the hot gas and stellar fractions of clusters in simulations with different physical heating mechanisms. These fail to reproduce cool-core systems but are successful in matching the hot gas profiles of non-cool-core clusters. Although there is immense scatter in the observational data, the simulated clusters broadly match the integrated gas fractions within r 500. In line with previous work, however, they fare much less well when compared to the stellar fractions, having a dependence on cluster mass that is much weaker than is observed. The evolution with redshift of the hot gas fraction is much larger in the simulation with early pre-heating than in one with continual feedback; observations favour the latter model. The strong dependence of hot gas fraction on cluster physics limits its use as a probe of cosmological parameters. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.