Evolution of x-ray cluster scaling relations in simulations with radiative cooling and nongravitational heating

We investigate the redshift dependence of X-ray cluster scaling relations drawn from three hydrodynamic simulations of the Lambda CDM cosmology: a Radiative'' model that incorporates radiative cooling of the gas, a Preheating'' model that additionally heats the gas uniformly at high redshift, and a Feedback'' model that self-consistently heats cold gas in proportion to its local star formation rate. While all three models are capable of reproducing the observed local L-X-T-X relation, they predict substantially different results at high redshift (to z = 1.5), with the Radiative, Preheating, and Feedback models predicting strongly positive, mildly positive, and mildly negative evolution, respectively. The physical explanation for these differences lies in the structure of the intracluster medium. All three models predict significant temperature fluctuations at any given radius due to the presence of cool subclumps and, in the case of the Feedback simulation, reheated gas. The mean gas temperature lies above the dynamical temperature of the halo for all models at z = 0, but differs between models at higher redshift, with the Radiative model having the lowest mean gas temperature at z = 1.5. We have not attempted to model the scaling relations in a manner that mimics the observational selection effects, nor has a consistent observational picture yet emerged. Nevertheless, evolution of the scaling relations promises to be a powerful probe of the physics of entropy generation in clusters. First indications are that early, widespread heating is favored over an extended period of heating, as is associated with galaxy formation.