Effects of preheating on X-ray scaling relations in galaxy clusters

The failure of purely gravitational and gasdynamical models of X-ray cluster formation to reproduce basic observed properties of the local cluster population suggests the need for one or more additional physical processes operating on the intracluster medium (ICM). We present results from 84 moderate-resolution gasdynamic simulations designed to investigate systematically the effects of preheating (an early elevated ICM adiabat) on the resultant, local X-ray size-temperature, luminosity-temperature, and ICM mass-temperature relations. Seven sets of 12 simulations are performed for a Lambda CDM cosmology, each set characterized by a different initial entropy level S-i. The slopes of the observable relations steepen monotonically as S-i is increased. Observed slopes for all three relations are reproduced by models with S-i epsilon 55-150 keV cm(2), levels that compare favorably to empirical determinations of core ICM entropy by Lloyd-Davies, Ponman, & Cannon. The redshift evolution for the case of a locally successful model with S-i = 106keV cm(2) is presented. At temperatures kT greater than or similar to 3keV, little or no evolution in physical isophotal sizes or bolometric luminosities is expected to z less than or similar to1. The ICM and total masses at fixed T are lower at higher z, as expected from the virial theorem. ICM mass fractions show a mild T dependence. Clusters with kT less than or similar to 3 keV contain ICM mass fractions depressed by modest amounts (less than or similar to 25%) below the cosmic mean baryon fraction Omega (b)/Omega (m); hot clusters subject to preheating remain good tracers of the cosmic mix of clustered mass components at redshifts z less than or similar to 1.