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

The failure of purely gravitational and gasdynamical models of X-ray cluste r formation to reproduce basic observed properties of the local cluster pop ulation suggests the need for one or more additional physical processes ope rating on the intracluster medium (ICM). We present results from 84 moderat e-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-tempera ture 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 in creased. Observed slopes for all three relations are reproduced by models w ith S-i epsilon 55-150 keV cm(2), levels that compare favorably to empirica l determinations of core ICM entropy by Lloyd-Davies, Ponman, & Cannon. The redshift evolution for the case of a locally successful model with S-i = 1 06keV cm(2) is presented. At temperatures kT greater than or similar to 3ke V, little or no evolution in physical isophotal sizes or bolometric luminos ities is expected to z less than or similar to1. The ICM and total masses a t fixed T are lower at higher z, as expected from the virial theorem. ICM m ass fractions show a mild T dependence. Clusters with kT less than or simil ar to 3 keV contain ICM mass fractions depressed by modest amounts (less th an 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.