Effects of Preheating on X‐Ray Scaling Relations in Galaxy Clusters

Abstract

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 ΛCDM 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 ∈ 55-150 keV cm², 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 = 106 keV cm² is presented. At temperatures kT 3 keV, little or no evolution in physical isophotal sizes or bolometric luminosities is expected to z 1. 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 3 keV contain ICM mass fractions depressed by modest amounts (25%) below the cosmic mean baryon fraction Ω_b/Ω_m; hot clusters subject to preheating remain good tracers of the cosmic mix of clustered mass components at redshifts z 1.