FORMATION AND EVOLUTION OF X-RAY-CLUSTERS - A HYDRODYNAMIC SIMULATION OF THE INTRACLUSTER MEDIUM

The X-ray-emitting intracluster medium (ICM) in clusters of galaxies is studied using a combined three-dimensional hydrodynamic and N-body simulation algorithn with the assumption that clusters formed via hierarchical clustering in a cold dark matter-dominated universe with ICM fraction ΩICM = 0.1. The ICM is treated as an ideal γ = 5/3 gas undergoing shock heating within the self-consistency evolving dark matter potential. The evolution of a single, Coma-richness cluster is examined in detail. A core of ∼ 1013 M⊙ of gas collapses by z = 1 and grows through mergers and accretion of surrounding material. The process can be crudely defined by a shock front moving outward at ∼400 km s-1 reaching a radius ∼5h50-1 Mpc at the present epoch. The 2 × 1014h50-1 M⊙ of gas within an Abell radius at z = 0 is close to isothermal at T = 7.2 keV with evidence for a modest temperature inversion in the cluster center. Polytropic models provide a poor description of the ICM thermodynamic state. The density profile of the cluster shows no resolved core radius, although a flattening of the logarithmic slope is apparent at radii r ≲ 500h50-1 kpc. The total 2-10 keV luminosity of 7.5 × 1044h50 ergs s-1 is therefore subject to an uncertain core contribution. A Sunyaev-Zel'dovich central decrement of roughly 0.5 mK would be expected from this cluster at redshifts z ≲ 0.25 at 1′ resolution. The signal drops below 0.1 mK at z = 0.5 Investigation of the standard hydrostatic isothermal β-model shows that estimates of the specific energy ratio β= σ2/(kT/μmp) based on surface brightness profiles are systematically biased. The discrepancy arises from the existence of residual kinetic energy in the shock-heated gas and poor modeling of the underlying binding mass distribution. Simple binding mass estimates based on this model underestimate the actual binding mass within an Abell radius by ∼ 30%. The mass contained within an Abell radius for Coma and A2256 is estimated to be 2.4 and 2.6 × 1015h50-1 M⊙, respectively, after correcting for these effects.