Exploring the energetics of intracluster gas with a simple and accurate model

Abstract

The state of the hot gas in clusters of galaxies is investigated with a set of model clusters, created by assuming a polytropic equation of state (Γ = 1.2) and hydrostatic equilibrium inside gravitational potential wells drawn from a dark matter (DM) simulation. Star formation, energy input, and nonthermal pressure support are included. To match the gas fractions seen in nonradiative hydrodynamical simulations, roughly 5% of the binding energy of the DM must be transferred to the gas during cluster formation; the presence of nonthermal pressure support increases this value. In order to match X-ray observations, scale-free behavior must be broken. This can be due to either variation of the efficiency of star formation with cluster mass M 500, or the input of additional energy proportional to the formed stellar mass MF . These two processes have similar effects on X-ray scalings. If 9% of the gas is converted into stars, independent of cluster mass, then feedback energy input of 1.2 × 10-5 MFc 2 (or ∼ 1.0keV per particle) is required to match observed clusters. Alternatively, if the stellar mass fraction varies as α M -0.26500 then a lower feedback of 4 × 10-6 MFc 2 is needed, and if the stellar fraction varies as steeply as α M -0.49500 then no additional feedback is necessary. The model clusters reproduce the observed trends of gas temperature and gas mass fraction with cluster mass, as well as observed entropy and pressure profiles; thus they provide a calibrated basis with which to interpret upcoming Sunyaev-Zeldovich surveys. One consequence of the increased gas energy is that the baryon fraction inside the virial radius is ≲90% of the cosmic mean, even for the most massive clusters. © 2009. The American Astronomical Society. All rights reserved..