Evolution of X‐Ray Clusters of Galaxies and Shock Heating of the Intracluster Medium

Abstract

Evolutions of X-ray clusters of galaxies are studied by N-body (shell model) + mesh code (TVD) simulations on the assumption of spherical symmetry. We consider a density perturbation of $10^{15} M_{\odot}$ composed of dark matter and gas in cold dark matter dominated universe with the cosmological density parameter, $\Omega_0 = 1$ or $0.2$. A shock front appears during its initial collapse, moving outward as ambient gas accretes towards cluster center. The shock front separates the inner X-ray emitting, hot region, where gas is almost in hydrostatic equilibrium but with small radial infall ($\sim 100$km s$^{-1}$) being left, from the outer cool region. Gas inside the shock is strongly compressed and heated by shock so that X-ray luminosity rapidly rises in the early stage. In the late stage, however, the X-ray luminosity rises only gradually due partly to the expansion of the inner high temperature region and partly to the increase of X-ray emissivity of gas as the results of continuous adiabatic compression inside the shock. The shock front structure, which was not clearly resolved in the previous SPH simulations, is clearly captured by the present simulations. In addition, we find a sound wave propagating outward, thereby producing spatial modulations with amplitudes of $\sim$ 10 % in the radial temperature and density profiles and time variations in the strength of the shock. Such modulations, if observed, could be used as a probe to investigate the structure of clusters.