Evolution of X-ray clusters of galaxies and shock heating of the intracluster medium

Evolutions of spherical X-ray clusters of galaxies are studied by using an N-body + total variation diminishing (TVD) mesh code. We consider a growth of density perturbation of 10(15) M-circle dot composed of dark matter and gas in a cold dark matter-dominated universe with Omega(0) = 1 or 0.2. When the perturbation collapsed at z similar to 1, a shock front appears at r similar to 0.1 Mpc, moving outward as ambient gas accretes toward cluster center. The shock front separates the inner X-ray-emitting, hot region from the outer cool region. In the former, gas is almost in hydrostatic equilibrium but with small radial infall (similar to 100 km s(-1)) being left, while in the latter, gas falls almost freely and emits no X-rays. Gas inside the shock is strongly compressed and heated by shock so that X-ray luminosity rapidly rises in the early stage (until temperature reaches about virial). In the late stage, on the other hand, 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 result of continuous adiabatic compression inside the shock. We also find that the density distribution is generally less concentrated in a lower density universe and, hence, X-ray luminosity rises more slowly than in a higher density universe. The shock front structure, which was not clearly reserved in the previous SPH simulations, is clearly captured by the present simulations. Our results confirm that shock heating plays an important role in the heating process of the intracluster medium. In addition, we find that a sound wave propagates outward, thereby producing modulations with amplitudes of similar to 10% in the radial temperature and density profiles which, in turn, cause time variations in the strength of the shock. Such modulations, if observed, could be used as a probe to investigate the internal structure of clusters and the initial temperature of gas.