The Effect of Cooling on the Density Profile of Hot Gas in Clusters of Galaxies: Is Additional Physics Needed?

Abstract

We use high-resolution hydrodynamic simulations to investigate the density profile of hot gas in clusters of galaxies, adopting a variant of cold dark matter cosmologies and employing a cosmological N-body/smoothed particle hydrodynamics code to follow the evolution of dark matter and gas. In addition to gravitational interactions, gas pressure, and shock heating, we include bremsstrahlung cooling in the computation. Dynamical time, two-body relaxation time, and cooling time in the simulations are examined to demonstrate that the results are free from artificial relaxation effects and that the time step is short enough to accurately follow the evolution of the system. In the simulation with nominal resolution of 66h^{-1} kpc the computed cluster appears normal, but in a higher (by a factor 2) resolution run, cooling is so efficient that the final gas density profile shows a steep rise toward the cluster center that is not observed in real clusters. Also, the X-ray luminosity of 7\times10^{45} ergs s^{-1} far exceeds that for any cluster of the computed temperature. The most reasonable explanation for this discrepancy is that there are some physical processes still missing in the simulations that actually mitigate the cooling effect and play a crucial role in the thermal and dynamical evolution of the gas near the center. Among the promising candidate processes are heat conduction and heat input from supernovae. We discuss the extent to which these processes can alter the evolution of gas.