The Variation of Gas Mass Distribution in Galaxy Clusters: Effects of Preheating and Shocks

We investigate the origin of the variation of the gas mass fraction in the core of galaxy clusters, which was indicated by our work on the X-ray fundamental plane. Applying a spherical-collapse model of cluster formation and considering the effect of shocks on preheated intracluster gas, we construct a simple model to predict the spatial gas distribution of clusters. As suggested by our previous work, we assume that the core structure of clusters determined at the cluster collapse has not changed much after that. The adopted model supposes that the gas distribution characterized by the slope parameter is related to the preheated temperature. Comparison with observations of relatively hot (≳3 keV) and low-redshift clusters suggests that the preheated temperature is about 0.5-2 keV, which is higher than expected from the conventional galactic wind model and possibly suggests the need for additional heating, such as from quasars or gravitational heating on the largest scales at high redshift. The dispersion of the preheated temperature may be attributable to gravitational heating in subclusters. We calculate the central gas fraction of a cluster from the gas distribution, assuming that the global gas mass fraction is constant within a virial radius at the time of the cluster collapse. We find that the central gas density thus calculated is in good agreement with the observed one, which suggests that the variation of the gas mass fraction in cluster cores appears to be explained by breaking the self-similarity in clusters due to preheated gas. We also find that this model does not change major conclusions regarding the fundamental plane and its cosmological implications obtained in previous papers, which strongly suggests that the core structure preserves information about the cluster formation not only for the dark halo, but also for the intracluster gas.