Core Structure of Intracluster Gas: Isothermal Hydrostatic Equilibrium

We investigated the core structure of intracluster hot gas based on the so-called β-model. We found that the virial temperature, T_{<small>vir</small>}, of a cluster may be represented better by βT_X than T_X, where β is a parameter obtained from the X-ray surface brightness, and T_X is the emission-weighted mean temperature of the gas. For 121 clusters observed by ROSAT and ASCA, the luminosity-temperature relation, L_X-βT_X, is found to be less steep than L_X-T_X. The clusters can be classified into large and small core groups, and the latter exhibits no significant correlation between the core and virial radii. We examined the properties of the small core clusters, and found that they have a shorter cooling time than the Hubble time, while no significant correlation was found with the presence of cD galaxies or asymmetry in the surface brightness. We carried out hydrodynamical calculations to simulate the β-model and confirmed the above results: T_{<small>vir</small>} simeq βT for the gas temperature, T, and the gas core formed due to a central cD galaxy is too small to account for the cores of the smaller core clusters.