UPDATING THE SIGMA-T RELATIONSHIP FOR GALAXY CLUSTERS

The relationship between the X-ray determined temperature T of the int racluster gas and the optical measured velocity dispersion a of the cl uster galaxies is often believed to be not only a straightforward but also robust test for the dynamical properties of galaxy clusters. Here , we present the sigma-T relationship using the 94 clusters drawn from the largest sample of 149 clusters in literature, for which both sigm a and T are observationally determined. Employment of the doubly weigh ted orthogonal distance regression to our sample yields sigma = 10(2.4 7+/-0.06)T(0.67+/-0.09), indicating an apparent deviation of dynamical state from that predicted by the isothermal and hydrostatic equilibri um model for galaxy clusters, though the average ratio beta(spec) of s pecific energy in galaxies to that in gas is found to be in excellent agreement with unity. It shows that a nonisothermal gas distribution w ith a mean polytropic index of gamma = 1.3 can account for the reporte d sigma-T relationship, while overall clusters can still be regarded a s dynamically-relaxed systems.