X-Ray Measurement of Dark Matter ``Temperature'' in A1795

Abstract

We present a method from an X-ray observation of a galaxy cluster to measure the radial profile of the dark matter velocity dispersion, σ_DM, and to compare the dark matter ``temperature'' defined as μm<sub>p</sub>σ<sup>2</sup><sub>DM</sub>/k<sub>B</sub> with the gas temperature. The method is applied to the XMM-Newton observation of A1795. The ratio between the specific energy of the dark matter and that of the intracluster medium (ICM), which can be defined as β<sub>DM</sub> in analogy with β<sub>spec</sub>, is found to be less than unity everywhere, ranging ~0.3-0.8. In other words, the ICM temperature is higher than the dark matter ``temperature,'' even in the central region where the radiative cooling time is short. A β_DM value smaller than unity can most naturally be explained by heating of the ICM. The excess energy of the ICM is estimated to be ~1-3 keV per particle.