Stellar kinematics of X-ray bright massive elliptical galaxies

We discuss a simple and fast method for estimating masses of early-type galaxies from optical data and compare the results with X-ray derived masses. The optical method relies only on the most basic observables such as the surface brightness I(R) and the line-of-sight velocity dispersion sigma(p)(R) profiles and provides an anisotropy-independent estimate of the galaxy circular speed V-c. The mass-anisotropy degeneracy is effectively overcome by evaluating V-c at a characteristic radius R-sweet defined from local properties of observed profiles. The sweet radius R-sweet is expected to lie close to R-2, where I(R) proportional to R-2, and not far from the effective radius R-eff. We apply the method to a sample of five X-ray bright elliptical galaxies observed with the 6 m telescope BTA-6 in Russia. We then compare the optical V-c estimate with the X-ray derived value, and discuss possible constraints on the non-thermal pressure in the hot gas and configuration of stellar orbits. We find that the average ratio of the optical V-c estimate to the X-ray one is equal to approximate to 0.98 with 11 per cent scatter, i.e. there is no evidence for the large non-thermal pressure contribution in the gas at similar to R-sweet. From analysis of the Lick indices H beta, Mgb, Fe5270 and Fe5335, we calculate the mass of the stellar component within the sweet radius. We conclude that a typical dark matter fraction inside R-sweet in the sample galaxies is similar to 60 per cent for the Salpeter initial mass function (IMF) and similar to 75 per cent for the Kroupa IMF.