Abstract
We analyse six X-ray bright elliptical galaxies, observed with Chandra and XMM-Newton, and approximate their gravitational potentials by isothermal spheres phi = v2(c) log r over a range of radii from similar to 0.5 to similar to 25 kpc. We then compare the circular speed v(c) derived from X-ray data with the estimators available from optical data. In particular, we discuss two simple and robust procedures for evaluating the circular speed of the galaxy using the observed optical surface brightness and the line-of-sight velocity dispersion profiles. The best-fitting relation between the circular speeds derived from optical observations of stars and X-ray observations of hot gas is v(c,opt) similar or equal to eta v(c,X), where eta = 1.10-1.15 (depending on the method), suggesting, albeit with large statistical and systematic uncertainties, that non-thermal pressure on average contributes similar to 20-30 per cent of the gas thermal pressure.