Reconciling stellar dynamical and hydrostatic X-ray mass measurements of an elliptical galaxy with gas rotation, turbulence and magnetic fields

Recent hydrostatic X-ray studies of the hot interstellar medium (ISM) in early-type galaxies underestimate the gravitating mass as compared to stellar dynamics, implying modest, but significant deviations from exact hydrostatic equilibrium. We present a method for combining X-ray measurements and stellar dynamical constraints in the context of Bayesian statistics that allows the radial distribution of the implied non-thermal pressure or bulk motions in the hot ISM to be constrained. We demonstrate the accuracy of the method with hydrodynamical simulations tailored to produce a realistic galaxy model. Applying the method to the nearby elliptical galaxy NGC 4649, we find a significant but subdominant non-thermal pressure fraction (0.27 ± 0.06) in the central (≲5 kpc) part of the galaxy, similar to the level of deviations from hydrostatic equilibrium expected in galaxy clusters. Plausible sources of systematic error, if important, may reduce this fraction. Our results imply ˜360 km s-1 random turbulence or a magnetic field B = (39 ± 6)(ne/0.1 cm- 3)0.59 ± 0.09 μG, whereas gas rotation alone is unlikely to explain the detailed non-thermal profile. Future observations with Astro-H will allow turbulence or gas rotation at this level to be detected.