Steepening mass profiles, dark matter and environment of X-ray bright elliptical galaxies

We use a new non-parametric Bayesian approach to obtain the most probable mass distributions and circular velocity curves along with their confidence ranges, given deprojected density and temperature profiles of the hot gas surrounding X-ray bright elliptical galaxies. For a sample of six X-ray bright ellipticals, we find that all circular velocity curves are rising in the outer parts due to a combination of a rising temperature profile and a logarithmic pressure gradient that increases in magnitude. Therefore at large radii, mass density profiles rise more steeply than isothermal profiles, implying that we are probing the more massive group-sized haloes in which these galaxies are embedded. Comparing the circular velocity curves we obtain from X-rays to those obtained from dynamical models, we find that the former are often lower in the central ∼10 kpc. This is probably due to a combination of (i) non-thermal contributions of up to ∼35 per cent in the pressure (with stronger effects in NGC 4486), (ii) multiple-temperature components in the hot gas, (iii) incomplete kinematic spatial coverage in the dynamical models and (iv) mass profiles that are insufficiently general in the dynamical modelling. Complementing the total mass information from the X-rays with photometry and stellar population models to infer the dark matter content, we find evidence for massive dark matter haloes with dark matter mass fractions of ∼35-80 per cent at 2R_e, rising to a maximum of 80-90 per cent at the outermost radii. We also find that the six galaxies follow a Tully-Fisher relation with a slope of ∼4 and that their circular velocities at 1R_e correlate strongly with the velocity dispersion of the local environment. As a result, the galaxy luminosity at 1R_e also correlates with the velocity dispersion of the environment. These relations suggest a close link between the properties of central X-ray bright elliptical galaxies and their environments.