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

TL;DR

This paper introduces a Bayesian method to reconcile X-ray and stellar dynamical mass measurements in elliptical galaxies by constraining nonthermal pressure, turbulence, and magnetic fields, validated with simulations and applied to NGC4649.

Contribution

The paper presents a novel Bayesian approach combining X-ray and stellar data to measure nonthermal pressures in galaxy gas, improving mass estimates.

Findings

Nonthermal pressure fraction in NGC4649's center is about 27%.

Turbulence or magnetic fields are needed to explain the nonthermal support.

Gas rotation alone is insufficient to account for observed deviations.

Abstract

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 nonthermal 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 NGC4649, we find a significant but subdominant nonthermal 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. This would imply >360 km/s random turbulence or a magnetic field B=(39+/-6)(n_e/0.1 cm^{-3})^{0.59+/-0.09} muG, whereas gas rotation alone is unlikely to explain the detailed nonthermal profile. Future observations with Astro-H will allow turbulence or gas rotation at this level to be detected.