Comparison of an approximately isothermal gravitational potentials of elliptical galaxies based on X-ray and optical data

TL;DR

This study compares gravitational potential estimates of elliptical galaxies from X-ray and optical data, finding a consistent relation that suggests non-thermal pressure contributes significantly to gas pressure.

Contribution

It introduces robust methods to estimate galaxy circular speeds from optical data and compares them with X-ray derived speeds, highlighting the role of non-thermal pressure.

Findings

Optical and X-ray circular speeds are closely related with a factor of about 1.10-1.15.

Non-thermal pressure contributes approximately 20-30% of the gas thermal pressure.

The methods provide a consistent framework for analyzing elliptical galaxy potentials.

Abstract

We analyze six X-ray bright elliptical galaxies, observed with Chandra and XMM-Newton, and approximate their gravitational potentials by isothermal spheres phi(r)=v_c^2 ln(r) over a range of radii from ~0.5 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}~ \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 ~20-30% of the gas thermal pressure.