Dark Matter in Elliptical Galaxies

TL;DR

This review discusses how X-ray observations of hot gas in elliptical galaxies provide evidence for dark matter, revealing details about mass profiles, baryon fractions, and the validity of hydrostatic equilibrium assumptions.

Contribution

It synthesizes current X-ray constraints on dark matter in elliptical galaxies, highlighting new insights into mass profiles and the reliability of hydrostatic equilibrium.

Findings

Dark matter is firmly established in elliptical galaxies.

Total mass profiles are close to isothermal, with a possible power-law variation.

Constraints on baryon fraction and dark matter halo shape are discussed.

Abstract

We review X-ray constraints on dark matter in giant elliptical galaxies (10^{12} M_sun <~ M_vir <~ 10^{13} M_sun) obtained using the current generation of X-ray satellites, beginning with an overview of the physics of the hot interstellar medium and mass modeling methodology. Dark matter is now firmly established in many galaxies, with inferred NFW concentration parameters somewhat larger than the mean theoretical relation. X-ray observations confirm that the total mass profile (baryons+DM) is close to isothermal (M ~ r), and new evidence suggests a more general power-law relation for the slope of the total mass profile that varies with the stellar half-light radius. We also discuss constraints on the baryon fraction, super-massive black holes, and axial ratio of the dark matter halo. Finally, we review constraints on non-thermal gas motions and discuss the accuracy of the hydrostatic equilibrium approximation in elliptical galaxies.