The Nature of Dark Matter in Elliptical Galaxies: Chandra Observations of NGC 4636

We determine the total enclosed mass profile from 0.7 to 35 kpc in the elliptical galaxy NGC 4636 based on the hot interstellar medium temperature profile measured using the Chandra X-ray Observatory, and other X-ray and optical data. The total mass increases as radius to the power 1.2 to a good approximation over this range in radii, attaining a total mass-to-light ratio of 40 solar masses per solar visual luminosity at 35 kpc. We find that at least half, and as much as 80%, of the mass within the optical half-light radius is non-luminous, implying an exceptionally low baryon fraction in NGC 4636. The large inferred dark matter concentration and central dark matter density, consistent with the upper end of the range expected for standard cold dark matter halos, imply that mechanisms proposed to explain low dark matter densities in less massive galaxies are not effective in elliptical galaxies.