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 r ^ { 1.2 } to a good approximation over this range in radii , attaining a total of \sim 1.5 \times 10 ^ { 12 } M _ { \odot } ( corresponding to M _ { tot } / L _ { V } = 40 ) 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 .