A CENSUS OF BARYONS AND DARK MATTER IN AN ISOLATED, MILKY WAY SIZED ELLIPTICAL GALAXY

We present a study of the dark and luminous matter in the isolated elliptical galaxy NGC 720, based on deep X-ray observations made with the Chandra and Suzaku observatories. The gas properties are reliably measured almost to R-2500, allowing us to place good constraints on the enclosed mass and baryon fraction (f(b)) within this radius (M-2500 = (1.6 +/- 0.2) x 10(12) M-circle dot, f(b),(2500) = 0.10 +/- 0.01; systematic errors are typically less than or similar to 20%). The data indicate that the hot gas is close to hydrostatic, which is supported by good agreement with a kinematical analysis of the dwarf satellite galaxies. We confirm at high significance (similar to 20 sigma) the presence of a dark matter (DM) halo. Assuming a Navarro-Frenk-White DM profile, our physical model for the gas distribution enables us to obtain meaningful constraints at scales larger than R-2500, revealing that most of the baryons are in the hot gas. We find that f(b) within the virial radius is consistent with the Cosmological value, confirming theoretical predictions that a similar to Milky Way mass (M-vir = 3.1(-0.3)(+0.4) x 10(12) M-circle dot ) galaxy can sustain a massive, quasi-hydrostatic gas halo. While f(b) is higher than the cold (cool gas plus stars) baryon fraction typically measured in similar-mass spiral galaxies, both the gas fraction (f(g)) and f(b) in NGC 720 are consistent with an extrapolation of the trends with mass seen in massive galaxy groups and clusters. After correcting for f(g), the entropy profile is close to the self-similar prediction of gravitational structure formation simulations, as observed in massive galaxy clusters. Finally, we find a strong heavy metal abundance gradient in the interstellar medium, qualitatively similar to those observed in massive galaxy groups.