2011ApJ...729...53H

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 R₂₅₀₀, allowing us to place good constraints on the enclosed mass and baryon fraction (f_b) within this radius (M₂₅₀₀= (1.6±0.2)x10¹² M_☉, f_b,2500= 0.10±0.01; systematic errors are typically ≲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 (∼20σ) 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 R₂₅₀₀, 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 ∼ Milky Way mass (M_vir = 3.1^+0.4_–0.3x10¹² M_☉) 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.