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We present a study of the dark and luminous matter in the isolated ellipticalgalaxy NGC720, based on deep X-ray observations made with Chandra and Suzaku.The gas is reliably measured to ~R2500, allowing us to place good constraintson the enclosed mass and baryon fraction (fb) within this radius(M2500=1.6e12+/-0.2e12 Msun, fb(2500)=0.10+/-0.01; systematic errors are<~20%). The data indicate that the hot gas is close to hydrostatic, which issupported by good agreement with a kinematical analysis of the dwarf satellitegalaxies. We confirm a dark matter (DM) halo at ~20-sigma. Assuming an NFW DMprofile, our physical model for the gas distribution enables us to obtainmeaningful constraints at scales larger than R2500, revealing that most of thebaryons are in the hot gas. We find that fb within Rvir is consistent with theCosmological value, confirming theoretical predictions that a ~Milky Way-mass(Mvir=3.1e12+/-0.4e12 Msun) galaxy can sustain a massive, quasi-hydrostatic gashalo. While fb is higher than the cold baryon fraction typically measured insimilar-mass spiral galaxies, both the gas fraction (fg) and fb in NGC720 areconsistent with an extrapolation of the trends with mass seen in massive galaxygroups and clusters. After correcting for fg, the entropy profile is close tothe self-similar prediction of gravitational structure formation simulations,as observed in galaxy clusters. Finally, we find a strong heavy metal abundancegradient in the ISM similar to those observed in massive galaxy groups.

A Census of Baryons and Dark Matter in an Isolated, Milky Way-sized Elliptical Galaxy