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

Abstract

We present a study of the dark and luminous matter in the isolated elliptical\ngalaxy NGC720, based on deep X-ray observations made with Chandra and Suzaku.\nThe gas is reliably measured to ~R2500, allowing us to place good constraints\non the enclosed mass and baryon fraction (fb) within this radius\n(M2500=1.6e12+/-0.2e12 Msun, fb(2500)=0.10+/-0.01; systematic errors are\n<~20%). The data indicate that the hot gas is close to hydrostatic, which is\nsupported by good agreement with a kinematical analysis of the dwarf satellite\ngalaxies. We confirm a dark matter (DM) halo at ~20-sigma. Assuming an NFW DM\nprofile, our physical model for the gas distribution enables us to obtain\nmeaningful constraints at scales larger than R2500, revealing that most of the\nbaryons are in the hot gas. We find that fb within Rvir is consistent with the\nCosmological value, confirming theoretical predictions that a ~Milky Way-mass\n(Mvir=3.1e12+/-0.4e12 Msun) galaxy can sustain a massive, quasi-hydrostatic gas\nhalo. While fb is higher than the cold baryon fraction typically measured in\nsimilar-mass spiral galaxies, both the gas fraction (fg) and fb in NGC720 are\nconsistent with an extrapolation of the trends with mass seen in massive galaxy\ngroups and clusters. After correcting for fg, the entropy profile is close to\nthe self-similar prediction of gravitational structure formation simulations,\nas observed in galaxy clusters. Finally, we find a strong heavy metal abundance\ngradient in the ISM similar to those observed in massive galaxy groups.\n