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 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 (fb ) within this radius (M [subscript 2500] = (1.6 ± 0.2) × 10[subscript 12] M ☉, f [subscript b,2500] = 0.10 ± 0.01; systematic errors are typically lsim20%). 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 R 2500, revealing that most of the baryons are in the hot gas. We find that fb within the virial radius is consistent with the Cosmological value, confirming theoretical predictions that a ~ Milky Way mass (M vir = 3.1[superscript +0.4] [subscript –0.3] × 10[superscript 12] M ☉) galaxy can sustain a massive, quasi-hydrostatic gas halo. While fb is higher than the cold (cool gas plus stars) baryon fraction typically measured in similar-mass spiral galaxies, both the gas fraction (fg ) and fb in NGC 720 are consistent with an extrapolation of the trends with mass seen in massive galaxy groups and clusters. After correcting for fg , 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.