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 (f_b ) within this radius (M ₂₅₀₀ = (1.6 ± 0.2) × 10¹² M _sun, f _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 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 _-0.3 × 10¹² M _sun) 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.