The ElIXr Galaxy Survey. II. Baryons and Dark Matter in an Isolated Elliptical Galaxy

The Elliptical Isolated X-ray (ElIXr) Galaxy Survey is a volume-limited (<110 Mpc) study of optically selected, isolated, L _* elliptical galaxies to provide an X-ray census of galaxy-scale (virial mass, M _vir <~ 10¹³ M _⊙) objects and identify candidates for detailed hydrostatic mass modeling. In this paper, we present a Chandra and XMM study of one such candidate, NGC 1521, and constrain its distribution of dark and baryonic matter. We find a morphologically relaxed hot gas halo, extending almost to R ₅₀₀, that is well described by hydrostatic models similar to the benchmark, baryonically closed, Milky Way mass elliptical galaxy NGC 720. We obtain good constraints on the enclosed gravitating mass (M ₅₀₀ = [3.8 ± 1.0] × 10¹² M _⊙, slightly higher than NGC 720) and baryon fraction (f _{b, 500} = 0.13 ± 0.03). We confirm at 8.2σ the presence of a dark matter (DM) halo consistent with ΛCDM. Assuming a Navarro-Frenk-White DM profile, our self-consistent, physical model enables meaningful constraints beyond R ₅₀₀, revealing that most of the baryons are in the hot gas. Within the virial radius, f_b is consistent with the cosmic mean, suggesting that the predicted massive, quasi-hydrostatic gas halos may be more common than previously thought. We confirm that the DM and stars conspire to produce an approximately power-law total mass profile (ρ_totvpropr ^-α) that follows the recently discovered scaling relation between α and optical effective radius. Our conclusions are insensitive to modest, observationally motivated, deviations from hydrostatic equilibrium. Finally, after correcting for the enclosed gas fraction, the entropy profile is close to the self-similar prediction of gravitational structure formation simulations, as observed in massive galaxy clusters.