James Lane, Jo Bovy
Abstract The Gaia - Sausage/Enceladus (GS/E) accretion remnant is one of the most important stellar populations in the Milky Way halo. Recent simulation-based work has suggested that the anisotropy profiles of remnants like GS/E decline towards the center and may be fit well by an Osipkov–Merritt-type distribution function (DF). We study the anisotropy profile of GS/E using a chemically selected sample of stars from Apache Point Galactic Evolution Experiment 17th data release and Gaia. We find that the anisotropy profile of GS/E is high and constant with β ∼ 0.9 beyond 8 kpc, dropping to β ∼ 0.4 at 2 kpc. We fit a two-component Osipkov–Merritt anisotropy profile to the GS/E data, finding that a superposition of profiles with scale radii r a = 2 kpc and 547 kpc, with a mixture fraction k om = 0.88, provide a better fit to the data than a constant anisotropy profile. Using this new model, we reassess the density profile and mass of the GS/E remnant from a previous work that assumed a constant anisotropy, finding an increase in the derived mass from 1.5 × 10 8 M ⊙ to 2.2 9 − 0.63 + 0.95 × 1 0 8 M ⊙ . In general, the superposition Osipkov–Merritt DF more satisfactorily matches the kinematics of the GS/E remnant than the constant anisotropy DF, and in the future will form a more reliable basis for modeling the remnant. Additionally, these new constraints on the kinematics of GS/E near the Galactic center are an important measurement for any future theoretical or simulation-based investigation into its nature and origin.