The orbital anisotropy profile of the Gaia-Sausage/Enceladus accretion remnant

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 well-fit by an Osipkov-Merritt-type distribution function (DF). We study the anisotropy profile of GS/E using a chemically-selected sample of stars from APOGEE DR17 and Gaia. We find that the anisotropy profile of GS/E is high and constant with ${\hat{I}}^2\sim 0.9$ beyond 8 kpc, dropping to ${\hat{I}}^2\sim 0.4$ 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 $r_{\mathrm{a}}=2$ kpc and 547 kpc, with a mixture fraction $k_{\mathrm{om}}=0.88$ provide a better fit to the data than a constant anisotropy profile. Using this new model, we re-assess the density profile and mass of the GS/E remnant from a previous work that assumed a contant anisotropy, finding an increase in the derived mass from $1.5\times 10^8{\mathrm{M}}_{\odot }$ to $2.29_{-0.63}^{+0.95}\times 10^8{\mathrm{M}}_{\odot }$. 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 modelling 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.