The stellar mass of the Gaia-Sausage/Enceladus accretion remnant

ABSTRACT The Gaia-Sausage/Enceladus (GS/E) structure is an accretion remnant that comprises a large fraction of the Milky Way’s stellar halo. We study GS/E using high-purity samples of kinematically selected stars from APOGEE DR16 and Gaia. Employing a novel framework to account for kinematic selection biases using distribution functions, we fit density profiles to these GS/E samples and measure their masses. We find that GS/E has a shallow density profile in the inner Galaxy, with a break between 15 and 25 kpc beyond which the profile steepens. We also find that GS/E is triaxial, with axis ratios 1: 0. 55: 0. 45 (nearly prolate), and the major axis is oriented about 80° from the Sun–Galactic centre line and 16° above the plane. We measure a stellar mass for GS/E of 1. 45\, ^+0. 92-₀. ₅₁\, (stat. ) \, ^+0. 13-₀. ₃₇ (sys. ) \ 10^8 M⊙. Our mass estimate is lower than others in the literature, a finding we attribute to the excellent purity of the samples we work with. We also fit a density profile to the entire Milky Way stellar halo, finding a mass in the range of 6. 7–8. 4 × 108 M⊙, and implying that GS/E could make up as little as 15–25 per cent of the mass of the Milky Way stellar halo. Our lower stellar mass combined with standard stellar mass-to-halo mass relations implies that GS/E constituted a minor 1: 8 mass-ratio merger at the time of its accretion.