The origin of accreted stellar halo populations in the Milky Way using APOGEE, Gaia, and the EAGLE simulations

Recent work indicates that the nearby Galactic halo is dominated by the debris from a major accretion event. We confirm that result from an analysis of APOGEE-DR14 element abundances and Gaia-DR2 kinematics of halo stars. We show that similar to 2/3 of nearby halo stars have high orbital eccentricities (e greater than or similar to 0.8), and abundance patterns typical of massive Milky Way dwarf galaxy satellites today, characterized by relatively low [Fe/H], [Mg/Fe], [Al/Fe], and [Ni/Fe]. The trend followed by high-e stars in the [Mg/Fe]-[Fe/H] plane shows a change of slope at [Fe/H]similar to -1.3, which is also typical of stellar populations from relatively massive dwarf galaxies. Low-e stars exhibit no such change of slope within the observed [Fe/H] range and show slightly higher abundances of Mg, Al, and Ni. Unlike their low-e counterparts, high-e stars show slightly retrograde motion, make higher vertical excursions, and reach larger apocentre radii. By comparing the position in [Mg/Fe]-[Fe/H] space of high-e stars with those of accreted galaxies from the EAGLE suite of cosmological simulations, we constrain the mass of the accreted satellite to be in the range 10(8.5)less than or similar to M-* less than or similar to 10(9) M-circle dot. We show that the median orbital eccentricities of debris are largely unchanged since merger time, implying that this accretion event likely happened at z less than or similar to 1.5. The exact nature of the low-e population is unclear, but we hypothesize that it is a combination of in situ star formation, high- vertical bar z disc stars, lower mass accretion events, and contamination by the low-e tail of the high-e population. Finally, our results imply that the accretion history of the Milky Way was quite unusual.