2019MNRAS.482.3426M

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 ∼2/3 of nearby halo stars have high orbital eccentricities (e >= 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] ∼ -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 <= M_* <= 10⁹ M_☉. We show that the median orbital eccentricities of debris are largely unchanged since merger time, implying that this accretion event likely happened at z <= 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-|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.