Origin of the excess of high-energy retrograde stars in the Galactic halo

We report on the very low $\alpha$-element abundances of a group of metal-poor stars with high orbital energy and with large retrograde motion in the Milky Way halo, whose excess has been reported recently from metallicity and kinematics. We constructed a sample of halo stars with measured abundances and precise kinematics, including $\sim 880$ stars with [{{Fe}/{H}}]$<-0.7$, by crossmatching the Stellar Abundances for Galactic Archaeology database to the second data release of Gaia. Three regions in the energy-angular momentum space have been selected: innermost halo, Gaia Enceladus/Sausage, and high-energy retrograde halo. While the innermost halo and Gaia Enceladus regions have chemical abundances consistent with high- and low-$\alpha$ populations in the halo, respectively, chemical abundances of stars in the high-energy retrograde halo are different from the two populations; their [{X}/{Fe}], where X represents Na, Mg, and Ca, are even lower than those in Gaia Enceladus. These abundances, as well as their low mean metallicity, provide a new support for the idea that the retrograde component is dominated by an accreted dwarf galaxy which has a longer star formation timescale and is less massive than Gaia Enceladus/Sausage.