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.
