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 .