A Hint of Three-section Halo As Seen from the APOGEE DR14

Based on the [M/H] versus [Mg/Fe] diagram and distances from APOGEE data release 14, we compare the spatial distributions, the l − Vlos diagram, and the abundance gradients between high-[Mg/Fe] and low-[Mg/Fe] sequences. The two sequences are clearly shown at in the metallicity range of −1.6 < [M/H] < −0.7, where the halo at consists of low-[Mg/Fe] stars only. In the intermediate-metallicity range of −1.1 < [M/H] < −0.7, a [Mg/Fe] gradient is detected for stars at and it flattens out at . The l − Vlos diagram is adopted to separate halo stars from the disk by defining the transition metallicity, which is of [M/H] ∼ −1.1 0.05 dex for the high-[Mg/Fe] sequence and of [M/H] ∼ −0.7 0.05 dex for the low-[Mg/Fe] sequence. The R and distributions for the high-[Mg/Fe] sequence, the thick disk at −1.1 < [M/H] < −0.7 and the in situ halo at −1.6 < [M/H] < −1.1, have a cutoff at R ∼ 15 kpc and , beyond which low-[Mg/Fe] halo stars are the main contributions. In the metallicity range of −1.6 < [M/H] < −0.7, there is a negative metallicity gradient for the high-[Mg/Fe] halo at , while only a marginal or no slope in the [M/H] versus diagram for the low-[Mg/Fe] halo at , beyond which both the high-[Mg/Fe] halo and low-[Mg/Fe] halo flatten out toward . These results indicate a complicated formation history of the Galaxy and we may see a hint of a three-section halo, i.e., the inner in situ halo within , the intermediately outer dual-mode halo at , and the extremely outer accreted halo with .