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

Abstract

Based on the [M/H] versus [Mg/Fe] diagram and distances from APOGEE data release 14, we compare the spatial distributions, the l - V _los diagram, and the abundance gradients between high-[Mg/Fe] and low-[Mg/Fe] sequences. The two sequences are clearly shown at 5< | Z| < 10 {kpc} in the metallicity range of -1.6 < [M/H] < -0.7, where the halo at | Z| > 10 {kpc} 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 | Z| =10{--}30 {kpc} and it flattens out at | Z| > 30 {kpc}. The l - V _los 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 | Z| 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 | Z| ∼ 10 {kpc}, 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 | Z| < 8{--}10 {kpc}, while only a marginal or no slope in the [M/H] versus | Z| diagram for the low-[Mg/Fe] halo at | Z| < 8{--}10 {kpc}, beyond which both the high-[Mg/Fe] halo and low-[Mg/Fe] halo flatten out toward | Z| > 20 {kpc}. 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 | Z| ∼ 8{--}10 {kpc}, the intermediately outer dual-mode halo at | Z| ∼ 10{--}30 {kpc}, and the extremely outer accreted halo with | Z| > 30 {kpc}.