A Hint of Three-section Halo as Seen from the APOGEE DR14
  Based on the [Fe/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 5<|Z|<10 kpc in the metallicity range of -1.6
<[Fe/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 <[Fe/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-Vlos diagram is adopted to separate halo stars from the disk by
defining the transition metallicity, which is of [Fe/H]~ -1.1 dex for the
high-[Mg/Fe] sequence and of [Fe/H]~-0.7 dex for the low-[Mg/Fe] sequence. The
R and |Z| distributions for the high-[Mg/Fe] sequence, the thick disk at
-1.1<[Fe/H]<-0.7 and the in situ halo at -1.6<[Fe/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<[Fe/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 [Fe/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.
