Disk-Like Chemistry of the Triangulum-Andromeda Overdensity as Seen by
  APOGEE
  The nature of the Triangulum-Andromeda (TriAnd) system has been debated since
the discovery of this distant, low-latitude Milky Way (MW) overdensity more
than a decade ago. Explanations for its origin are either as a halo
substructure from the disruption of a dwarf galaxy or a distant extension of
the Galactic disk. We test these hypotheses using chemical abundances of a
dozen TriAnd members from the Sloan Digital Sky Survey's 14th Data Release of
Apache Point Observatory Galactic Evolution Experiment (APOGEE) data to compare
to APOGEE abundances of stars with similar metallicity from both the
Sagittarius (Sgr) dSph, and the outer MW disk. We find that TriAnd stars are
chemically distinct from Sgr across a variety of elements, (C+N), Mg, K, Ca,
Mn, and Ni, with a separation in [X/Fe] of about 0.1 to 0.4 dex depending on
the element. Instead, the TriAnd stars, with a median metallicity of about
-0.8, exhibit chemical abundance ratios similar to those of the lowest
metallicity ([Fe/H] ~ -0.7) stars in the outer Galactic disk, and are
consistent with expectations of extrapolated chemical gradients in the outer
disk of the MW. These results suggest that TriAnd is associated with the MW
disk, and, therefore, that the disk extends to this overdensity --- i.e., past
a Galactocentric radius of 24 kpc --- albeit vertically perturbed about 7 kpc
below the nominal disk midplane in this region of the Galaxy.
