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The nature of the Triangulum-Andromeda (TriAnd) system has been debated sincethe discovery of this distant, low-latitude Milky Way (MW) overdensity morethan a decade ago. Explanations for its origin are either as a halosubstructure from the disruption of a dwarf galaxy or a distant extension ofthe Galactic disk. We test these hypotheses using chemical abundances of adozen TriAnd members from the Sloan Digital Sky Survey's 14th Data Release ofApache Point Observatory Galactic Evolution Experiment (APOGEE) data to compareto APOGEE abundances of stars with similar metallicity from both theSagittarius (Sgr) dSph, and the outer MW disk. We find that TriAnd stars arechemically 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 onthe element. Instead, the TriAnd stars, with a median metallicity of about-0.8, exhibit chemical abundance ratios similar to those of the lowestmetallicity ([Fe/H] ~ -0.7) stars in the outer Galactic disk, and areconsistent with expectations of extrapolated chemical gradients in the outerdisk of the MW. These results suggest that TriAnd is associated with the MWdisk, and, therefore, that the disk extends to this overdensity --- i.e., pasta Galactocentric radius of 24 kpc --- albeit vertically perturbed about 7 kpcbelow the nominal disk midplane in this region of the Galaxy.

Disk-Like Chemistry of the Triangulum-Andromeda Overdensity as Seen by APOGEE