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 ] \sim - 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 .