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

TL;DR

This study uses APOGEE chemical abundance data to determine that the Triangulum-Andromeda overdensity is likely an extension of the Milky Way's disk, rather than a disrupted dwarf galaxy, based on its chemical similarity to outer disk stars.

Contribution

It provides the first detailed chemical abundance comparison showing TriAnd's similarity to the outer Galactic disk, supporting its origin as a disk extension.

Findings

TriAnd stars are chemically distinct from Sagittarius dwarf galaxy stars.

TriAnd's chemical ratios are similar to outer Milky Way disk stars.

TriAnd likely represents a perturbed extension of the Galactic disk.

Abstract

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.