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

Christian R. Hayes, Steven R. Majewski, Sten Hasselquist, Rachael L. Beaton, Katia Cunha, Verne V. Smith, Adrian M. Price-Whelan, Borja Anguiano, Timothy C. Beers, Ricardo Carrera, J. G. Fernández-Trincado, Peter M. Frinchaboy, D. A. Garcia-Hernández, Richard R. Lane, David L. Nidever, Christian Nitschelm, Alexandre Roman-Lopes, Olga Zamora

Research output: Contribution to journalArticlepeer-review

18 Scopus citations


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 the chemical abundances of a dozen TriAnd members from the Sloan Digital Sky Survey-IV's (SDSS-IV's) 14th Data Release (DR14) 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.

Original languageEnglish (US)
Article numberL8
JournalAstrophysical Journal Letters
Issue number1
StatePublished - May 20 2018
Externally publishedYes


  • Galaxy: disk
  • Galaxy: evolution
  • Galaxy: halo
  • Galaxy: structure
  • stars: abundances Supporting material: machine-readable table

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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