The GALAH Survey: No chemical evidence of an extragalactic origin for
  the Nyx stream
  The results from the ESA Gaia astrometric mission and deep photometric
surveys have revolutionized our knowledge of the Milky Way. There are many
ongoing efforts to search these data for stellar substructure to find evidence
of individual accretion events that built up the Milky Way and its halo. One of
these newly identified features, called Nyx, was announced as an accreted
stellar stream traveling in the plane of the disk. Using a combination of
elemental abundances and stellar parameters from the GALAH and APOGEE surveys,
we find that the abundances of the highest likelihood Nyx members are entirely
consistent with membership of the thick disk, and inconsistent with a dwarf
galaxy origin. We conclude that the postulated Nyx stream is most probably a
high-velocity component of the Milky Way's thick disk. With the growing
availability of large data sets including kinematics, stellar parameters, and
detailed abundances, the probability of detecting chance associations
increases, and hence new searches for substructure require confirmation across
as many data dimensions as possible.
