The GALAH Survey: No chemical evidence of an extragalactic origin for the Nyx stream

TL;DR

This study uses elemental abundances from large surveys to investigate the Nyx stellar stream, finding no chemical evidence of extragalactic origin and suggesting it is part of the Milky Way's thick disk.

Contribution

The paper provides a chemical analysis that challenges the extragalactic origin hypothesis of the Nyx stream, proposing it is likely a thick disk feature.

Findings

Nyx members are consistent with thick disk abundances.

No chemical evidence supports an extragalactic origin for Nyx.

Nyx is probably a high-velocity component of the Milky Way's thick disk.

Abstract

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.