High-Resolution Chemical Abundances of the Nyx Stream

TL;DR

This study uses high-resolution spectroscopy to analyze the chemical composition of Nyx stream stars, revealing their similarity to the thick disk and exploring possible origins related to galaxy mergers or disk evolution.

Contribution

It provides detailed chemical abundance data for Nyx stars, challenging previous merger hypotheses and proposing new formation scenarios based on chemodynamic evidence.

Findings

Most Nyx stars are metal-rich and chemically similar to the thick disk.

Some Nyx stars are metal-poor with thick disk-like chemistry.

Nyx likely formed from early minor mergers or disk spin-up, not a single dwarf galaxy merger.

Abstract

Nyx is a nearby, prograde, and high-eccentricity stellar stream physically contained in the thick disk but with an unknown origin. Nyx could be the remnant of a disrupted dwarf galaxy, in which case the associated dark matter substructure could affect terrestrial dark matter direct detection experiments. Alternatively, Nyx could be a signature of the Milky Way's disk formation and evolution. To determine the origin of Nyx, we obtained high-resolution spectroscopy of 34 Nyx stars using Keck/HIRES and Magellan/MIKE. A differential chemical abundance analysis shows that most Nyx stars reside in a metal-rich ($\mbox{[Fe/H]} > -1$) high-$\alpha$ component that is chemically indistinguishable from the thick disk. This rules out an originally suggested scenario that Nyx is the remnant of a single massive dwarf galaxy merger. However, we also identify five substantially more metal-poor stars ($\mbox{[Fe/H]} \sim -2.0$) that have chemical abundances similar to the metal-weak thick disk. It remains unclear how stars chemically identical to the thick disk can be on such prograde, high-eccentricity orbits. We suggest two most likely scenarios: that Nyx is the result of an early minor dwarf galaxy merger or that it is a record of the early spin-up of the Milky Way disk -- although neither perfectly reproduces the chemodynamic observations. The most likely formation scenarios suggest that future spectroscopic surveys should find Nyx-like structures outside of the Solar Neighborhood.