Kavli Affiliate: Lina Necib
| First 5 Authors: Shuyu Wang, Lina Necib, Alexander P. Ji, Xiaowei Ou, Mariangela Lisanti
| Summary:
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.
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