Evidence for a Vast Prograde Stellar Stream in the Solar Vicinity
2019
Massive
dwarf galaxiesthat merge with the
Milky Wayon
progradeorbits can be dragged into the disk plane before being completely disrupted. Such mergers can contribute to an accreted stellar disk and a dark matter disk. We present evidence for Nyx, a vast new stellar stream in the vicinity of the Sun, that may provide the first indication that such an event occurred in the
Milky Way. We identify about 500 stars that have coherent radial and
progrademotion in this stream using a catalog of accreted stars built by applying deep learning methods to the second Gaia data release. Nyx is concentrated within ±2 kpc of the Galactic midplane and spans the full radial range studied (6.5-9.5 kpc). The kinematics of Nyx stars are distinct from those of both the thin and
thick disk. In particular, its rotational speed lags the disk by ∼80 km/s and its stars follow more eccentric orbits. A small number of Nyx stars have chemical abundances or inferred ages; from these, we deduce that Nyx stars have a peak metallicity of [Fe/H] ∼−0.5 and ages ∼10-13 Gyr. Taken together with the kinematic observations, these results strongly favor the interpretation that Nyx is the remnant of a disrupted
dwarf galaxy. To further justify this interpretation, we explicitly demonstrate that metal-rich,
progradestreams like Nyx can be found in the disk plane of
Milky Way-like galaxies using the FIRE hydrodynamic simulations. Future spectroscopic studies will be able to validate whether Nyx stars originate from a single progenitor.
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