Abstract

Massive dwarf galaxies that merge with the Milky Way on prograde orbits 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 prograde motion 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 \pm 2 \text { 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 \sim 80 \text { 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 \text { [ Fe / H ] } \sim - 0.5 and ages \sim 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 , prograde streams 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 .