The Milky Way’s Shell Structure Reveals the Time of a Radial Collision

@article{Donlon2020TheMW,
  title={The Milky Way’s Shell Structure Reveals the Time of a Radial Collision},
  author={Thomas Donlon and Heidi Jo Newberg and Robyn E. Sanderson and Lawrence M. Widrow},
  journal={The Astrophysical Journal},
  year={2020},
  volume={902}
}
We identify shell structures in the Milky Way for the first time. We find two shells in the Virgo Overdensity region and two shells in the Hercules Aquila Cloud region using Sloan Digital Sky Survey, Gaia, and LAMOST data. These shell stars are a subset of the substructure previously identified as the Virgo Radial Merger (VRM). Timing arguments for these shells indicate that their progenitor dwarf galaxy passed through the Galactic center 2.7 ± 0.2 Gyr ago. Based on the time of collision, it is… 

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