The Milky Way's Shell Structure Reveals the Time of a Radial Collision
  We identify shell structures in the Milky Way for the first time. We find 2
shells in the Virgo Overdensity (VOD) region and 2 shells in the Hercules
Aquila Cloud (HAC) 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 also possible that the VRM
is related to the phenomenon that created phase-space spirals in the vertical
motion of the disk and/or the Splash, and could have caused a burst of star
formation in the inner disk.
  We analyze phase mixing in a collection of radial merger N-body simulations,
and find that shell structure similar to that observed in Milky Way data
disappears by 5 Gyr after collision with the Galactic center. The method used
to calculate the merger time of the VRM was able to reliably recover the
correct merger times for these simulations.
  Previous work supports the idea that the VRM and the Gaia
Sausage/Gaia-Enceladus Merger are the same. However, the Gaia Sausage is widely
believed to be 8--11 Gyr old. The disparate ages could be reconciled if the
larger age is associated with an infall time when the progenitor crossed the
virial radius; we do not constrain the time at which the progenitor became
bound to the Milky Way. Alternatively, the Gaia Sausage could be younger than
previously thought.
