The structure of accreted stellar streams
  Many of the Milky Way's globular clusters are likely accreted from satellite
galaxies that have long since merged with the Milky Way. When these globular
clusters are susceptible to tidal disruption, this process likely starts
already inside the parent satellite leading to an early stellar stream within
the satellite. When the parent satellite merges with the Milky Way, the
globular cluster and its pre-merger stellar stream are accreted in a somewhat
chaotic process. Here, we investigate the properties of the accreted stream
after the merger as we would see it today using a suite of simulations of
accretion events. We find that the accretion process leads to a wide range of
behaviors, but generally scatters the accreted stream over a wide,
two-dimensional area of the sky. The behavior ranges from a set of a few or
more well-defined "sub-streams" extending out from the post-merger thin stream
by tens of degrees, to more widely dispersed debris over much of the sky,
depending on how close to the center of the MilkyWay the merger happened. Using
mock Gaia-like observations of the simulated streams, we demonstrate that an
accreted-stream component can explain the off-track features observed in the
GD-1 stream. Sub-streams can appear like thin tidal streams themselves that are
seemingly unassociated with the post-merger stream, raising the possibility
that some of the progenitor-less streams observed in the Milky Way are part of
a single or a few accreted streams created in an ancient merger event.
