A spectroscopic survey of Andromeda's Western Shelf
  The Andromeda galaxy (M31) shows many tidal features in its halo, including
the Giant Southern Stream (GSS) and a sharp ledge in surface density on its
western side (the W Shelf). Using DEIMOS on the Keck telescope, we obtain
radial velocities of M31's giant stars along its NW minor axis, in a radial
range covering the W Shelf and extending beyond its edge. In the space of
velocity versus radius, the sample shows the wedge pattern expected from a
radial shell, which is detected clearly here for the first time. This confirms
predictions from an earlier model of formation of the GSS, which proposed that
the W Shelf is a shell from the third orbital wrap of the same tidal debris
stream that produces the GSS, with the main body of the progenitor lying in the
second wrap. We calculate the distortions in the shelf wedge pattern expected
from its outward expansion and angular momentum, and show that these effects
are echoed in the data. In addition, a hot, relatively smooth spheroid
population is clearly present. We construct a bulge-disk-halo N-body model that
agrees with surface brightness and kinematic constraints, and combine it with a
simulation of the GSS. From the contrasting kinematic signatures of the hot
spheroid and shelf components, we decompose the observed stellar metallicity
distribution into contributions from each component using a non-parametric
mixture model. The shelf component's metallicity distribution matches previous
observations of the GSS superbly, further strengthening the evidence they are
connected and bolstering the case for a massive progenitor of this stream.
