Kinematics and Metallicity of Red Giant Branch Stars in the Northeast
  Shelf of M31
  We obtained Keck/DEIMOS spectra of 556 individual red giant branch stars in 4
spectroscopic fields spanning $13-31$ projected kpc along the Northeast (NE)
shelf of M31. We present the first detection of a complete wedge pattern in the
space of projected M31-centric radial distance versus line-of-sight velocity
for this feature, which includes the returning stream component of the shelf.
This wedge pattern agrees with expectations of a tidal shell formed in a radial
merger and provides strong evidence in favor of predictions of Giant Stellar
Stream (GSS) formation models in which the NE shelf originates from the second
orbital wrap of the tidal debris. The observed concentric wedge patterns of the
NE, West (W), and Southeast (SE) shelves corroborate this interpretation
independently of the models. We do not detect a kinematical signature in the NE
shelf region corresponding to an intact progenitor core, favoring GSS formation
models in which the progenitor is completely disrupted. The shelf's photometric
metallicity distribution implies that it is dominated by tidal material, as
opposed to the phase-mixed stellar halo or the disk. The metallicity
distribution ([Fe/H]$_{\rm phot}$ = $-0.42$ $\pm$ $0.01$) also matches the GSS,
and consequently the W and SE shelves, further supporting a direct physical
association between the tidal features.
