Project AMIGA: Distance and Metallicity Gradients Along Andromeda's
  Giant Southern Stream from the Red Clump
  The Giant Southern Stream (GSS) of M31, a keystone signature of a major
accretion event, yields crucial constraints on M31 formation and evolution
models. Currently, our understanding of the GSS, in terms of both its geometry
and its chemistry, results from either wide-field imaging probing only a few
magnitudes below the red giant branch tip, or deep imaging or spectroscopy of
isolated regions. Here, we take an alternative approach, using Hubble Space
Telescope (HST) imaging to characterize the horizontal branch red clump (RC)
using unbinned maximum likelihood fits to luminosity functions (LFs) from
observed color-magnitude diagrams (CMDs). Comparing the RC mean magnitude
across three fields at projected distances of 21, 52 and 80 kpc from M31, we
find a line of sight distance gradient identical to recent literature
measurements in fields along the core. We also find tentative evidence that the
line of sight distance dispersion increases with projected distance from M31.
Meanwhile, the metallicity in the 52 kpc field westward of the GSS core is at
least as high as that in the 21 kpc GSS core field, and the peak colors of the
RC in these two fields imply identical metallicities to within 0.2 dex. We
discuss implications for distance and metallicity gradients both along and
perpendicular to the GSS in the context of recent ground-based photometric and
spectroscopic results, including evidence for a dropoff in metallicity moving
westward from the GSS, as well as prospects for further constraining stellar
populations in the vicinity of the GSS.
