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The Giant Southern Stream (GSS) of M31, a keystone signature of a majoraccretion event, yields crucial constraints on M31 formation and evolutionmodels. Currently, our understanding of the GSS, in terms of both its geometryand its chemistry, results from either wide-field imaging probing only a fewmagnitudes below the red giant branch tip, or deep imaging or spectroscopy ofisolated regions. Here, we take an alternative approach, using Hubble SpaceTelescope (HST) imaging to characterize the horizontal branch red clump (RC)using unbinned maximum likelihood fits to luminosity functions (LFs) fromobserved color-magnitude diagrams (CMDs). Comparing the RC mean magnitudeacross three fields at projected distances of 21, 52 and 80 kpc from M31, wefind a line of sight distance gradient identical to recent literaturemeasurements in fields along the core. We also find tentative evidence that theline of sight distance dispersion increases with projected distance from M31.Meanwhile, the metallicity in the 52 kpc field westward of the GSS core is atleast as high as that in the 21 kpc GSS core field, and the peak colors of theRC in these two fields imply identical metallicities to within 0.2 dex. Wediscuss implications for distance and metallicity gradients both along andperpendicular to the GSS in the context of recent ground-based photometric andspectroscopic results, including evidence for a dropoff in metallicity movingwestward from the GSS, as well as prospects for further constraining stellarpopulations in the vicinity of the GSS.

Project AMIGA: Distance and Metallicity Gradients Along Andromeda's Giant Southern Stream from the Red Clump