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

TL;DR

This study uses Hubble Space Telescope imaging to analyze the distance and metallicity gradients along the Andromeda Galaxy's Giant Southern Stream, providing new insights into its structure and composition.

Contribution

It introduces an alternative method using unbinned maximum likelihood fits to characterize the red clump in the GSS, revealing gradients in distance and metallicity.

Findings

Distance gradient along the GSS consistent with previous measurements

Tentative evidence of increasing distance dispersion with projected distance

Similar metallicities in the 21 kpc and 52 kpc fields, suggesting uniformity

Abstract

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.