Major substructure in the M31 outer halo: Distances and metallicities along the giant stellar stream

Abstract

We present a renewed look at M31's giant stellar stream along with the nearby structures streams C and D, exploiting a new algorithm capable of fitting to the red giant branch (RGB) of a structure in both colour and magnitude space. Using this algorithm, we are able to generate probability distributions in distance, metallicity and RGB width for a series of subfields spanning these structures. Specifically, we confirm a distance gradient of approximately 20 kpc per degree along a 6 deg extension of the giant stellar stream, with the farthest subfields from M31 lying ~120 kpc more distant than the innermost subfields. Further, we find a metallicity that steadily increases from-0.7-0.1+0.1 to-0.2-0.1+0.2 dex along the inner half of the stream before steadily dropping to a value of-1.0-0.2+0.2 dex at the farthest reaches of our coverage. The RGB width is found to increase rapidly from 0.4-0.1+0.1 to 1.1-0.1+0.2 dex in the inner portion of the stream before plateauing and decreasing marginally in the outer subfields of the stream. In addition, we estimate stream C to lie at a distance between 794 and 862 kpc and stream D between 758 and 868 kpc. We estimate the median metallicity of stream C to lie in the range-0.7 to-1.6 dex and a metallicity of-1.1-0.2+0.3 dex for stream D. RGB widths for the two structures are estimated to lie in the range 0.4-1.2 dex and 0.3-0.7 dex, respectively. In total, measurements are obtained for 19 subfields along the giant stellar stream, four along stream C, five along stream D and three general M31 spheroid fields for comparison. We thus provide a higher resolution coverage of the structures in these parameters than has previously been available in the literature.