Substructure Along M 31’s Southeast Minor Axis: The Forward Continuation of the Giant Southern Stream

Abstract

We present evidence of substructure along the southeastern minor axis of the Andromeda galaxy (M 31) that is likely the forward continuation of M 31’s giant southern stream (GSS). M 31 red giant branch (RGB) stars are separated from foreground Milky Way (MW) dwarf stars using a likelihood method based on photometric and spectroscopic diagnostics. The separation is achieved without using the velocity of the stars, allowing an unbiased study of M 31’s stellar kinematics in eight fields located 9 – 30 kpc from M 31’s center (in projection). The line-of-sight velocity distribution of the 1013 M 31 RGB stars is composed of two components: a broad (hot) component (σsphv = 129 kms-1) which presumably represents M 31’s virialized spheroid, and a narrow (cold) component centered near M 31’s systemic velocity with a velocity dispersion that decreases with increasing radial distance. The spatial and velocity distribution of the cold component is similar to that of the “Southeast shelf” predicted by the orbital model of the progenitor of the GSS presented in [1].