Stellar Kinematics in the Complicated Inner Spheroid of M31: Discovery of Substructure along the Southeastern Minor Axis and Its Relationship to the Giant Southern Stream

Abstract

We present the discovery of a kinematically cold stellar population along the southeastern minor axis of the Andromeda galaxy (M31) that is likely the forward continuation of M31's giant southern stream. This discovery was made during an on-going spectroscopic survey of M31 red giant branch (RGB) stars using the DEIMOS instrument on the Keck II 10 m telescope. Stellar kinematics are investigated in eight fields located 9-30 kpc from M31's center (in projection). A likelihood method based on photometric and spectroscopic diagnostics is used to isolate confirmed M31 RGB stars from foreground Milky Way dwarf stars: for the first time, this is done without using radial velocity as a selection criterion, allowing an unbiased study of M31's stellar kinematics. The radial velocity distribution of the 1013 M31 RGB stars contains two components. The broad (hot) component (σ = 129 km s-1) presumably represents M31's virialized spheroid. A significant fraction (19%) of the population is in a narrow (cold) component centered near M31's systemic velocity, with a velocity dispersion that decreases with increasing radial distance, from σ = 55.5 km s-1 at Rproj = 12 kpc to σ = 10.6 km s-1 (9.5 km s-1 after accounting for measurement error) at Rproj = 18 kpc. The spatial and velocity distribution of the cold component matches that of the Southeast shelf predicted by the Fardal et al. orbital model of the progenitor of M31's giant southern stream. The metallicity distribution of the cold component matches that of the giant southern stream but is about 0.2 dex more metal rich than that of the hot spheroidal component. We discuss the implications of our discovery on the interpretation of the intermediate-age spheroid population found in this region in recent ultradeep HST imaging studies.