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

We present the discovery of a kinematically-cold stellar population along theSE minor axis of the Andromeda galaxy (M31) that is likely the forwardcontinuation of M31's giant southern stream. This discovery was made in thecourse of an on-going spectroscopic survey of red giant branch (RGB) stars inM31 using the DEIMOS instrument on the Keck II 10-m telescope. Stellarkinematics are investigated in eight fields located 9-30 kpc from M31's center(in projection). A likelihood method based on photometric and spectroscopicdiagnostics is used to isolate confirmed M31 RGB stars from foreground MilkyWay dwarf stars: for the first time, this is done without using radial velocityas a selection criterion, allowing an unbiased study of M31's stellarkinematics. The radial velocity distribution of the 1013 M31 RGB stars showsevidence for the presence of two components. The broad (hot) component has avelocity dispersion of 129 km/s and presumably represents M31's virializedspheroid. A significant fraction (19%) of the population is in a narrow (cold)component centered near M31's systemic velocity with a velocity dispersion thatdecreases with increasing radial distance, from 55.5 km/s at R_proj=12 kpc to10.6 km/s at R_proj=18 kpc. The spatial and velocity distribution of the coldcomponent matches that of the "Southeast shelf" predicted by the Fardal et al.(2007) orbital model of the progenitor of the giant southern stream. Themetallicity distribution of the cold component matches that of the giantsouthern stream, but is about 0.2 dex more metal rich on average than that ofthe hot spheroidal component. We discuss the implications of our discovery onthe interpretation of the intermediate-age spheroid population found in thisregion in recent ultra-deep HST imaging studies.Comment: 23 pages, 16 figures, 2 tables, accepted for publication in the Astrophysical Journal. Changes from previous version: expanded discussion in sections 4.2 and 7.2, removal of section 7.1.4 and associated figure (discussion moved to section 7.1.2