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 SE minor axis of the Andromeda galaxy (M31) that is likely the forward continuation of M31's giant southern stream. This discovery was made in the course of an on-going spectroscopic survey of red giant branch (RGB) stars in M31 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 shows evidence for the presence of two components. The broad (hot) component has a velocity dispersion of 129 km/s and 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 55.5 km/s at R_proj=12 kpc to 10.6 km/s at R_proj=18 kpc. The spatial and velocity distribution of the cold component matches that of the "Southeast shelf" predicted by the Fardal et al. (2007) orbital model of the progenitor of the 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 on average 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 ultra-deep HST imaging studies.

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)

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Cited by in corpus (9)

• The Dual Origin of Stellar Halos
• Modeling The Structure And Dynamics of Dwarf Spheroidal Galaxies with Dark Matter And Tides
• The Nature and Origin of Substructure in the Outskirts of M31. I. Surveying the Stellar Content with HST/ACS
• Was the Andromeda Stream Produced by a Disk Galaxy?
• The Stellar Content of Galaxy Halos: A Comparison between LambdaCDM Models and Observations of M31
• The Extended Star Formation History of the Andromeda Spheroid at 35 Kpc on the Minor Axis
• Wide-Field Survey of Globular Clusters in M31. II. Kinematics of the Globular Cluster System
• Constraining the Mass Profiles of Stellar Systems: Schwarzschild Modeling of Discrete Velocity Datasets
• The kinematic footprints of five stellar streams in Andromeda's halo