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)

References in corpus (8)

• The Origin of the Bifurcation in the Sagittarius Stream
• The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31)
• A deep kinematic survey of planetary nebulae in the Andromeda Galaxy using the Planetary Nebula Spectrograph
• A kinematically selected, metal-poor stellar halo in the outskirts of M31
• Exploring Halo Substructure with Giant Stars X. Extended Dark Matter or Tidal Disruption?: The Case for the Leo I Dwarf Spheroidal Galaxy
• Investigating the Andromeda Stream: III. A Young Shell System in M31
• A New Method for Isolating M31 Red Giant Stars: The Discovery of Stars out to a Radial Distance of 165 Kiloparsecs
• The Extended Star Formation History of the Andromeda Spheroid at Twenty One Kiloparsecs on the Minor Axis

Cited by in corpus (32)

• The Dual Origin of Stellar Halos
• The M31 Velocity Vector. II. Radial Orbit Towards the Milky Way and Implied Local Group Mass
• Modeling The Structure And Dynamics of Dwarf Spheroidal Galaxies with Dark Matter And Tides
• Global Properties of M31's Stellar Halo from the SPLASH Survey. I. Surface Brightness Profile
• The Nature and Origin of Substructure in the Outskirts of M31. I. Surveying the Stellar Content with HST/ACS
• Global Properties of M31's Stellar Halo from the SPLASH Survey: II. Metallicity Profile
• Was the Andromeda Stream Produced by a Disk Galaxy?
• The Stellar Content of Galaxy Halos: A Comparison between LambdaCDM Models and Observations of M31
• A clear age-velocity dispersion correlation in Andromeda's stellar disk
• 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
• DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of our Nearest Neighbor
• Elemental Abundances in M31: A Comparative Analysis of Iron and Alpha Element Abundances in the Outer Disk, Giant Stellar Stream, and Inner Halo of M31
• Constraining the Mass Profiles of Stellar Systems: Schwarzschild Modeling of Discrete Velocity Datasets
• Elemental Abundances in M31: First Alpha and Iron Abundance Measurements in M31's Giant Stellar Stream
• Global Properties of M31's Stellar Halo from the SPLASH Survey: III. Measuring the Stellar Velocity Dispersion Profile
• Elemental Abundances in M31: Gradients in the Giant Stellar Stream
• The kinematic footprints of five stellar streams in Andromeda's halo
• Accretion in action: phase space coherence of stellar debris and globular clusters in Andromeda's South-West Cloud
• Elemental Abundances in M31: Properties of the Inner Stellar Halo
• Elemental Abundances in M31: Iron and Alpha Element Abundances in M31's Outer Halo
• Carbon Stars in the Satellites and Halo of M31
• Elemental abundances in M31: Individual and Coadded Spectroscopic [Fe/H] and [α/Fe] throughout the M31 Halo with SPLASH
• Tracing the Metal-Poor M31 Stellar Halo with Blue Horizontal Branch Stars
• Kinematics and Metallicity of Red Giant Branch Stars in the Northeast Shelf of M31
• Resolved SPLASH Chemodynamics in Andromeda's PHAT Stellar Halo and Disk: On the Nature of the Inner Halo Along the Major Axis
• A catalogue of H$\alpha$ emission-line point sources in the vicinity fields of M31 and M33 from the LAMOST survey
• The survey of planetary nebulae in Andromeda (M31) VII. Predictions of a major merger simulation model compared with chemodynamical data of the disc and inner halo substructures
• The origin of the metallicity distributions of the NE and W stellar shelves in the Andromeda Galaxy
• First resolved stellar halo kinematics of a MW-mass galaxy outside the Local Group: A flat counter-rotating halo in NGC 4945
• On the Use of Field RR Lyrae as Galactic Probes -- VIII. Early Formation of the Galactic Spheroid
• Architecture of the Andromeda: a quantitative analysis of clustering in the inner stellar halo