The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31)

Abstract

We present spectroscopic observations of red giant branch (RGB) stars over a large expanse in the halo of the Andromeda spiral galaxy (M31), acquired with the DEIMOS instrument on the Keck II 10-m telescope. Using a combination of five photometric/spectroscopic diagnostics -- (1) radial velocity, (2) intermediate-width DDO51 photometry, (3) Na I equivalent width (surface gravity sensitive), (4) position in the color-magnitude diagram, and (5) comparison between photometric and spectroscopic [Fe/H] estimates -- we isolate over 250 bona fide M31 bulge and halo RGB stars located in twelve fields ranging from R = 12-165kpc from the center of M31 (47 of these stars are halo members with R > 60 kpc). We derive the photometric and spectroscopic metallicity distribution function of M31 RGB stars in each of these fields. The mean of the resulting M31 spheroid (bulge and halo) metallicity distribution is found to be systematically more metal-poor with increasing radius, shifting from <[Fe/H]> = -0.47+/-0.03 (sigma = 0.39) at R < 20 kpc to <[Fe/H]> = -0.94+/-0.06 (sigma = 0.60) at R ~ 30 kpc to <[Fe/H]> = -1.26+/-0.10 (sigma = 0.72) at R > 60 kpc, assuming [alpha/Fe] = 0.0. These results indicate the presence of a metal-poor RGB population at large radial distances out to at least R = 160 kpc, thereby supporting our recent discovery of a stellar halo in M31: its halo and bulge (defined as the structural components with R^{-2} power law and de Vaucouleurs R^{1/4} law surface brightness profiles, respectively) are shown to have distinct metallicity distributions. If we assume an alpha-enhancement of [alpha/Fe] = +0.3 for M31's halo, we derive <[Fe/H]> = -1.5+/-0.1 (sigma = 0.7). Therefore, the mean metallicity and metallicity spread of this newly found remote M31 RGB population are similar to those of the Milky Way halo.

Accepted for publication in ApJ on May 4th, 2006 (submitted on Jan 30, 2006). 16 pages, 13 figures, 3 tables

Cited by in corpus (57)

• The Haunted Halos of Andromeda and Triangulum: A panorama of galaxy formation in action
• Tracing Galaxy Formation with Stellar Halos II: Relating Substructure in Phase- and Abundance-Space to Accretion Histories
• The Dual Origin of Stellar Halos
• The M31 Velocity Vector. II. Radial Orbit Towards the Milky Way and Implied Local Group Mass
• The stellar halo of the Galaxy
• The SAGES Legacy Unifying Globulars and GalaxieS Survey (SLUGGS): Sample definition, methods, and initial results
• Halo Mass and Assembly History Exposed in the Faint Outskirts: the Stellar and Dark Matter Haloes of Illustris Galaxies
• A trio of new Local Group galaxies with extreme properties
• A kinematically selected, metal-poor stellar halo in the outskirts of M31
• Investigating the Andromeda Stream: III. A Young Shell System in M31
• Metallicity and Alpha-Element Abundance Measurement in Red Giant Stars from Medium Resolution Spectra
• The Panchromatic Hubble Andromeda Treasury XI: The Spatially-Resolved Recent Star Formation History of M31
• Global Properties of M31's Stellar Halo from the SPLASH Survey. I. Surface Brightness Profile
• A New Method for Isolating M31 Red Giant Stars: The Discovery of Stars out to a Radial Distance of 165 Kiloparsecs
• An updated survey of globular clusters in M31. II Newly discovered bright and remote clusters
• Global Properties of M31's Stellar Halo from the SPLASH Survey: II. Metallicity Profile
• The Nature and Origin of Substructure in the Outskirts of M31 -- II. Detailed Star Formation Histories
• 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
• The Hubble Space Telescope Survey of M31 Satellite Galaxies I. RR Lyrae-based Distances and Refined 3D Geometric Structure
• The Need for Speed: Escape velocity and dynamical mass measurements of the Andromeda galaxy
• 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
• ACS photometry of newly-discovered globular clusters in the outer halo of M31
• The Extended Star Formation History of the Andromeda Spheroid at Twenty One Kiloparsecs on the Minor Axis
• Wide-Field Survey of Globular Clusters in M31. II. Kinematics of the Globular Cluster System
• An HST/ACS investigation of the spatial and chemical structure and sub-structure of NGC 891, a Milky Way analogue
• Comparing the observable properties of dwarf galaxies on and off the Andromeda plane
• DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of our Nearest Neighbor
• The difference in metallicity distribution functions of halo stars and globular clusters as a function of galaxy type: A tracer of globular cluster formation and evolution
• A Global Star Forming Episode in M31 2-4 Gyr Ago
• The M31 microlensing event WeCAPP-GL1/Point-AGAPE-S3: evidence for a MACHO component in the dark halo of M31?
• Global Properties of M31's Stellar Halo from the SPLASH Survey: III. Measuring the Stellar Velocity Dispersion Profile
• The Outer Halo Globular Clusters of M31
• Spectroscopic versus Photometric Metallicities: Milky Way Dwarf Spheroidal Companions as a Test Case
• Probing the Nature of the G1 Clump Stellar Overdensity in the Outskirts of M31
• The Identification of New Stellar Groupings in the M81 Debris Field
• The Strikingly Metal-Rich Halo of the Sombrero Galaxy
• Elemental abundances in M31: [Fe/H] and [α/Fe] in M31 Dwarf Galaxies Using Coadded Spectra
• [$\alpha$/Fe] Abundances of Four Outer M 31 Halo Stars
• The kinematic footprints of five stellar streams in Andromeda's halo
• Unearthing Foundations of a Cosmic Cathedral: Searching the Stars for M33's Halo
• Globular cluster content and evolutionary history of NGC147
• 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
• Discovery of an extended halo of metal-poor stars in the Andromeda spiral galaxy
• Elemental abundances in M31: Individual and Coadded Spectroscopic [Fe/H] and [α/Fe] throughout the M31 Halo with SPLASH
• Metallicity distribution of the progenitor of the Giant Stellar Stream in the Andromeda Galaxy
• Tracing the Metal-Poor M31 Stellar Halo with Blue Horizontal Branch Stars
• Astrophysics in 2006
• Red giants in the outer halo of the elliptical galaxy NGC 5128 / Centaurus A
• Metallicities of Outer Halo M31 Globular Clusters from Integrated Light Calcium-II Triplet Spectroscopy
• A catalogue of H$\alpha$ emission-line point sources in the vicinity fields of M31 and M33 from the LAMOST survey
• Emission-line stars in M31 from the SPLASH and PHAT surveys
• Detection of a Stellar Halo in NGC 4244
• Stellar halos of bright central galaxies II: Scaling relations, colors and metallicity evolution with redshift
• The Structure of Galaxies at Faint Light Levels: Probing Galaxy Assembly