The Extended Star Formation History of the Andromeda Spheroid at 35 Kpc on the Minor Axis

Abstract

Using the HST ACS, we have obtained deep optical images reaching well below the oldest main sequence turnoff in fields on the southeast minor-axis of the Andromeda Galaxy, 35 kpc from the nucleus. These data probe the star formation history in the extended halo of Andromeda -- that region beyond 30 kpc that appears both chemically and morphologically distinct from the metal-rich, highly-disturbed inner spheroid. The present data, together with our previous data for fields at 11 and 21 kpc, do not show a simple trend toward older ages and lower metallicities, as one might expect for populations further removed from the obvious disturbances of the inner spheroid. Specifically, the mean ages and [Fe/H] values at 11 kpc, 21 kpc, and 35 kpc are 9.7 Gyr and -0.65, 11.0 Gyr and -0.87, and 10.5 Gyr and -0.98, respectively. In the best-fit model of the 35 kpc population, one third of the stars are younger than 10 Gyr, while only ~10% of the stars are truly ancient and metal-poor. The extended halo thus exhibits clear evidence of its hierarchical assembly, and the contribution from any classical halo formed via early monolithic collapse must be small.

Accepted for publication in The Astrophysical Journal Letters. 4 pages, latex, 2 color figures

References in corpus (7)

• The Haunted Halos of Andromeda and Triangulum: A panorama of galaxy formation in action
• On the mass-to-light ratio of the local Galactic disc and the optical luminosity of the Galaxy
• The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31)
• 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
• 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
• The Extended Star Formation History of the Andromeda Spheroid at Twenty One Kiloparsecs on the Minor Axis

Cited by in corpus (19)

• The Panchromatic Hubble Andromeda Treasury X. Ultraviolet to Infrared Photometry of 117 Million Equidistant Stars
• 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
• Global Properties of M31's Stellar Halo from the SPLASH Survey: II. Metallicity Profile
• A forming, dust enshrouded disk at z=0.43: the first example of a late type disk rebuilt after a major merger?
• Chronology of our Galaxy from Gaia Colour-Magnitude Diagram-fitting (ChronoGal). I. The formation and evolution of the thin disk from the Gaia Catalogue of Nearby Stars
• Elemental Abundances in M31: Properties of the Inner Stellar Halo
• Radial Star Formation Histories in 32 Nearby Galaxies
• Elemental Abundances in M31: Iron and Alpha Element Abundances in M31's Outer Halo
• The elusive stellar halo of the Triangulum Galaxy
• 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
• Harnessing the Hubble Space Telescope Archives: A Catalogue of 21,926 Interacting Galaxies
• The Blue Straggler Population in Dwarf Galaxies
• The origin of the metallicity distributions of the NE and W stellar shelves in the Andromeda Galaxy
• The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) II. The Spatially Resolved Recent Star Formation History of M33
• The intermediate age population of the Galactic halo
• The Panchromatic Hubble Andromeda Southern Treasury (PHAST). II. The Spatially Resolved Recent Star Formation History in M31