The Nature and Origin of Substructure in the Outskirts of M31 -- II. Detailed Star Formation Histories

Abstract

While wide-field surveys of M31 have revealed much substructure at large radii, understanding the nature and origin of this material is not straightforward from morphology alone. Using deep HST/ACS data, we have derived further constraints in the form of quantitative star formation histories (SFHs) for 14 inner halo fields which sample diverse substructures. In agreement with our previous analysis of colour-magnitude diagram morphologies, we find the resultant behaviours can be broadly separated into two categories. The SFHs of 'disc-like' fields indicate that most of their mass has formed since z~1, with one quarter of the mass formed in the last 5 Gyr. We find 'stream-like' fields to be on average 1.5 Gyr older, with <10 percent of their stellar mass formed within the last 5 Gyr. These fields are also characterised by an age--metallicity relation showing rapid chemical enrichment to solar metallicity by z=1, suggestive of an early-type progenitor. We confirm a significant burst of star formation 2 Gyr ago, discovered in our previous work, in all the fields studied here. The presence of these young stars in our most remote fields suggests that they have not formed in situ but have been kicked-out from through disc heating in the recent past.

14 pages, 10 figures (+12 figures in appendix). MNRAS, in press

References in corpus (15)

• The Star Formation Histories of Local Group Dwarf Galaxies I. Hubble Space Telescope / Wide Field Planetary Camera 2 Observations
• The Haunted Halos of Andromeda and Triangulum: A panorama of galaxy formation in action
• The ACS Survey of Galactic Globular Clusters: M54 and Young Populations in the Sagittarius Dwarf Spheroidal Galaxy
• The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31)
• A Wide-field High Resolution HI Mosaic of Messier 31: I. Opaque Atomic Gas and Star Formation Rate Density
• Cold Dark Matter Substructure and Galactic Disks II: Dynamical Effects of Hierarchical Satellite Accretion
• Investigating the Andromeda Stream: III. A Young Shell System in M31
• Structural parameters for globular clusters in M31 and generalizations for the fundamental plane
• Recovering the star formation rate in the solar neighborhood
• 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 Once and Future Andromeda Stream
• Substructure around M31 : Evolution and Effects
• Identifying Contributions to the Stellar Halo from Accreted, Kicked-Out, and In Situ Populations
• Probing the Nature of the G1 Clump Stellar Overdensity in the Outskirts of M31

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