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

TL;DR

This study uses deep HST imaging to analyze the star formation history of Andromeda's extended halo at 35 kpc, revealing a complex, hierarchical assembly with diverse stellar ages and metallicities.

Contribution

First detailed star formation history analysis of Andromeda's halo beyond 30 kpc, showing a non-monotonic age and metallicity trend and evidence of hierarchical assembly.

Findings

Mean stellar ages at 11, 21, and 35 kpc are 9.7, 11.0, and 10.5 Gyr.

Approximately one-third of stars at 35 kpc are younger than 10 Gyr.

The halo shows signs of hierarchical assembly with minimal contribution from classical early-collapse halo.

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.