Young accreted globular clusters in the outer halo of M31

TL;DR

This study reports the discovery and analysis of two young globular clusters in the outer halo of M31, providing evidence of their accretion and contributing to understanding galaxy formation.

Contribution

It presents the first evidence of young globular clusters in M31's outer halo, supporting the accretion scenario similar to the Milky Way.

Findings

Both clusters are likely at least 2 Gyr younger than Milky Way globulars.

Clusters are associated with a halo substructure, the South-West Cloud.

Horizontal branch morphology indicates younger ages.

Abstract

We report on Gemini/GMOS observations of two newly discovered globular clusters in the outskirts of M31. These objects, PAndAS-7 and PAndAS-8, lie at a galactocentric radius of ~87 kpc and are projected, with separation ~19 kpc, onto a field halo substructure known as the South-West Cloud. We measure radial velocities for the two clusters which confirm that they are almost certainly physically associated with this feature. Colour-magnitude diagrams reveal strikingly short, exclusively red horizontal branches in both PA-7 and PA-8; both also have photometric [Fe/H] = -1.35 +/- 0.15. At this metallicity, the morphology of the horizontal branch is maximally sensitive to age, and we use the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both objects are very likely to be at least 2 Gyr younger than the oldest Milky Way globular clusters. Our observations provide strong evidence for young globular clusters being accreted into the remote outer regions of M31 in a manner entirely consistent with the established picture for the Milky Way, and add credence to the idea that similar processes play a central role in determining the composition of globular cluster systems in large spiral galaxies in general.