The outer halo globular cluster system of M31 - III. Relationship to the stellar halo

Abstract

We utilize the final catalogue from the Pan-Andromeda Archaeological Survey to investigate the links between the globular cluster system and field halo in M31 at projected radii R_proj = 25-150 kpc. In this region the cluster radial density profile exhibits a power-law decline with index Γ = -2.37 ± 0.17, matching that for the stellar halo component with [Fe/H] < -1.1. Spatial density maps reveal a striking correspondence between the most luminous substructures in the metal-poor field halo and the positions of many globular clusters. By comparing the density of metal-poor halo stars local to each cluster with the azimuthal distribution at commensurate radius, we reject the possibility of no correlation between clusters and field overdensities at 99.95{{ per cent}} significance. We use our stellar density measurements and previous kinematic data to demonstrate that ≈ 35-60 per cent of clusters exhibit properties consistent with having been accreted into the outskirts of M31 at late times with their parent dwarfs. Conversely, at least {∼ }40{{ per cent}} of remote clusters show no evidence for a link with halo substructure. The radial density profile for this subgroup is featureless and closely mirrors that observed for the apparently smooth component of the metal-poor stellar halo. We speculate that these clusters are associated with the smooth halo; if so, their properties appear consistent with a scenario where the smooth halo was built up at early times via the destruction of primitive satellites. In this picture the entire M31 globular cluster system outside R_proj = 25 kpc comprises objects accumulated from external galaxies over a Hubble time of growth.