Origin of the elevated carbon-star density in the Andromeda II halo field

Determine the cause of the unusually high density of candidate carbon-rich asymptotic giant branch stars in the halo field surrounding Andromeda II, including whether it arises from M31 substructure, residual Milky Way contamination, or an intrinsic stellar population.

Background

The halo field surrounding Andromeda II contains an unusually high density of candidate carbon stars compared with the other M31 halo fields. The field lies behind the Triangulum–Andromeda structure, and a stream-like feature in that Milky Way structure is present in the same region.

The authors test whether the corresponding Milky Way main-sequence population could contaminate the TP-AGB selection, but find that the relevant isochrone stars are too blue. Despite this check, the physical origin of the elevated carbon-star density is not resolved.