Determine the age of Gep I relative to Andromeda XXV

Determine whether Gep I formed contemporaneously with, or later than, the bulk of Andromeda XXV’s stellar mass by obtaining an age estimate based on photometry that reaches the cluster’s main-sequence turn-off.

Background

The existing Hubble Space Telescope photometry does not reach the main-sequence turn-off of Gep I, preventing a direct age determination. This leaves unresolved whether the cluster formed along with the predominantly old stellar population of Andromeda XXV or formed subsequently, a distinction that is particularly important because Gep I has a very red horizontal branch despite its low metallicity.

References

On the other hand, the current dataset does not allow us to reach a conclusion regarding the age similarity between the cluster and the dSph, as our photometry does not reach the main-sequence turn-off (MSTO).

— The solitary star cluster of the Andromeda XXV dwarf spheroidal  (2608.26959 - Crociati et al., 27 Aug 2026) in Section 3.1; Summary and conclusions