The solitary star cluster of the Andromeda XXV dwarf spheroidal

Abstract

We present Hubble Space Telescope Advanced Camera for Surveys observations of Gep I, a globular cluster (GC) candidate in the low-mass ($M_{\star}\sim 6.5 \times 10^5\,M_{\odot}$) M31 dwarf spheroidal (dSph) satellite Andromeda XXV (And XXV). We confirm the nature of this object and provide the first detailed characterisation of its resolved stellar populations using a colour-magnitude diagram (CMD) that reaches 2 magnitudes below the horizontal branch. We compare Gep I's metallicity and distance with those of the surrounding And XXV stellar population, and find them to be strikingly similar, consistent with a physical association between the GC and the dSph. Gep I is very extended ($R_h=24^{+5}_{-4}$ pc) and faint ($M_V = -4.5 \pm 0.2$ mag), similar to the star clusters residing in other low-mass dwarf galaxies. It is characterised by a very low metallicity ($\rm [Fe/H] = -2.4^{+0.3}_{-0.4}\,$dex) and a red horizontal branch morphology, a combination also seen in suspected accreted GCs in the M31 halo and in the Local Group dwarf irregular galaxy NGC 6822. While Gep I is most likely a genuine star cluster, the current data do not exclude the tantalising possibility that it consists of And XXV stars temporarily captured by a dark subhalo orbiting within the dSph's potential well.