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 ($R_h=24_{-4}^{+5}$ 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 ($[Fe/H]=-2.4_{-0.4}^{+0.3}$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.

Accepted for publication in MNRAS. 11 pages, 6 figures, 2 tables