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PreprintCC BY 4.0arXiv

The solitary star cluster of the Andromeda XXV dwarf spheroidal

  • Chiara Crociati
  • Annette M. N. Ferguson
  • Gracie McGill
  • Raffaele Pascale
  • Anna Genina
  • Pete B. Kuzma
  • Dougal Mackey
  • Alan W. McConnachie
  • Rokas Žemaitis
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Abstract

We present Hubble Space Telescope Advanced Camera for Surveys observations of Gep I, a globular cluster (GC) candidate in the low-mass (M⋆∼ 6.5 × 105 M⊙) 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 (Rh=24+5−4 pc) and faint (MV = −4.5 ± 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+0.3−0.4dex) 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.

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Cite

BibTeXRIS
BibTeX
@misc{crociati2026solitary,
  title = {{The solitary star cluster of the Andromeda XXV dwarf spheroidal}},
  author = {Chiara Crociati and Annette M. N. Ferguson and Gracie McGill and Raffaele Pascale and Anna Genina and Pete B. Kuzma and Dougal Mackey and Alan W. McConnachie and Rokas Žemaitis},
  howpublished = {arXiv},
  year = {2026},
  month = aug,
  doi = {10.48550/arxiv.2608.26959},
  eprint = {2608.26959},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2608.26959},
  note = {CC BY 4.0},
}
RIS
TY  - UNPB
TI  - The solitary star cluster of the Andromeda XXV dwarf spheroidal
AU  - Chiara Crociati
AU  - Annette M. N. Ferguson
AU  - Gracie McGill
AU  - Raffaele Pascale
AU  - Anna Genina
AU  - Pete B. Kuzma
AU  - Dougal Mackey
AU  - Alan W. McConnachie
AU  - Rokas Žemaitis
PY  - 2026
DA  - 2026/08/31
PB  - arXiv
DO  - 10.48550/arxiv.2608.26959
UR  - https://arxiv.org/abs/2608.26959
AB  - We present Hubble Space Telescope Advanced Camera for Surveys observations of Gep I, a globular cluster (GC) candidate in the low-mass (M_⋆∼ 6.5 × 10^5 M_⊙) 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 ± 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^+0.3_−0.4dex) 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.
ER  -