发表机构
Institute for Astronomy, University of Edinburgh; INAF - Osservatorio di Astrofisica e Scienza dello Spazio di Bologna; National Research Council Herzberg Astronomy and Astrophysics（爱丁堡大学天文研究所; 意大利国家天体物理研究所博洛尼亚天文与空间科学观测站; 国家研究委员会赫尔茨伯格天文学与天体物理学研究所）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
研究对仙女座XXV矮椭球星系中的Gep I候选体开展观测，确认其为星团或暗亚晕捕获的恒星集合，表征其参数并探讨其物理关联。
AI 中文摘要
我们呈现了哈勃空间望远镜高级巡天相机对Gep I的观测结果，Gep I是低质量（恒星质量M★~6.5×10^5 M⊙）M31矮椭球星系（dSph）卫星仙女座XXV（And XXV）中的一个球状星团（GC）候选体。我们确认了该天体的性质，并利用达到水平分支以下2个星等的色-星等图（CMD）首次对其分辨恒星族群进行了详细表征。我们将Gep I的金属丰度和距离与周围And XXV恒星族群的相应参数进行比较，发现二者极为相似，符合该球状星团与矮椭球星系存在物理关联的结论。Gep I延展度很高（半光半径Rh=24^(+5)_(-4) pc）且光度较暗（绝对星等MV=-4.5±0.2 mag），与其他低质量矮星系中的星团类似。其特征为金属丰度极低（[Fe/H]=-2.4^(+0.3)_(-0.4) dex），并呈现红色水平分支形态，这种组合也见于M31晕中疑似吸积形成的球状星团以及本星系群矮不规则星系NGC 6822中。尽管Gep I极有可能是一个真正的星团，但当前数据并未排除其由And XXV恒星被矮椭球星系引力势阱内运行的暗亚晕临时捕获形成的诱人可能性。
英文摘要
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.
CommentsAccepted for publication in MNRAS. 11 pages, 6 figures, 2 tables
Journal refMon Not R Astron Soc (2026)
