发表机构
University of Edinburgh; Independent Researcher; University of Exeter; University of Cambridge; National Research Council Herzberg Astronomy and Astrophysics; University of California, Berkeley; University of Leicester（爱丁堡大学; 独立研究者; 埃克塞特大学; 剑桥大学; 国家研究委员会赫茨伯格天文学与天体物理学研究所; 加州大学伯克利分校; 莱斯特大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
该研究通过HST测光表征NGC6822、NGC147、NGC185的26个球状星团，发现三者平均金属丰度相近但HB形态差异大，识别出3个特殊异常星团及SC7的潮汐畸变，为研究星系吸积历史提供了线索。
AI 中文摘要
我们给出了本星系群三个矮星系中26个球状星团（GCs）的哈勃空间望远镜（HST）深场测光数据，这三个星系分别是孤立矮不规则（dIrr）星系NGC6822，以及M31的矮椭圆（dE）卫星星系NGC147和NGC185。从它们的色-星等图（CMDs）中，我们量化了它们的红巨星分支（RGB）和水平分支（HB）形态，并采用新的经验关系推导了金属丰度、红化和距离的测量值。此外，我们还从它们的积分光轮廓中测量了大小和V波段星等。我们发现这些星团的金属丰度范围为[Fe/H]≈-0.7至≤-2；然而，尽管这三个矮星系的演化历史差异极大，它们的球状星团平均金属丰度却非常相似（≈-1.7 dex）。相比之下，我们发现几乎所有NGC6822的球状星团都呈现红HB形态，而两个矮椭圆星系中的球状星团则以蓝HB为主。我们重点关注NGC6822中三个金属丰度极低但HB非常红的异常球状星团；它们也是我们样本中最延展的星团，半光半径约为13-17 pc。如果这些星团是年轻的，考虑到它们位置非常偏远，且金属丰度显著低于NGC6822中古老和中年恒星的金属丰度，其起源将难以解释。这三个星团与M31外晕中与次结构相关的几个球状星团极为相似，表明要么M31近期吸积了一个类似NGC6822的矮星系，要么M31和NGC6822近期都吸积了一个类似的低质量系统。此外，我们发现有力证据表明，NGC6822的另一个球状星团SC7存在潮汐畸变。
英文摘要
We present deep HST photometry for 26 globular clusters (GCs) residing in three Local Group dwarf galaxies: the isolated dwarf irregular (dIrr) galaxy, NGC6822, and the M31 dwarf elliptical (dE) satellites, NGC147 and NGC185. From their colour-magnitude diagrams (CMDs), we quantify their red giant branch (RGB) and horizontal branch (HB) morphologies, and employ new empirical relationships to derive measurements of metallicity, reddening and distance. Additionally, we measure sizes and $V$-band magnitudes from their integrated light profiles. We find that the clusters span a range of metallicities, from [Fe/H] $\sim-0.7$ to $\lesssim-2$; however, the three dwarfs have very similar mean GC metallicities ($\sim-1.7$ dex) despite their very different evolutionary histories. In contrast, we find that almost all of NGC6822's GCs exhibit red HB morphologies, whereas those in the two dEs are predominantly blue. We highlight three outlying GCs in NGC6822 that have very low metallicities yet very red HBs; they are also the most extended clusters in our sample, with half-light radii of $\sim 13-17$pc. If these clusters are young, their origin is difficult to explain given their very remote locations, and metallicities which are significantly lower than those of the old and intermediate-age stars in NGC6822. These three clusters are strikingly similar to several GCs linked to substructure in the outer halo of M31, suggesting either the recent accretion of an NGC6822-like dwarf by M31, or that both M31 and NGC6822 have recently accreted a similar low-mass system. Additionally, we find compelling evidence that another NGC6822 cluster, SC7, is tidally distorted.
CommentsAccepted for publication in MNRAS. 20 pages, 16 figures
