标题： 被吸积的球状星团和仙女座星系外晕中的水平分支形态 显示英文标题

标题： Accreted Globular Clusters and Horizontal Branch Morphology in the Outer Halo of M31

摘要： M31拥有丰富的外晕（$R_{\rm{proj}} > 25$kpc）球状星团（GCs）群体，其中许多显示出与大尺度潮汐碎片特征的空间和/或运动学关联的强烈证据。 我们展示了对48个晕球状星团的深度哈勃空间望远镜测光数据，其中包括18个与恒星流有明确联系的星团和13个可能有关联的星团。 利用颜色-星等图（CMDs），我们量化了水平分支（HB）的形态，并采用新的经验关系，这些关系基于银河系（MW）球状星团校准，以一致的方式推导出金属丰度和视线消光。 我们发现水平分支形态与子结构的存在之间存在显著相关性：具有非常红色水平分支的星团几乎都与子结构相关，而非子结构星团则具有延伸的蓝色水平分支。 这提供了红水平分支晕星团起源于卫星吸积的第一直接证据，这一观点近50年前由银河系研究提出，但直到现在才得到证实。 除了更富金属的尾部，子结构星团样本中还包含一些金属含量极低且具有红色水平分支的星团，这在银河系中没有任何已知天体类似。 我们认为这些是最近被吸积的年轻星团，支持了越来越多的证据，表明仙女座星系（M31）经历的吸积历史比银河系更为漫长。 显示英文摘要

摘要： M31 hosts a rich population of outer halo ($R_{\rm{proj}} > 25$ kpc) globular clusters (GCs), many of which show strong evidence for spatial and/or kinematical associations with large-scale tidal debris features. We present deep Hubble Space Telescope photometry of 48 halo GCs, including 18 with clear ties to stellar streams and 13 with potential associations. Using the colour-magnitude diagrams (CMDs), we quantify the horizontal branch (HB) morphologies and employ new empirical relationships, calibrated on Milky Way (MW) GCs, to consistently derive metallicities and line-of-sight extinctions. We find a remarkable correlation between HB morphology and the presence of substructure: GCs with very red HBs are almost exclusively associated with substructure, while non-substructure GCs have extended blue HBs. This provides the first direct evidence that red HB halo clusters originate from satellite accretion, a notion introduced nearly 50 years ago from MW studies which has remained unconfirmed until now. In addition to a more metal-rich tail, the substructure GC sample also contains a few clusters with very low metallicities and red HBs, unlike any objects known in the MW. We suggest these are recently accreted young clusters, supporting the growing evidence that M31 has experienced a more prolonged accretion history than the MW.