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

**Authors:** Gracie McGill, Annette M. N. Ferguson, Dougal Mackey, Avon P. Huxor, Geraint F. Lewis, Nicolas F. Martin, Alan W. McConnachie, Charli M. Sakari, Nial R. Tanvir, Kim A. Venn  
**arXiv:** [2506.21267](https://arxiv.org/abs/2506.21267)  
**Date:** 2025-06-26  
**Summary page:** https://www.summarizepaper.com/en/paper/2506.21267v1/

## Key points

- Investigated outer halo of M31, focusing on 48 globular clusters (GCs) beyond 25 kpc from the center
- Analyzed color-magnitude diagrams (CMDs) to determine HB morphologies, metallicities, and extinctions
- Found correlation between HB morphology and substructure in outer halo GCs, indicating connection to satellite accretion events
- Discovered diverse properties within substructure GC sample, including very low metallicities and red HBs not typical in Milky Way objects
- Suggested that unique clusters may represent recently accreted young clusters, implying prolonged history of satellite accretion in M31 compared to the Milky Way

## Summary

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In this study, we investigate the outer halo of M31, a galaxy renowned for its abundance of globular clusters (GCs) located at distances greater than 25 kpc from the center. Utilizing deep observations from the Hubble Space Telescope, we focus on 48 halo GCs and examine their connections to stellar streams and potential associations with tidal debris features. By analyzing the color-magnitude diagrams (CMDs) of these clusters, we are able to determine their horizontal branch (HB) morphologies, metallicities, and line-of-sight extinctions using newly calibrated empirical relationships based on Milky Way GCs. One notable finding is the correlation between HB morphology and substructure in these outer halo GCs. We observe that GCs with very red HBs are predominantly linked to substructure, indicating a strong connection to satellite accretion events. This discovery provides direct evidence supporting a theory introduced in Milky Way studies nearly half a century ago but never confirmed until now. Furthermore, our analysis reveals a diverse range of properties within the substructure GC sample. In addition to having a more metal-rich tail compared to non-substructure GCs, we identify several clusters with very low metallicities and red HBs - characteristics not typically seen in Milky Way objects. These unique clusters may represent recently accreted young clusters, suggesting that M31 has experienced a more prolonged history of satellite accretion compared to our own galaxy. Overall, our study sheds light on the complex formation history of M31's outer halo globular clusters and emphasizes the importance of understanding their origins through detailed observational analyses. This work contributes valuable insights into the dynamics of galaxy evolution and provides key evidence for ongoing research into the accretion processes shaping galaxies in our cosmic neighborhood.