# In-situ vs accreted Milky Way globular clusters: a new classification method and implications for cluster formation

**Authors:** Vasily Belokurov, Andrey Kravtsov  
**arXiv:** [2309.15902](https://arxiv.org/abs/2309.15902)  
**Date:** 2023-09-27  
**Summary page:** https://www.summarizepaper.com/en/paper/2309.15902v1/

## Key points

- Belokurov and Kravtsov present a new scheme for classifying in-situ and accreted globular clusters (GCs) in the Milky Way
- The classification scheme uses total energy and the z-component of orbital angular momentum, calibrated using the [Al/Fe] abundance ratio
- In-situ GCs are distributed within the central 10 kpc of the Galaxy in a flattened configuration aligned with the Milky Way disc
- Accreted GCs have a wide distribution of distances and a spatial distribution close to spherical
- In-situ and accreted GCs have different distributions of iron abundance ([Fe/H]), with only in-situ GCs showing bimodality
- Accreted and in-situ GCs can be effectively separated based on their [Al/Fe]-[Mg/Fe] abundance ratios
- In-situ and accreted GCs follow distinct sequences in the age-[Fe/H] plane
- There is a clear disc spin-up signature in the median rotational velocity of in-situ GCs at metallicities around [Fe/H] ≈ -1.3 to -1, indicating formation of Milky Way's disc approximately 11.7–12.7 billion years ago (or at redshift z ≈ 3.1–5.3)
- High-metallicity in-situ GCs were born within Milky Way's disc while lower metallicity ones originated during early turbulent stages of Galaxy's evolution as part of its Aurora stellar component
- The authors invite comments on their classification scheme to improve understanding of formation and evolution processes related to globular clusters within our galaxy

## Summary

In their study, Belokurov and Kravtsov (2023) present a new scheme for classifying the in-situ and accreted globular clusters (GCs) in the Milky Way. The classification scheme utilizes the total energy ($E$) and $z$-component of the orbital angular momentum, calibrated using the [Al/Fe] abundance ratio. The authors demonstrate that this classification results in distinct spatial, kinematic, and chemical abundance distributions for the GC populations. The in-situ GCs are found to be distributed within the central 10 kpc of the Galaxy in a flattened configuration aligned with the Milky Way disc. On the other hand, the accreted GCs exhibit a wide distribution of distances and have a spatial distribution close to spherical. Furthermore, it is observed that in-situ and accreted GCs have different distributions of iron abundance ([Fe/H]), with only the metallicity distribution of in-situ GCs showing a well-known bimodality. Additionally, Belokurov and Kravtsov find that accreted and in-situ GCs can be effectively separated based on their [Al/Fe]-[Mg/Fe] abundance ratios. These two populations also follow distinct sequences in the age-[Fe/H] plane. Notably, they observe a clear disc spin-up signature in the median rotational velocity ($V_\phi$) of in-situ GCs at metallicities around [Fe/H] ≈ -1.3 to -1, similar to what is observed for in-situ field stars. This signature indicates that formation of Milky Way's disc occurred approximately 11.7–12.7 billion years ago (or at redshift z ≈ 3.1–5.3), as inferred from GC ages. Based on these findings, it is concluded that high-metallicity in-situ GCs were born within Milky Way's disc while lower metallicity ones originated during early turbulent stages of Galaxy's evolution as part of its Aurora stellar component. The authors invite comments on their classification scheme regarding any misclassified individual globular clusters which could help improve our understanding of formation and evolution processes related to globular clusters within our galaxy as well as provide insights into distinct properties between its two populations: In situ vs Accreted ones . Related studies by Belokurov and Kravtsov (2022) & (2023) discovered stars born In Situ & stars from dwarf galaxies exhibit distinct distributions of aluminum & sodium ratios which were utilized to identify & investigate kinematic & chemical properties associated with Milky Way's In Situ stellar populations & Globular Clusters respectively; thus enhancing our knowledge about different origins & characteristics associated with them while shedding light on their formation mechanisms contributing towards better understanding galactic evolution overall..