# The effect of environment on the mass assembly history of the Milky Way and M31

**Authors:** Ewoud Wempe, Amina Helmi, Simon D. M. White, Jens Jasche, Guilhem Lavaux  
**arXiv:** [2501.08089](https://arxiv.org/abs/2501.08089)  
**Date:** 2025-01-14  
**Summary page:** https://www.summarizepaper.com/en/paper/2501.08089v1/

## Key points

- Wempe et al. study mass growth histories of Milky Way and M31 analogues in Local Group simulations
- Comprehensive analysis tailored to match key properties of main galaxies in the Local Group
- Differences between M31 and Milky Way halos in mass growth patterns and timing of formation
- Role of major and minor mergers in shaping evolutionary paths of Milky Way and M31 analogues
- Bias towards early epochs for Milky Ways in mass accretion through mergers compared to M31s
- Significant portion of Milky Ways undergo Gaia-Enceladus/Sausage-like mergers and host objects resembling LMC
- Origins of material comprising Milky Way and M31 halos traced back to a plane initially aligned with the Local Sheet and Supergalactic plane
- Accretion events influenced significantly by tidal interactions with M31 for Milky Ways as cosmic time progresses beyond redshift z ~ 2

## Summary

In their study, Wempe et al. investigate the mass growth histories of Milky Way and M31 analogues within constrained cosmological simulations of the Local Group. By analyzing a comprehensive set of $\Lambda$CDM realizations tailored to match key properties of the main galaxies in the Local Group, such as their masses, relative separation, dynamics, and environment, the researchers offer valuable insights into the formation and evolution of these galactic halos. A comparison with isolated analogues from TNG dark-matter-only simulations reveals intriguing differences: while M31 halos exhibit similar mass growth patterns to their isolated counterparts, Milky Ways tend to form earlier and experience suppressed growth in later stages. The study also sheds light on the role of major and minor mergers in shaping the evolutionary paths of Milky Way and M31 analogues. Interestingly, the authors find that mass accretion through mergers is biased towards early epochs for Milky Ways compared to M31s, with a majority of accretion events occurring between 1 to 4 billion years after the Big Bang. Notably, a significant portion (32%) of Milky Ways in the simulations undergo Gaia-Enceladus/Sausage (GES)-like mergers, while 13% host objects resembling the Large Magellanic Cloud (LMC), with 5% featuring both types of mergers. Remarkably, one simulation even includes analogues for both the Small Magellanic Cloud (SMC) and Sagittarius dwarf galaxy. Furthermore, Wempe et al. trace back the origins of material comprising Milky Way and M31 halos to a plane initially aligned with the Local Sheet and Supergalactic plane. As cosmic time progresses beyond redshift $z \sim 2$, most accretion events occur within this plane, influenced significantly by tidal interactions with M31 for Milky Ways. The findings highlight how environmental factors play a crucial role in shaping the mass assembly histories of galaxies within complex cosmic structures like the Local Group. This detailed analysis not only enhances our understanding of galactic evolution but also demonstrates how key merger events observed in our own Milky Way can be reproduced within a $\Lambda$CDM framework.