# The merger and assembly histories of Milky Way- and M31-like galaxies with TNG50: disk survival through mergers

**Authors:** Diego Sotillo-Ramos, Annalisa Pillepich, Martina Donnari, Dylan Nelson, Lukas Eisert, Vicente Rodriguez-Gomez, Gandhali Joshi, Mark Vogelsberger, Lars Hernquist  
**arXiv:** [2211.00036](https://arxiv.org/abs/2211.00036)  
**Date:** 2022-10-31  
**Summary page:** https://www.summarizepaper.com/en/paper/2211.00036v1/

## Key points

- The paper analyzes the merger and assembly histories of Milky Way (MW) and Andromeda (M31)-like galaxies using the TNG50 simulation.
- The goal is to determine how often disk galaxies of this mass can survive recent major mergers with a stellar mass ratio greater than or equal to 1:4.
- 198 analog galaxies in TNG50 were identified based on their $z=0$ stellar mass ($10^{10.5-11.2} {\rm M_{\odot}}$), disky stellar morphology, and local environment.
- Major mergers are common, with 85% (168) of MW/M31-like galaxies in TNG50 having undergone at least one major merger across their lifetime, and 16% having undergone a recent major merger within the last 5 Gyr.
- Higher merger mass ratios are more likely to destroy the stellar disks, but roughly half of the cases result in the pre-existing stellar disk being destroyed but reforming thanks to star formation.
- MW/M31-like galaxies with recent major mergers have somewhat thicker stellar disks, more massive and somewhat shallower stellar haloes, larger ex-situ mass fractions but similarly massive kinematically-defined bulges compared to those with more ancient massive mergers.
- A recent quiet merger history is not necessary for obtaining a relatively thin stellar disk at $z=0$.
- These findings are consistent with observed properties of the Galaxy and Andromeda as well as constraints on their most recent major mergers.

## Summary

This paper presents an analysis of the merger and assembly histories of Milky Way (MW) and Andromeda (M31)-like galaxies using the cosmological magneto-hydrodynamical simulation TNG50. The goal is to quantify how often disk galaxies of this mass can survive recent major mergers with a stellar mass ratio greater than or equal to 1:4. The authors identify 198 analog galaxies in TNG50, selected based on their $z=0$ stellar mass ($10^{10.5-11.2} {\rm M_{\odot}}$), disky stellar morphology, and local environment. The study finds that major mergers are common, with 85% (168) of MW/M31-like galaxies in TNG50 having undergone at least one major merger across their lifetime. In fact, 31 galaxies (16%) have undergone a recent major merger within the last 5 Gyr. During these mergers, the gas available is sufficient to induce starbursts at pericentric passages or sustain prolonged star formation after coalescence. Roughly half of the cases result in the pre-existing stellar disk being destroyed but reforming thanks to star formation. Higher merger mass ratios are more likely to destroy the stellar disks, and MW/M31-like galaxies with recent major mergers have somewhat thicker stellar disks, more massive and somewhat shallower stellar haloes, larger ex-situ mass fractions but similarly massive kinematically-defined bulges compared to those with more ancient massive mergers. The paper also compares the 𝑧 = 0 structural and global properties of these galaxies with the rest of the TNG50 MW/M31-like objects and finds that a recent quiet merger history is not necessary for obtaining a relatively thin stellar disk at $z=0$. These findings are consistent with observed properties of the Galaxy and Andromeda as well as constraints on their most recent major mergers 8-11 Gyr ago for our Galaxy and ~2 Gyr ago for Andromeda respectively. Overall, this study provides valuable insights into how disk galaxies can survive through major mergers while shedding light on their properties in a cosmological context. The high resolution TNG50 simulation used here allows for large unbiased samples at relatively high resolution providing a unique opportunity to explore these questions in detail.