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

TL;DR

This study uses the TNG50 simulation to investigate how Milky Way- and Andromeda-like galaxies can survive or reform their stellar disks after major mergers, revealing that disk survival is common despite frequent mergers.

Contribution

It provides new insights into the merger histories of MW/M31-like galaxies and demonstrates that disk survival or reformation is possible even after recent major mergers.

Findings

85% of MW/M31-like galaxies experienced at least one major merger.

16% had a recent major merger within the last 5 Gyr.

Disks can reform after mergers due to star formation, despite initial destruction.

Abstract

We analyze the merger and assembly histories of Milky Way (MW) and Andromeda (M31)-like galaxies to quantify how, and how often, disk galaxies of this mass can survive recent major mergers (stellar mass ratio $\ge$ 1:4). For this, we use the cosmological magneto-hydrodynamical simulation TNG50 and identify 198 analog galaxies, selected based on their $z=0$ stellar mass ($10^{10.5-11.2} {\rm M_{\odot}}$), disky stellar morphology and local environment. Firstly, major mergers are common: 85 per cent (168) of MW/M31-like galaxies in TNG50 have undergone at least one major merger across their lifetime. In fact, 31 galaxies (16 per cent) have undergone a recent major merger, i.e. in the last 5 Gyr. The gas available during the merger suffices to either induce starbursts at pericentric passages or to sustain prolonged star formation after coalescence: in roughly half of the cases, the pre-existing stellar disk is destroyed because of the merger but reforms thanks to star formation. Moreover, higher merger mass ratios are more likely to destroy the stellar disks. In comparison to those with more ancient massive mergers, MW/M31-like galaxies with recent major mergers have, on average, somewhat thicker stellar disks, more massive and somewhat shallower stellar haloes, larger stellar ex-situ mass fractions, but similarly massive kinematically-defined bulges. All this is qualitatively consistent with the different observed properties of the Galaxy and Andromeda and with the constraints on their most recent major mergers, 8-11 and ~2 Gyr ago, respectively. According to contemporary cosmological simulations, a recent quiet merger history is not a pre-requisite for obtaining a relatively-thin stellar disk at $z=0$.