A 2-3 billion year old major merger paradigm for the Andromeda galaxy and its outskirts

TL;DR

This study uses high-resolution simulations to propose that a major 4:1 merger event 1.8-3 Gyr ago explains many observed features of the Andromeda galaxy, including its disk structure, star formation history, and halo substructures.

Contribution

It demonstrates that a single major merger can account for the complex features observed in M31, providing a unified explanation for its disk, halo, and substructures.

Findings

A 4:1 merger model reproduces M31's disk properties.

The Giant Stream can form with various merger mass ratios.

A recent major merger explains M31's complex halo and substructures.

Abstract

Recent observations of our neighbouring galaxy M31 have revealed that its disk was shaped by widespread events. The evidence for this includes the high dispersion ($V/\sigma$ $\le$ 3) of stars older than 2 Gyr, and a global star formation episode, 2-4 Gyr ago. Using the modern hydrodynamical code, GIZMO, we have performed 300 high-resolution simulations to explore the extent to which these observed properties can be explained by a single merger. We find that the observed M31 disk resembles models having experienced a 4:1 merger, in which the nuclei coalesced 1.8-3 Gyr ago, and where the first passage took place 7 to 10 Gyr ago at a large pericentre distance (32 kpc). We also show that within a family of orbital parameters, the Giant Stream (GS) can be formed with various merger mass-ratios, from 2:1 to 300:1. A recent major merger may be the only way to create the very unusual age-dispersion relation in the disk. It reproduces and explains the long-lived 10 kpc ring, the widespread and recent star formation event, the absence of a remnant of the GS progenitor, the apparent complexity of the 3D spatial distribution of the GS, the NE and G Clumps and their formation process, and the observed slope of the halo profile. These modelling successes lead us to propose that the bulk of the substructure in the M31 halo, as well as the complexity of the inner galaxy, may be attributable to a single major interaction with a galaxy that has now fully coalesced with Andromeda.