The impact of two massive early accretion events in a Milky Way-like galaxy: repercussions for the buildup of the stellar disc and halo

TL;DR

This study uses simulations to analyze how two massive early mergers shaped a Milky Way-like galaxy's stellar disc and halo, revealing their effects on galaxy structure, chemistry, and star formation history.

Contribution

It demonstrates that two significant high-redshift mergers can produce observable signatures in galaxy structure and star formation, challenging previous chemical abundance interpretations.

Findings

The mergers are gas-rich and deposit mass in different galaxy regions.

Chemical signatures of the mergers are indistinguishable from in-situ populations.

Star formation history shows bursts approximately 2 Gyr apart, linked to mergers.

Abstract

We identify and characterise a Milky Way-like realisation from the Auriga simulations with two consecutive massive mergers $\sim2\,$Gyr apart at high redshift, comparable to the reported Kraken and Gaia-Sausage-Enceladus. The Kraken-like merger ($z=1.6$, $M_{\rm Tot} = 8\times10^{10}\,$M$_{\odot}$) is gas-rich, deposits most of its mass in the inner $10\,$kpc, and is largely isotropic. The Sausage-like merger ($z=1.14$, $M_{\rm Tot} = 1\times10^{11}\,$M$_{\odot}$) leaves a more extended mass distribution at higher energies, and has a radially anisotropic distribution. For the higher redshift merger, the stellar mass ratio of the satellite to host galaxy is 1:3. As a result, the chemistry of the remnant is indistinguishable from contemporaneous in-situ populations, making it challenging to identify this component through chemical abundances. This naturally explains why all abundance patterns attributed so far to Kraken are in fact fully consistent with the metal-poor in-situ so-called Aurora population and thick disc. However, our model makes a falsifiable prediction: if the Milky Way underwent a gas-rich double merger at high redshift, then this should be imprinted on its star formation history with bursts about $\sim2\,$Gyrs apart. This may offer constraining power on the highest-redshift major mergers.