Group accretion in Milky Way-like stellar haloes

Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands

^★ Corresponding author: t.m.callingham@astro.rug.nl

Received: 1 April 2025
Accepted: 2 July 2025

Abstract

Context. As galaxies form hierarchically, larger satellites can accrete alongside smaller companions in group infall events. This coordinated accretion is likely to have left signatures in the Milky Way’s stellar halo that we can see today.

Aims. Our goal is to characterise the possible groups of companions that accompanied larger known accretion events of our Galaxy, and infer where their stellar material could be in physical and dynamical space today.

Methods. We used the AURIGA simulation suite of Milky Way-like haloes to identify analogues to these large accretion events and their group infall companions, and we followed their evolution in time.

Results. We find that most of the material from larger accretion events is deposited on much more bound orbits than their companions. This implies a weak dynamical association between companions and debris, but it is strongest with the material lost first. As a result, the companions of the Milky Way’s earliest building blocks likely contributed stars to the solar neighbourhood, whilst the companions of our last major merger are likely found in both the solar neighbourhood and the outer halo. More recent infall groups of satellites, or those of a lower mass, are more likely to retain dynamical coherence, for example through clustering in the orientation of angular momentum.

Conclusions. Group infall has likely shaped the Milky Way’s stellar halo. Disentangling this will be challenging for the earliest accretion events, although overlap with their less bound debris may be particularly telling.