Progenitor diversity in the accreted stellar halos of Milky Way-like galaxies

TL;DR

This study compares predictions from different galaxy formation models regarding the number of progenitor galaxies contributing to the Milky Way's stellar halo, revealing that the Milky Way's halo is unusually dominated by few early-accreted progenitors.

Contribution

It provides a comparative analysis of progenitor diversity in simulated Milky Way-like galaxies across multiple models, highlighting the Milky Way's unique halo composition.

Findings

The Milky Way's halo is dominated by about 2 progenitors within 8-45 kpc.

Model predictions vary significantly beyond expected ΛCDM scatter.

Galaxies with ongoing mergers show similar diversity to the Milky Way.

Abstract

Ongoing large stellar spectroscopic surveys of the Milky Way seek to reconstruct the major events in the assembly history of the Galaxy. Chemical and kinematic observations can be used to separate the contributions of different progenitor galaxies to the present-day stellar halo. Here we compute the number of progenitors that contribute to the accreted stellar halos of simulated Milky Way-like galaxies as a function of radius (the radial diversity) in three suites of models: Bullock & Johnston, Aquarius and Auriga. We show that there are significant differences between the predictions of these three models, beyond the halo-to-halo scatter expected in $\Lambda$CDM. Predictions of diversity from numerical simulations are sensitive to model-dependent assumptions regarding the efficiency of star formation in dwarf galaxies. We compare, at face value, to current constraints on the radial diversity of the Milky Way's accreted halo. These constraints imply that the halo of our Galaxy is dominated by $\sim2$ progenitors in the range $8-45\,\mathrm{kpc}$, in contrast to averages of $7$ progenitors in the Bullock & Johnston models, $3.5$ in Aquarius and $4.2$ in Auriga over the same region. We additionally find that the models with radial diversity most similar to that of the Milky Way are predominantly those with ongoing merger events. The Milky Way therefore appears unusual in having an accreted stellar halo dominated by a small number of progenitors accreted at very early times.