The hunt for the Milky Way's accreted disc

TL;DR

This study develops a chemo-dynamical method to detect an accreted stellar disc in the Milky Way, finding no evidence in current data, which suggests a quiet merger history and a light dark disc, but larger samples are needed for confirmation.

Contribution

The paper introduces a new chemo-dynamical template to identify accreted stars, applied to existing data, providing constraints on the Milky Way's merger history and dark matter structure.

Findings

No evidence for accreted stars in the current sample.

Supports a quiescent Milky Way with no major mergers since disc formation.

Indicates a likely light dark matter 'dark disc'.

Abstract

The Milky Way is expected to host an accreted disc of stars and dark matter. This forms as massive >1:10 mergers are preferentially dragged towards the disc plane by dynamical friction and then tidally shredded. The accreted disc likely contributes only a tiny fraction of the Milky Way's thin and thick stellar disc. However, it is interesting because: (i) its associated `dark disc' has important implications for experiments hoping to detect a dark matter particle in the laboratory; and (ii) the presence or absence of such a disc constrains the merger history of our Galaxy. In this work, we develop a chemo-dynamical template to hunt for the accreted disc. We apply our template to the high-resolution spectroscopic sample from Ruchti et al. (2011), finding at present no evidence for accreted stars. Our results are consistent with a quiescent Milky Way with no >1:10 mergers since the disc formed and a correspondingly light `dark disc'. However, we caution that while our method can robustly identify accreted stars, our incomplete stellar sample makes it more challenging to definitively rule them out. Larger unbiased stellar samples will be required for this.