The Global Dynamical Atlas of the Milky Way mergers: Constraints from Gaia EDR3 based orbits of globular clusters, stellar streams and satellite galaxies

TL;DR

This paper uses Gaia EDR3 data to identify and characterize six major merger groups in the Milky Way halo, revealing their properties, metallicity distributions, and contributing to understanding galaxy formation.

Contribution

It introduces a new dynamical atlas of Milky Way mergers by detecting groups in action and energy space, including a newly identified merger named Pontus.

Findings

Six merger groups identified, including a new one called Pontus.

Approximately 25% of the studied objects are associated with these mergers.

The LMS-1/Wukong merger is the most metal-poor among the identified groups.

Abstract

The Milky Way halo was predominantly formed by the merging of numerous progenitor galaxies. However, our knowledge of this process is still incomplete, especially in regard to the total number of mergers, their global dynamical properties and their contribution to the stellar population of the Galactic halo. Here, we uncover the Milky Way mergers by detecting groupings of globular clusters, stellar streams and satellite galaxies in action ($\mathbf{J}$) space. While actions fully characterize the orbits, we additionally use the redundant information on their energy ($\textit{E}$) to enhance the contrast between groupings. For this endeavour, we use $\textit{Gaia}$ EDR3 based measurements of $170$ globular clusters, $41$ streams and $46$ satellites to derive their $\mathbf{J}$ and $\textit{E}$. To detect groups, we use the $\texttt{ENLINK}$ software, coupled with a statistical procedure that accounts for the observed phase-space uncertainties of these objects. We detect a total of $N=6$ groups, including the previously known mergers $\textit{Sagittarius}$, $\textit{Cetus}$, $\textit{Gaia-Sausage/Enceladus}$, $\textit{LMS-1/Wukong}$, $\textit{Arjuna/Sequoia/I'itoi}$ and one new merger that we call $\textit{Pontus}$. All of these mergers, together, comprise $62$ objects ($\approx 25\%$ of our sample). We discuss their members, orbital properties and metallicity distributions. We find that the three most metal-poor streams of our Galaxy -- "C-19" ([Fe/H]$=-3.4$ dex), "Sylgr" ([Fe/H]$=-2.9$ dex) and "Phoenix" ([Fe/H]$=-2.7$ dex) -- are associated with $\textit{LMS-1/Wukong}$; showing it to be the most metal-poor merger. The global dynamical atlas of Milky Way mergers that we present here provides a present-day reference for galaxy formation models.