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