The Global Dynamical Atlas of the Milky Way mergers: Constraints from Gaia EDR3 based orbits of globular clusters, stellar streams and satellite galaxies (2202.07660v1)

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.

• The paper leverages Gaia EDR3 data to identify and categorize 170 globular clusters, 41 stellar streams, and 46 satellite galaxies, pinpointing six distinct accretion events.
• The study employs energy and action space analysis with the ENLINK software to classify merger groups, including a newly identified candidate, Pontus.
• The findings enhance models of Milky Way formation by linking merger events with the galaxy’s chemical enrichment and dynamical evolution.

Overview of the Global Dynamical Atlas of Milky Way Mergers from Gaia EDR3

The paper "The Global Dynamical Atlas of the Milky Way mergers: constraints from Gaia EDR3 based orbits of globular clusters, stellar streams and satellite galaxies" by Khyati Malhan et al. offers a substantial analysis of the mergers that have contributed to the formation of the Milky Way's halo. Utilizing the precise astrometric data provided by Gaia Early Data Release 3 (EDR3), the authors categorize and paper 170 globular clusters, 41 stellar streams, and 46 satellite galaxies as a means to uncover the accretion history of the Galactic halo.

Detection and Classification of Mergers

The research identifies Milky Way mergers by detecting clusters, streams, and satellites in their dynamical action space. Actions, represented as integrals of motion, provide a robust framework for identifying merger events thanks to their conservation over time in a potential that evolves adiabatically. This approach is complemented by the inclusion of energy as an additional discriminator in their group-identification methodology, allowing for enhanced contrast in the detection of groupings. For the data processing, the authors deployed the ENLINK software, which facilitates the identification of hierarchical groups in multi-dimensional datasets.

Key Findings and Current Mergers

The analysis detects six distinct groups of accreted objects that are interpreted as the results of past mergers. These include the well-known mergers: Sagittarius, Cetus, Gaia-Sausage/Enceladus, LMS-1/Wukong, and Arjuna/Sequoia/I'itoi. Notably, the paper documents one new merger candidate, named Pontus. These mergers collectively account for approximately 25% of the sampled objects, underscoring their significant contribution to the structure of the Galactic halo. The results notably reaffirm some established associations while proposing revised or novel group memberships based on the newly leveraged Gaia DR3 dataset.

Implications and Broader Understanding

The findings carry profound implications for the understanding of the Milky Way's formation and evolution. The meticulous cataloging of these mergers enhances the current models of hierarchical galaxy formation and reveals insights into the chemical enrichment and dynamical histories of the involved structures. In particular, the Milky Way's most metal-poor streams, such as C-19 with [Fe/H] = -3.4 dex, are linked to the LMS-1/Wukong merger, presenting opportunities for future studies examining the potential primordial nature and earliest star-forming environments within these progenitor galaxies.

Concluding Perspective and Future Directions

This paper not only provides a significant addition to the understanding of the Milky Way's accretion history but is also instrumental in forecasting the trajectories of future research. The outlined global dynamical atlas serves as a foundational reference for developing advanced models of galaxy formation and furthering the capability to detect and characterize additional merger events with upcoming Gaia data releases. As various spectroscopic studies enrich the chemical and kinematic datasets, the groundwork laid here will likely contribute to increasingly detailed reconstructions of the Milky Way's evolutionary tapestry.