A new member of the Milky Way's family tree: Characterizing the Pontus merger of our Galaxy

Abstract

We study the $\textit{Pontus}$ structure -- a recently discovered merger that brought in $\sim7$ globular clusters in the course of the hierarchical build-up of the Milky Way's halo. Here, we analyse the stellar population of $\textit{Pontus}$ and examine (1) its phase-space distribution using the ESA/$\textit{Gaia}$ dataset, (2) its metallicity and chemical abundances (i.e., [Fe/H], [$\alpha$/Fe], [Mg/Fe], [Al/Fe]) using the spectroscopic catalogue of APOGEE DR17, and (3) the colour-magnitude diagram that shows interesting features, including a possibly double horizontal branch and a small population of blue stragglers. In sum, the $\textit{Pontus}$ stars show some unique properties that suggest they likely originated from the merging of an independent satellite galaxy; future analysis will shed more light on the true nature of this structure. This chemo-dynamical analysis of $\textit{Pontus}$ stars is another step forward in our bigger quest to characterize $\textit{all}$ the merging events of our Milky Way.