A relic from a past merger event in the Large Magellanic Cloud

TL;DR

This paper presents evidence of a past merger event in the Large Magellanic Cloud, identified through a globular cluster with unique chemical properties, supporting hierarchical galaxy formation theories.

Contribution

It reports the discovery of a dissolved satellite galaxy in the LMC, evidenced by the globular cluster NGC 2005's peculiar chemistry, indicating a historical merger event.

Findings

Globular cluster NGC 2005 shows unique chemical signatures.

Evidence supports hierarchical assembly in the LMC.

A former satellite galaxy has been identified through its debris.

Abstract

According to the standard cosmological scenario, the large galaxies that we observe today have reached their current mass via mergers with smaller galaxy satellites (Moore et al.1999). This hierarchical process is expected to take place on smaller scales for the satellites themselves, that should build-up from the accretion of smaller building blocks (D'Onghia & Lake 2008). The best chance we have to test this prediction is by looking at the most massive satellite of the Milky Way (MW): the Large Magellanic Cloud (LMC). Smaller galaxies have been revealed to orbit around the LMC (Erkal & Belokurov 2020, Patel et al. 2020), but so far the only evidence for mutual interactions is related to the orbital interplay with the nearby Small Magellanic Cloud (SMC), which is the most massive LMC satellite. In this work, we report the likely discovery of a past merger event that the LMC experienced with a galaxy with a low star formation efficiency and likely having a stellar mass similar to those of dwarf spheroidal galaxies. This former LMC satellite has now completely dissolved, depositing the old globular cluster (GC) NGC 2005 as part of its debris. This GC is the only surviving witness of this ancient merger event, recognizable through its peculiar chemical composition. This discovery is the observational evidence that the process of hierarchical assembly has worked also in shaping our closest satellites.