  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.
