According to the standard cosmological scenario, the large galaxies that we observe today have reached their current mass via mergers with smaller galaxy satellites¹. This hierarchical process is expected to take place on smaller scales for the satellites themselves, which should build up from the accretion of smaller building blocks². The best chance we have to test this prediction is by looking at the most massive satellite of the Milky Way: the Large Magellanic Cloud (LMC). Smaller galaxies have been revealed to orbit around the LMC^3,4, but so far the only evidence for mutual interactions is related to the orbital interplay with the nearby Small Magellanic Cloud, 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 a stellar mass similar to those of dwarf spheroidal galaxies. This former LMC satellite has now completely dissolved, depositing the old globular cluster NGC 2005 as part of its debris. This globular cluster, the only surviving witness of this ancient merger event, is recognizable through its peculiar chemical composition. This discovery is observational evidence that the process of hierarchical assembly has worked also in shaping our closest satellites.

根据标准的宇宙学情景，我们今天观察到的大型星系通过与较小的星系卫星¹的合并达到了它们目前的质量。这种分层过程预计将在卫星本身的较小规模上发生，这应该由较小的构建块²的积累建立起来。我们必须检验这一预测的最佳机会是观察银河系最大的卫星：大麦哲伦星云（LMC）。已发现较小的星系围绕 LMC ^3,4运行，但到目前为止，相互相互作用的唯一证据与附近的小麦哲伦星云的轨道相互作用有关，小麦哲伦星云是质量最大的 LMC 卫星。在这项工作中，我们报告了可能发现了 LMC 与一个恒星形成效率低且恒星质量可能与矮球状星系相似的星系所经历的过去合并事件。这颗前 LMC 卫星现已完全溶解，将旧球状星团 NGC 2005 作为其碎片的一部分沉积下来。这个球状星团是这次古老合并事件唯一幸存的见证，通过其独特的化学成分可以识别。这一发现是观察性证据，表明分层组装过程也有助于塑造我们最近的卫星。