我們的母星系銀河系是透過吞噬較小的星系而成長至現今規模的。最近，NASA 和 ESA 的哈伯太空望遠鏡提供了明確的證據，表明在銀河系演化的最早期階段，有一個名為 LKH 的矮星系與年輕的銀河系發生了合併。這項發表在《自然天文學》上的新發現，將我們對銀河系歷史的了解向更久遠的過去推進了18億年。

隨著科學家試圖回溯更久遠的星系歷史，辨識合併的痕跡變得更加困難。研究團隊利用哈伯望遠鏡觀測了銀河系內部的39個球狀星團，這些星團就像宇宙的考古遺址，保存著來自其他星系的古老恆星。透過精確測量這些星團的年齡和金屬含量，研究人員成功區分出了一組與眾不同的球狀星團。

這些獨特的星團比起100億年前的蓋亞-香腸-恩克拉多斯合併事件還要古老。它們來自約118億年前的一場更早期的合併，當時銀河系吸收了 LKH 矮星系，其恆星質量大約是太陽的5億倍。這一發現表明，在銀河系演化的最早期階段，外部星系誕生的恆星也扮演了關鍵角色。

Our home galaxy, the Milky Way, grew to its current size by consuming smaller galaxies. Recently, the NASA/ESA Hubble Space Telescope provided definitive evidence of a dwarf galaxy named LKH merging with the young Milky Way during its earliest evolutionary phases. This new discovery, published in Nature Astronomy, pushes our understanding of the galaxy's history 1.8 billion years further back in time.

As scientists attempt to look further back into the galaxy's history, identifying signs of past mergers becomes increasingly difficult. The research team used the Hubble telescope to observe 39 globular clusters in the inner Milky Way, which act as cosmic archaeological sites preserving ancient stars from other galaxies. By precisely measuring the age and metal content of these clusters, researchers successfully distinguished a unique population of globular clusters.

These distinct clusters are older than those from the Gaia-Sausage-Enceladus merger that occurred 10 billion years ago. They originated from an even earlier merger about 11.8 billion years ago, when the Milky Way absorbed the LKH dwarf galaxy, which contained a stellar mass about 500 million times that of the Sun. This finding demonstrates that stars born in external galaxies also played a crucial role in the earliest phases of the Milky Way's evolution.