우리 은하는 처음부터 '섞인' 은하였다…120억년 전 병합 흔적 발견

A groundbreaking discovery has unveiled the earliest cosmic merger detected so far, shedding light on the formation of our Milky Way galaxy. The Italian National Institute for Astrophysics (INAF), led by an international team of researchers, has found compelling evidence that our galaxy devoured a dwarf galaxy 120 billion years ago.

This discovery, published in the prestigious journal Nature Astronomy, indicates that this merger occurred 18 billion years before the Gaia-Sausage-Enceladus (GSE) collision, which is believed to have significantly shaped our galaxy's current form.

Our Milky Way, once a small galaxy, grew by swallowing smaller galaxies and gathering gas clouds to form new stars. Recent mergers, such as the Sgr dwarf spheroidal galaxy, continue to this day. The largest merger ever recorded happened 10.5 billion years ago, when our galaxy absorbed the Gaia-SSC galaxy, dramatically altering its star disk. Scientists had previously believed that earlier mergers might have existed, but locating their traces proved challenging.

To uncover these elusive remnants, the research team used the Hubble Space Telescope to analyze 39 globular clusters within 20,000 light-years of the Milky Way's center. Globular clusters are dense collections of millions of stars that formed almost simultaneously in the early universe, acting like a cosmic fossil record. Dr. Davide Massari, the lead researcher, stated that this study revealed the first major building block of our galaxy's composition.

The team identified 12 "third-type globular clusters" that differ from existing satellite clusters and those formed during the GSE merger. These clusters predate the GSE clusters and are younger than our galaxy's satellite clusters. Calculations suggest that the swallowed dwarf galaxy contained stars up to five billion times the mass of the Sun, representing a significant portion of our galaxy's mass at the time.

The scientists named this dwarf galaxy LKH (Low-energy-Kraken-Heracles) in honor of previous research on early galaxy mergers.

Dr. Chiara Gerin, a co-author from the University of Bologna, explained that combining Hubble's precise observations with the European Space Agency's Gaia telescope data allowed them to distinguish the LKH clusters from other remnants. Dr. Massari emphasized that this research confirms that external galaxies played a crucial role in shaping the Milky Way's history, challenging the notion that only internal star formation defined the galaxy's early development.

The team plans to continue using the Hubble telescope to investigate other unexplored globular clusters and map out all significant mergers in the universe's history.