Astronomers using the Hubble Space Telescope have discovered definitive evidence that our home galaxy experienced a significant collision and merger with a dwarf galaxy 11.8 billion years ago. The discovery adds a vital new event to the history of the Milky Way, shedding light on the tumultuous early period of cosmic evolution just two billion years after the Big Bang.
Hubble Uncovers Ancient LKH Galaxy Collision
Researchers named the devoured dwarf galaxy LKH, which stands for Low-energy–Kraken–Heracles. The name serves as a reference to three earlier research papers that tangled with the idea of galactic mergers early in the Milky Way’s history. According to NASA, the team analyzed Hubble observations of 39 globular clusters situated within the inner 20,000 light-years of our galaxy.
Tracing Stellar Fossils with Precision Imaging
Figuring out the history of the Milky Way requires painstaking detective work. Because the early galaxy was smaller, closer in size to the galaxies it clashed with, and far more chaotic, the physical signs of ancient impacts can be erased over billions of years. To peer into this murky past, scientists relied on globular clusters, which are immense, roughly spherical collections containing tens of thousands to a few million stars. These clusters act as cosmic archaeological sites holding some of the oldest stars in our galaxy.
Thanks to the high resolution and depth of Hubble imaging, we could measure the age and the metal content of these clusters with unprecedented precision,
said Chiara Zerbinati, a study co-author from the University of Bologna in Italy, according to Space. Astronomers use the term “metals” to describe elements heavier than hydrogen and helium, including oxygen, carbon, iron, and aluminum.
When coupled with measurements from the European Space Agency mission NASA, researchers were able to isolate a distinct population of globular clusters that belonged neither to the Milky Way nor to the later Gaia–Sausage–Enceladus (GSE) galaxy. These clusters revealed when LKH was devoured and how massive it was.
Massive Impact on Milky Way Evolution
Calculations based on the number and mass of the imported globular clusters show that LKH possessed a total mass of roughly 500 million times the mass of the sun in stars, gas, and dark matter. While this is an order of magnitude less massive than the Small Magellanic Cloud, it represented a significant fraction of the Milky Way’s total mass at the time, given that our galaxy was much smaller 11.8 billion years ago.
Davide Massari of the Astrophysics and Space Science Observatory of Bologna in Italy, who served as the lead author of the research paper published in Nature Astronomy, noted that the discovery challenges previous assumptions. Some past studies have argued that the earliest phases of our galaxy’s evolution were defined by stars born only in our galaxy,
Massari said in a statement reported by Space. Here, we have shown that stars born in external galaxies also need to be considered.
The collision likely spurred a fresh burst of star formation, helping the Milky Way grow. Furthermore, the chemistry of the stars from LKH would have impacted the overall chemical evolution of our galaxy as those stars died and passed their remains into the interstellar medium to form new generations of stars.
