The Milky Way galaxy absorbed a smaller dwarf galaxy approximately 11.8 billion years ago in the earliest known merger in its history, researchers have discovered.
Astronomers studying star clusters near the galactic center found that the collision took place when the universe was less than 15 percent of its current age, roughly two billion years after the Big Bang. The discovery shows that our galaxy grew not only by producing its own stars, but also by capturing and consuming smaller galactic neighbors through its gravitational pull.
The absorbed dwarf galaxy was designated Low-energy Kraken-Hercules, or LKH, in recognition of three earlier scientific studies that examined the possibility of such an ancient collision. At the time of the merger, LKH contained stars with a total mass equal to 500 million Suns. That mass represented about a quarter of the total size of the Milky Way at that point in cosmic history.
Evidence from space telescopes
Researchers identified the ancient event using data gathered by the Hubble Space Telescope and the Gaia orbital observatory. The scientific team examined star clusters containing hundreds of thousands of stars located within a region spanning 20,000 light-years from the Milky Way's center. By analyzing these clusters, astronomers determined the precise ages, chemical compositions, and orbital trajectories of the stars.
The Hubble Space Telescope is a joint mission between NASA and the European Space Agency that has orbited Earth since 1990, while Gaia is an ESA space observatory dedicated to constructing a high-precision three-dimensional map of stars across the galaxy. A light-year is the astronomical unit measuring the distance light travels in one year, which equals roughly 5.88 trillion miles.
Astrophysicist Davide Massari of the National Institute for Astrophysics in Bologna, Italy, served as the lead author of the study, which was published in the journal Nature Astronomy and reported by Reuters. Massari said the principal result of the investigation was the definitive discovery of the first merger event experienced by the Milky Way at the dawn of its existence.
Gas collisions and star formation
Massari noted that whether the early growth of the Milky Way was driven mainly by internal star formation or by galactic mergers had long been a subject of debate among scientists. The new findings demonstrate that the early collision flooded the young galaxy with large quantities of stars, interstellar gas to fuel new star formation, and dark matter.
Massari explained that from the perspective of individual stars, the merger was relatively peaceful because stars did not physically collide with one another. However, he described the impact on gas as far more explosive, noting that collisions between gas from LKH and gas within the Milky Way likely triggered a major wave of star creation. In astrophysics, dark matter refers to the invisible matter that makes up the majority of the mass in the cosmos.
Many stars that originally formed inside the dwarf galaxy are believed to remain within the inner regions of the Milky Way today, including inside star clusters. Massari pointed out, however, that identifying these specific stars remains exceptionally difficult due to dense clouds of interstellar dust obscuring the central galactic bulge.
Subsequent mergers and Titan orbit
The LKH collision was the first of three major galactic mergers known to have shaped the Milky Way. Our galaxy later absorbed another dwarf galaxy named Gaia-Sausage-Enceladus roughly 1.8 billion years after the LKH merger. Over the past six billion years, the Milky Way has also been merging with the Sagittarius dwarf galaxy.
Massari emphasized that each of these galactic mergers influenced the sequence of events that ultimately formed our Solar System and Earth. He said reconstructing these historical fragments of the Milky Way helps scientists answer fundamental questions about human origins. The National Institute for Astrophysics is Italy's primary public research body for astronomy.
In related astronomical developments, space researchers previously reported that the distance between Earth and Titan, the largest moon of Saturn, is constantly changing. Depending on the relative orbital positions of Earth and Saturn around the Sun, the distance between Earth and Titan ranges from approximately 1.2 billion kilometers at closest approach to about 1.65 billion kilometers at its furthest point.
