The Milky Way, a large and spiral-shaped galaxy that is home to hundreds of billions of stars including our sun, has taken a long and complicated path to achieve its current dimensions, as illustrated by new research documenting a milestone event early in its history.
Scientists observing clusters of stars congregated near the Milky Way's center have discovered evidence that our galaxy swallowed a smaller galaxy roughly 11.8 billion years ago, when the universe was less than 15% of its current age. The event represents the Milky Way's earliest-known galactic merger and shows how it grew not only by forming its own stars but by merging with smaller galaxies caught in its gravitational pull.
The smaller galaxy is classified as a dwarf galaxy, differentiating it from larger galaxies like the Milky Way. The researchers estimate that it contained stars with a mass equivalent to 500 million times that of the sun at the time of the merger, about a quarter the size of what the Milky Way was at the time.
The researchers used the orbiting Hubble and Gaia telescopes to study stellar clusters, each containing hundreds of thousands of stars, inhabiting a region spanning the Milky Way's innermost 20,000 light-years, gauging their age, chemical composition and trajectory. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).
These stars became part of the Milky Way after a galactic merger that occurred about 2 billion years after the Big Bang event that initiated the universe, they said. The dwarf galaxy where they originated was given the name Low-energy-Kraken-Heracles, or LKH, in recognition of three earlier studies that examined the possibility of such a primordial merger.
"The key finding of our research is the unambiguous discovery of the first merger event experienced by our galaxy in its infancy," said astrophysicist Davide Massari of the National Institute for Astrophysics, or INAF, in Bologna, Italy, lead author of the study published in the journal Nature Astronomy.
Whether the Milky Way's early growth was fueled by homegrown star formation or a galactic merger has been a matter of debate, Massari said. The new study indicates that the merger infused the Milky Way early in its history with a large volume of stars and interstellar gas that feeds star formation, as well as the mysterious substance called dark matter.
"I would say that from the point of view of stars, the merger was rather peaceful, with no star-to-star crashes. But from the point of view of the gas, the merger was more 'explosive' — likely, the collision between the gas from LKH and that from the Milky Way triggered the formation of many stars," Massari said.
Massari said there are probably many stars that originally formed in the dwarf galaxy that still are buried in the inner regions of the Milky Way, some of them inside stellar clusters. Because of all the interstellar dust in this region, however, it is very difficult to identify individual stars, Massari said.
The Milky Way is known to have been involved in two other massive galactic mergers. It absorbed a dwarf galaxy called Gaia-Sausage-Enceladus about 1.8 billion years after the LKH merger. And over the past approximately 6 billion years it has been combining with the Sagittarius dwarf galaxy.
"Every single one of these mergers had an influence on the events that have led to the formation of our own solar system, and ultimately of Earth," Massari said. "This is why I think that reconstructing the pieces of the Milky Way's past effectively contributes to answering the big question about, 'Where do we come from?'"
