Hubble finds ancient galaxy collision that shaped the Milky Way

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The Hubble Space Telescope has discovered evidence that our galaxy experienced a significant collision and merger with a dwarf galaxy 11.8 billion years ago.

Mergers between galaxies are one of the main ways in which galaxies can grow. However, while we can witness mergers taking place in other galaxies, figuring out our Milky Way galaxy's history requires painstaking detective work. Even now, the story is only partly known.

"Our home is the Milky Way galaxy, but we do not know how our house was built," Davide Massari of the Astrophysics and Space Science Observatory of Bologna in Italy said in a statement . Massari is lead author of a paper describing the discovery of an ancient galactic merger. "In this paper we discover where the first significant batch of bricks came from: a dwarf galaxy that we call LKH."

LKH stands for Low-energy–Kraken–Heracles, which is an obtuse reference to three earlier research papers that tangled with the idea of mergers early in our galaxy's history.

In recent years, one other significant merger has come to light, which is a collision with a galaxy referred to as the Gaia–Sausage–Enceladus (GSE) galaxy about 10 billion years ago. The GSE had its own family of globular star clusters that became integrated with the Milky Way's own globular clusters during the collision.

Now, Massari's team have discovered a third population of globular clusters that do not seem to be native to the Milky Way, nor to have belonged to the GSE galaxy. Instead, they seem to have arrived 11.8 billion years ago, which is before the GSE merger.

"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 astronomer Chiara Zerbinati of the University of Bologna, who is a co-author on the paper. Metals are astronomer-speak for elements heavier than the hydrogen and helium that formed in the Big Bang , so they include the likes of oxygen and carbon as well as iron and aluminum.

"Coupled with measurements from [the European Space Agency mission] Gaia , this made it possible to distinguish a population of globular clusters that are different from the others," said Zerbinati. "These are the clusters that were born in LKH and they tell us when that galaxy was devoured by ours, and how massive it was."

Based on the number and mass of imported globular clusters, the team calculated that LKH had a total mass of 500 million times the mass of our sun . This is only slightly more massive than the Sagittarius Dwarf galaxy (approximately 400 million solar masses), which the Milky Way is currently cannibalizing and an order of magnitude less massive than the Small Magellanic Cloud . However, 11.8 billion years ago, which is just two billion years after the Big Bang, our Milky Way galaxy was much smaller and LKH would have contributed a sizable chunk of mass, in terms of stars, gas and dark matter , to the Milky Way.

The discovery of the merger adds an important new event to the Milky Way's history, one that would have undoubtedly influenced the evolution of our galaxy. For example, the collision could have spurred a fresh burst of star formation, helping the Milky Way to grow, while the chemistry of the stars from LKH will have impacted the Milky Way's overall chemical evolution as those stars died and passed their remains into the interstellar medium to be reborn in new stars.

"Some past studies have argued that the earliest phases of our galaxy's evolution were defined by stars born only in our galaxy," said Massari. "Here, we have shown that stars born in external galaxies also need to be considered."

The discovery of the merger with LKH was reported on Aug. 17 in the journal Nature Astronomy .