Rare Merger of Ancient Galaxies Solves Mystery of Galaxy Evolution

Years ago when the universe was still young, two massive galaxies collided. Now, ESA's Herschel space observatory has spotted the rare encounter between the two gas-rich galaxies, which may provide clues to galaxy formation.

Most large galaxies fall into one of two major categories. They're either spiral-shaped, like our own Milky Way, or they're gas-poor ellipticals that are populated by old, cool red stars and show few signs of ongoing star formation. It's these elliptical galaxies that have caused the real mystery among astronomers.

It was long assumed that large elliptical galaxies were built up gradually over time through the gravitational acquisition of many small dwarf galaxies. The gas in the galaxies eventually converted into cool, low-mass stars; this means that the galaxies would have exhausted all of their star-forming material by today, leaving them red and lifeless. When astronomers discovered that very massive elliptical galaxies had managed to form during the first three to four billion years of the universe's history, they were understandably perplexed. The galaxies had somehow assembled vast quantities of stars very quickly and then had suddenly "switched off."

Now there may be an explanation for this phenomenon. Herschel data revealed a galaxy pair in the form of a certain bright source, named HXMM01. After follow-up observations, though, astronomers found that it wasn't just one bright source but was instead two distinct galaxies. Each one boasted a stellar mass equal to about 100 billion suns and an equivalent amount of gas.

What is more interesting, though, is the bridge of gas that links the two galaxies. This bridge actually indicates that the two are merging. Not only that, but the merger seems to have sparked a star-formation frenzy. The system spawns roughly 2,000 stars like our sun every year.

"This monster system of interacting galaxies is the most efficient star-forming factory ever found in the universe at a time when it was only three billion years old," said Hai Fu from the University of California in a news release.

Yet the current rate of high star formation isn't sustainable; eventually, the reservoir of gas will run out and lead to an aging population of stars. This, in turn, will create a massive elliptical galaxy.

"We were very lucky to catch this extreme system in such a critical transitional phase," said Seb Oliver from the University of Sussex in a news release. "It shows that a merger of gas-rich and actively star-forming galaxies is a possible mechanism to form the most massive ellipticals that are observed in the young universe."

The findings were published in the journal Nature.