The Violent Origins of Disc Galaxies Revealed: Galactic Mergers

Why are there so many disc galaxies like our Milky Way in the universe? That's a question that has puzzled astronomers for years. Now, though, they may have found the reason. Using ALMA, researchers have found that merging galaxies can form disc galaxies instead of elliptical galaxies and that this phenomenon is actually fairly common.

For years, it's been widely accepted that merging disc galaxies eventually form an elliptical shaped galaxies. During the violent interactions that occur when two galaxies merge, the galaxies do not only gain mass, but also change their shape through time. Yet now, it appears that elliptical galaxies aren't always the result.

The scientists examined the distribution of gas in 37 galaxies that are in the final stages of merging in our nearby universe. More specifically, they used the ALMA and other radio telescopes to observe emission from carbon monoxide, which is an indicator of molecular gas.

So what did they find? It turns out that, surprisingly, most galaxy collisions in the nearby universe-within 40 to 600 million light-years from Earth-result in disc galaxies. These disc galaxies are defined by pancake-shaped regions of dust and gas.

"For the first time, there is observational evidence for merging galaxies that could result in disc galaxies," said Junko Ueda, the team leader, in a news release. "This is a large and unexpected step towards understanding the mystery of the birth of disc galaxies."

Currently, the researchers plan to continue investigations of disc galaxies. This should shed more light on galaxy formation and our own Milky Way.

"We have to start focusing on the formation of stars in these gas discs," said Ueda. "Furthermore, we need to look farther out in the more distant universe. We know that the majority of galaxies in the more distant universe also have discs. We however do not yet know whether galaxy mergers are also responsible for those, or whether they are formed by cold gas gradually falling into the galaxy. Maybe we have found a general mechanism that applies throughout the history of the universe."

The findings can be found online here.