Clash of

A pair of massive, hyper-luminous galaxies are merging in front of astronomer's eyes for the first time and revealing secrets of cosmic creation.

"Discovering a hyper-luminous starburst galaxy is an extraordinary feat, but discovering two - this close to each other - is amazing," said Dominik Riechers, assistant professor of astronomy and lead author on new research published in the Astrophysical Journal. "It's nearly 13 billion light-years away and in its frenzied star-forming action, we may be seeing the most extreme galaxy merger known."

Found in the Southern Hemisphere's Dorado constellation - known as the swordfish - the ADFS-27 galactic pair is located about 12.7 billion light-years away. Astronomers are seeing these galaxies in their infancy - and the light from the galaxies have taken nearly 13 billion years to reach our eyes.

In the paper, "Rise of the Titans: A Dusty, Hyper-luminous '870-micron Riser' Galaxy at z~6," Riechers, doctoral candidate T. K. Daisy Leung and their colleagues captured coalescing galaxies - likely the most massive systems in the universe - by using the Atacama Large Millimeter/submillimeter Array (ALMA), a high-elevation radio telescope in Chile, to detect their merger into a single galaxy. The merger of the two galaxies has triggered violent, ongoing star formation and lead to the growth of a very massive galaxy in later cosmic epochs, Riechers said.

Leung explained that this pair must have formed efficiently in early cosmic time, forming the foundation of massive galaxies and clusters astronomers see today. "These massive systems in the early universe are showing us snapshots of their early evolution," she said.

"Finding these galaxies - about 30,000 light-years apart - helps astronomers to understand how very extreme structures form, as they continue to birth stars and become even more massive," said Riechers.

"These galactic progenitors help us to understand massive galaxies of the present day, as we've tried to understand how these actually form. In other words, this discovery is helping astronomers to understand the timeline of the cosmos."

Riechers explained that his group first detected these systems with the European Space Agency's Herschel Space Observatory. It appeared as a red dot.

"Galaxies usually look bluer or greener. This one popped out because of its color. It was literally really red, which means it's a brighter object at longer wavelengths and it is farther away than most galaxies," Riechers said.

Earlier this year, this group of astronomers using the ALMA radio telescope examined the red dot and saw two galaxies that have about 50 times the amount of star-forming gas as the Milky Way.

Riechers said an enormous amount of observed gas will be converted into new stars quickly as the two merging galaxies produce stars at a "breakneck pace," about 1,000 times faster than in the current Milky Way.

Leung said that the ALMA telescope has revolutionized our understanding of young galaxies with its unprecedented resolution. "We can now see distant galaxies in exquisite detail, as we were able to uncover the compact, starburst nature of this merger pair - known only as a dusty blob in the good old days."

Research Report: "Rise of the Titans: a Dusty, Hyper-luminous '870-micron Riser' Galaxy at z~6," D. Riechers, 2017 Nov. 13, Astrophysical Journal

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