Astroboffins spy the most ancient protocluster of galaxies yet found post Big Bang

The giant group of 12 galaxies formed when the universe was just 800 million years old

The oldest protocluster of galaxies found to date began clumping together some 13 billion years ago, when the universe was just 6 per cent of its current age.

A global team of 36 astrophysicists led by the National Astronomical Observatory of Japan (NAOJ) studying data taken from the Keck and Gemini Observatory and the Subaru Telescope in Hawaii and Chile respectively, have discovered twelve galaxies gravitating towards one another. The collection, codenamed z66OD, has been classified as a protocluster, a type of cosmic structure describing the initial stages of a larger system that can attract over hundreds of galaxies.

It’s the oldest protocluster spotted yet; the finding confirms that these complex structures began forming when the universe was a mere 800 million years old.

“A protocluster is a rare and special system with an extremely high density, and not easy to find,” said Yuichi Harikane, a research fellow at the NAOJ who led the study to be published in The Astrophysical Journal (here’s the free preprint).

The protocluster is a whopping 13 billion light years away after all. “To overcome this problem, we used the wide field of view of the Subaru Telescope to map a large area of the sky and look for protoclusters," he added.


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Z66OD was spotted when the team found an area where galaxies were 15 times more dense than normal. Further observations revealed that the location contained an unusually high number of young stars - five times larger than other galaxies of similar size - spread out across 12 galaxies.

One was identified as Himiko, a giant galaxy swelling with gas discovered a decade ago. “It is reasonable to find a protocluster near a massive object, such as Himiko,” said Masami Ouchi, co-author of the paper and a researcher at the NAOJ. “However, we're surprised to see that Himiko was located not in the center of the protocluster, but on the edge 500 million light-years away from the center."

The group dynamics of a galaxy cluster are still a mystery, it’s not clear how they evolve and form over time. “Tracing the formation of the largest structures in the universe, and the galaxies inside them, is the new frontier in extragalactic astronomy,” said Dave Clements, co-author of the paper and a senior lecturer at Imperial College London.

“This result pushes that frontier back still further, and provides some hints as to the processes behind protocluster galaxy formation.” ®

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