A galaxy protocluster dating back to 13 billion years ago was discovered

The protocluster z66OD with its galaxies in the insets (Image courtesy NAOJ/Harikane et al.)
The protocluster z66OD with its galaxies in the insets (Image courtesy NAOJ/Harikane et al.)

An article published in “The Astrophysical Journal” reports the discovery of a galaxy protocluster dating back to about 13 billion years ago. A team of astronomers led by Yuichi Harikane of the National Astronomical Observatory of Japan (NAOJ), used the Subaru, Keck and Gemini telescopes to find a group of 12 early galaxies including a giant one nicknamed Himiko that form a precursor of today’s galaxy clusters cataloged as z66OD. Its study will help to better understand the evolution of these cosmic structures and the influences among the galaxies that form them, especially the more massive ones.

This research started from a map of the universe created using the Subaru telescope, in which astronomers found the candidate protocluster z66OD. Inside z66OD the galaxies have a concentration 15 times higher than the average for their era. Follow-up spectroscopic observations conducted with the W.M. Keck Observatory and the Gemini North Telescope allowed to confirm that the 12 galaxies are about 13 billion years old. These are early galaxies that show that 800 million years after the Big Bang there were already galaxy protoclusters. In the past few years, protoclusters were already discovered but not so ancient.

A surprising discovery concerns the galaxy nicknamed Himiko, discovered in 2009 again using the Subaru telescope. It’s a giant galaxy so astronomers expected to find it at the center of the protocluster z66OD, on the contrary it’s at its edge, at a distance of about 500 million years from the center. Understanding the reason for Himiko’s position could be a key to understand the mechanisms of a protocluster’s evolution into a modern galaxy cluster.

The star formation within the protocluster z66OD is very high, estimated at about five times higher than that of other early galaxies with similar masses. According to the astronomers this is due to the large amount of gas present in those galaxies and available to form new stars.

The researchers also identified another protocluster, cataloged as z57OD, but z66OD remains the most interesting for its age. The next step of the study will take place with observations carried out with the ALMA radio telescope which, with its power and sensitivity, is among the best existing instruments to investigate such distant objects.

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