Record-breaking protocluster takes fast-track

The discovery of the most distant large-scale cluster of galaxies in the very young universe has astronomers puzzling about how it formed so rapidly.

Crucial observations by a collaboration of telescopes including Gemini Observatory, Subaru Telescope and W.M. Keck Observatory, all on Maunakea, helped an international team of astronomers to discover the most distant cosmic collection of galaxies caught in the act of forming in the early universe.


Known as z660D, this puzzling protocluster — a large and scattered collection of young gas-rich galaxies — is feverishly creating stars some 13 billion light years away. It is the most distant large-scale structure of galaxies ever detected. Full details are published in the Sept. 30 issue of The Astrophysical Journal.

“This discovery suggests that large-scale structures already existed when our universe was only about 800 million years old,” explained lead author Yuichi Harikane from the National Astronomical Observatory of Japan.

“That’s just 6 percent of the universe’s current age of 13.8 billion years. In addition to its impressive age, this protocluster is in the fast lane to formation.”

“As we continue to find rare and essential objects like this protocluster of galaxies in the early history of our universe, collaboration of major facilities often becomes necessary,” added Chris Davis of the National Science Foundation, which provides support for the international Gemini Observatory. “Big telescopes can bring something unique to the table. In Gemini’s case, optical spectroscopy has always been a great strength and a powerful tool for discovery.”

The study began with a wide-field search for protocluster candidates using the wide-field Hyper Suprime-Cam imager on the 8-meter Subaru telescope on Maunakea. The team encountered the cluster, where galaxies are 15 times more concentrated than average, and used the sensitive Gemini Multi-Object Spectrograph on the 8-meter Gemini North telescope on Maunakea to capture the chemical fingerprints of half of the individual galaxies in the protocluster.

These data, coupled with similar spectroscopic data from the neighboring Keck Observatory and the Magellan Telescope in Chile, confirmed that z660D was unmistakably the most distant protocluster ever detected.

The galaxies in z66OD appear to be powered by prodigious bursts of star formation.

The number of stars forming per year in the protocluster is a startling five times larger than that in other galaxy groupings with similar masses observed in the early universe.


Equally curious is that while previous observations suggested protoclusters this early in the universe should contain a massive dusty galaxy, z660D does not appear to have one.

Objects such as z660D pose a formidable challenge to astronomers trying to understand the formation of some of the largest structures in the universe.

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