Oldest Galaxy Protocluster Forms “Queen’s Court”

Maunakea, Hawaii – Using multiple Maunakea Observatories – the Subaru Telescope, W. M. Keck Observatory, and Gemini Observatory – an international team of astronomers has discovered the earliest protocluster ever found; it is comprised of a collection of 12 galaxies in the constellation Cetus, which were observed as they were about 13 billion years ago.

One of the 12 galaxies is a giant object known as Himiko, which was discovered a decade ago by the Subaru Telescope and named for a legendary queen in ancient Japan. This discovery suggests that large structures such as protoclusters already existed when the universe was only about 800 million years old – 6 percent of its present age.

In the present universe, galaxy clusters can contain thousands of members, but how these clusters form is a big question in astronomy. To understand the formation of clusters, astronomers search for possible progenitors in the ancient universe, known as protoclusters.

A protocluster is a dense system of dozens of galaxies in the early universe, growing into a cluster. The previous record holder was the SDF protocluster, discovered by the Subaru Telescope in the Subaru Deep Field (SDF) near the constellation Coma Berenices.

Yuichi Harikane, a JSPS fellow at the National Astronomical Observatory of Japan (NAOJ) who led the team of astronomers explains, “A protocluster is a rare and special system with an extremely high density, and not easy to find. 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.”

In the map of the universe made by the Subaru Telescope, the team discovered a protocluster candidate, z66OD, where galaxies are 15 times more concentrated than normal for that era.

The team then conducted follow-up spectroscopic observations using Keck Observatory’s DEep Imaging and Multi-Object Spectrograph (DEIMOS) as well as Gemini North telescope’s Gemini Multi-Object Spectrograph (GMOS) to confirm the 12 galaxies existed 13 billion years ago, making the protocluster the earliest one known to date.

Figure 1: The most distant protocluster discovered by the Subaru Telescope. The blue shading shows the calculated extent of the protocluster, and the bluer color indicates higher density of galaxies in the protocluster. The red objects in zoom-in figures are the 12 galaxies found in it. This figure shows a square field-of-view 24 arcminutes along each side (corresponding to 198 million light-years along each side at a distance of 13 billion light-years). Each zoom-in figure is 16 arcseconds along each side (corresponding to 2.2 million light-years). Credit: NAOJ/Harikane et al.

Yoshiaki Ono at the University of Tokyo, Japan, who conducted the spectroscopic observations, explains, “The z66OD protocluster is the earliest protocluster, breaking the record set by the SDF protocluster by 100 million years.”

“This is a wonderful example of the cutting-edge science done from observatories on Maunakea,” said Keck Observatory Chief Scientist John O’Meara. “Results like this are constantly testing and driving our theories of structure in the universe, and I look forward to more results like this in the future.”

Interestingly, one of the 12 galaxies in z66OD was a giant object with a huge body of gas, known as Himiko, which was found previously by the Subaru Telescope in 2009.

“It is reasonable to find a protocluster near a massive object, such as Himiko. 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,” said Masami Ouchi, a team member at NAOJ and the University of Tokyo, who discovered Himiko in 2009.

Ironically, the legendary queen Himiko is also said to have lived cloistered away from her people. Ouchi continues, “It is still not understood why Himiko is not located in the center. These results will be key to understanding the relationship between clusters and massive galaxies.”

Another surprise was that the team found very active star formation in the z66OD protocluster using observational results from the Subaru Telescope, as well as another Maunakea observatory – the United Kingdom Infra-Red Telescope (UKIRT) – and the Spitzer Space Telescope.

“The total amount of stars forming in the galaxies in z660D is five times larger than in other galaxies with similar masses in the same age of the universe. The galaxies in z66OD form stars very efficiently, probably because the large mass of the system helps it to collect a large amount of gas, the material for stars,” explained Darko Donevski, a team member at SISSA Institute, Trieste, Italy.

Figure 2: Three-dimensional map of galaxies obtained in this research. The black points indicate locations of galaxies, and bluer color means higher density. The red arrow indicates the most distant protocluster discovered in this research. Credit: NAOJ/Harikane et al.

Team member Seiji Fujimoto at Waseda University, Japan, says, “Recent observations are revealing that protoclusters can also contain massive galaxies obscured by dust. Although we didn’t find any such galaxies in z66OD, future observations, such as by ALMA (Atacama Large Millimeter/submillimeter Array), may find some, and reveal the entire structure of z66OD.”

This research is published in the September 30, 2019 issue of The Astrophysical Journal as Yuichi Harikane, et al. “SILVERRUSH. VIII. Spectroscopic Identifications of Early Large Scale Structures with Protoclusters Over 200 Mpc at z~6-7: Strong Associations of Dusty Star-Forming Galaxies.”

This work was supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and the Japan Society for the Promotion of Science.


DEIMOS (the DEep Imaging and Multi-Object Spectrograph) boasts the largest field of view (16.7 arcmin by 5 arcmin) of any of the Keck instruments, and the largest number of pixels (64 Mpix). It is used primarily in its multi-object mode, obtaining simultaneous spectra of up to 130 galaxies or stars. Astronomers study fields of distant galaxies with DEIMOS, efficiently probing the most distant corners of the universe with high sensitivity.


The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two, 10-meter optical/infrared telescopes on the summit of Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. 

Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community.  We are most fortunate to have the opportunity to conduct observations from this mountain.


The Maunakea Observatories are a collaboration of independent institutions with telescopes located on Maunakea on the Island of Hawaii. Together, the Observatories make Maunakea the most scientifically productive site for astronomy world-wide. The Maunakea Observatories include: Caltech Submillimeter Observatory, Canada-France-Hawai‘i Telescope, Gemini International Observatory, James Clerk Maxwell Telescope (EAO), NASA Infrared Telescope Facility, Subaru Telescope, Submillimeter Array, United Kingdom Infrared Telescope, University of Hawai‘i Hilo Educational Telescope, University of Hawai‘i 2.2 Meter Telescope, Very Long Baseline Array and W. M. Keck Observatory (Keck I and Keck II). Learn more: www.maunakeaobservatories.org