Oldest Galaxy Protocluster Discovered

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.

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% 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, which are 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 said, “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.

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

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,” said Masami Ouchi, a team member at NAOJ and the University of Tokyo, who discovered Himiko in 2009. “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.”

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.

Ironically, the legendary Queen Himiko is also said to have lived cloistered away from her people.

“It is still not understood why Himiko is not located in the center,” Ouchi continued. “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,” explained Darko Donevski, a team member at SISSA Institute, Trieste, Italy. “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.”