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 13.0 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 the Subaru, Keck, and Gemini Telescopes, an international team of
astronomers has discovered a collection of 12 galaxies in the
constellation Cetus which existed about 13.0 billion years ago. This is
the earliest protocluster ever found. 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 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 the W.M. Keck Observatory and
Gemini North telescope, and confirmed 12 galaxies which existed 13.0
billion years ago, making it the earliest protocluster known to date.
Yoshiaki Ono at the University of Tokyo, Japan, who conducted the
spectroscopic observations, explains, "The z66OD protocluster is the
earliest protoclutser, breaking the record set by the SDF protocluster
by 100 million years."
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 the National Astronomical Observatory of Japan
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 a key for
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, United Kingdom Infra-Red Telescope, and 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.
Team member Seiji Fujimoto at Waseda University, Japan, comments,
"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 will be published on September 30, 2019 in 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.
Links:
- Subaru Telescope Discovers the Most Distant Protocluster of Galaxies (Subaru Telescope April 23, 2012 Press Release)
- Mysterious Space Blob Discovered at Cosmic Dawn (Subaru Telescope April 22, 2009 Press Release)
- Oldest Galaxy Protocluster Forms "Queen's Court" (Press Release from Waseda University)
- Record-Breaking Protocluster Takes Fast-track (Press Release from Gemini Observatory)
- Oldest Galaxy Protocluster Forms "Queen's Court" (Press Release from W. M. Keck Observatory)