A team of astronomers from the National Astronomical Observatory of Japan (NAOJ) and the University of Tokyo has detected the strong infrared emissions from protoclusters at 12 billion years ago by using the deep and wide extragalactic survey with Hyper Suprime-Cam (HSC) on the Subaru Telescope and archival infrared data taken by various space telescopes. The detected infrared emissions were brighter than that which was expected from the galaxy population observed in the visible. These results indicate dramatic star formation and super massive black hole growth which are not observable in the optical but luminous in the infrared.
There are many galaxies in our Universe. There is an environmental
dependence of galaxy type in that giant elliptical galaxies dominate
galaxy overdense regions (clusters of galaxies) while spiral galaxies
dominate general fields. To investigate how the environmental dependence
of galaxies developed, the team studied progenitors of modern clusters
of galaxies, called protoclusters.
To constrain the typical properties of protoclusters, they needed to
observe many protoclusters statistically. However, the surface number
density of protoclusters is too low to search for them easily.
Therefore, only a small number of protoclusters which existed over 10
billion years ago have been known.
HSC on the Subaru Telescope enables an effective search for
protoclusters. HSC is an optical camera with an extremely wide field of
view (about 1.8 square degrees, equivalent to the area of nine full
moons) and high sensitivity. Now, a deep and wide extragalactic survey
is on-going with HSC (HSC-SSP).
From the 120 square degree survey of the first HSC-SSP results, the
team selected about 180 protocluster candidates. This is the largest
catalog of protoclusters ever (March 4, 2018, Press Release from Subaru Telescope).
The Subaru Telescope is an optical telescope, but to study galaxy
properties in more detail, observations at various wavelengths are
needed. In the case of a rapidly star forming galaxy, most of the light
from its young stars, which is an indicator of the star formation rate,
is absorbed by its surrounding dust. The team needed to observe the
infrared/radio emissions re-emitted from the dust to evaluate the star
formation rate of a galaxy correctly. However, it is hard to observe in
the infrared with ground-based telescopes because most of the infrared
emissions are absorbed by water vapor in the atmosphere. There is no
working space telescope with the sensitivity required to observe
galaxies at 12 billion years ago in the infrared. ALMA can observe
distant galaxies at radio wavelengths, however, its available wavelength
range is limited and it is impractical to observe over 100
protoclusters.
To investigate the infrared properties of galaxies in protoclusters,
the science team focused on the infrared data in public archives freely
accessible to everyone. They used the archival full sky survey images at
mid-far infrared taken by five (one is not a full sky survey) infrared
space telescopes like Planck (Note 1). The spatial resolution and
sensitivity of these images are too low to resolve distant galaxies
individually, however, by stacking the images of the 180 protocluster
candidates selected with HSC, they successfully constrained the average
total infrared flux of a protocluster. Especially, the mid-far infrared
flux (30-200 microns) of galaxies at 12 billion years ago has been
unknown.
Surprisingly, the average total infrared flux of a protocluster is
brighter than that which was expected from the galaxy population found
by HSC. This implies that there are galaxies which are not observable in
the optical but which are luminous in the infrared.
What is the origin of this strong infrared emission? The team
investigated the flux to wavelength distribution in the infrared and
found that the average dust temperature of the protoclusters is warmer
than that of a typical star forming galaxy. That implies that there are
not only typical star forming galaxies but also growing super massive
black holes at the centers of the galaxies (so called active galactic
nuclei) and/or young hot dusty starburst galaxies which heat dust to
higher temperatures. This study demonstrates the need to study
protoclusters at wavelengths outside of the wavelength coverage of
ground-based large telescopes like the Subaru Telescope and ALMA.
To study the galaxies in protoclusters in more detail, individual
protocluster galaxies need to be resolved in the mid to far-infrared,
however, there is no telescope capable of such observations at this
point. The leader of the team, Mariko Kubo (postdoctoral fellow at NAOJ)
says, "In the future, SPICA, the future space telescope planned by
Japan and ESA, will reveal the mid-far infrared forms of distant
galaxies. On the other hand, unlike HSC, SPICA will not be made for wide
field surveys. Our results partly complement SPICA's science."
This study was published in The Astrophysical Journal on December 20,
2019 (Mariko Kubo, Jun Toshikawa, Nobunari Kashikawa, Yi-Kuan Chiang,
Roderik Overzier, Hisakazu Uchiyama, David L. Clements, David M.
Alexander, Yuichi Matsuda, Tadayuki Kodama, Yoshiaki Ono, Tomotsugu
Goto, Tai-An Cheng, and Kei Ito, 2019, ApJ 887, 214, "Planck Far-infrared Detection of Hyper Suprime-Cam Protoclusters at z∼4: Hidden AGN and Star Formation Activity").
This work was funded, in part, by the Japan Society for the Promotion
of Science Grants-in-Aid for Scientific Research (JP15H03645,
JP17H04831, JP17KK0098, JP19H00697).
Note 1:
The science team used the data archived by the Planck, IRAS, WISE, Herschel, and AKARI space telescopes. These telescopes finished their missions a long time ago, but the data taken by them is in public data archives. Infrared space telescopes generally finish their lives within a few years because they have to load coolant (which should be exhausted) and operate in a harsh environment. For mid-infrared wavelength observations, the James Webb Space Telescope will be launched next year. However, there is no far-infrared observatory for distant galaxies between the Herschel space telescope, which finished its mission in 2013, and SPICA (planned to be launched in around 2030).
The science team used the data archived by the Planck, IRAS, WISE, Herschel, and AKARI space telescopes. These telescopes finished their missions a long time ago, but the data taken by them is in public data archives. Infrared space telescopes generally finish their lives within a few years because they have to load coolant (which should be exhausted) and operate in a harsh environment. For mid-infrared wavelength observations, the James Webb Space Telescope will be launched next year. However, there is no far-infrared observatory for distant galaxies between the Herschel space telescope, which finished its mission in 2013, and SPICA (planned to be launched in around 2030).
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