Distant Universe

A team of astronomers has discovered 83 quasars powered by supermassive black holes (SMBHs) in the distant universe, from an epoch when the universe was less than 10 percent of its present age. This finding was made by using the wide-field camera Hyper Suprime-Cam (HSC) mounted on the Subaru Telescope. The discovery increases the number of black holes known at that epoch considerably, and reveals, for the first time, how common SMBHs are early in the universe’s history. In addition, it provides new insight into the effect of black holes on the physical state of gas in the early universe in its first billion years. Figure 1 shows an example of a discovered SMBH.

Supermassive black holes are found at the centers of galaxies, and have masses millions or even billions of times that of the Sun. While they are prevalent in the present-day universe, it is unclear when they first formed, and how many of them exist in the distant early universe. While distant SMBHs are identified as quasars, which shine as gas accretes onto them (see Figure 2 for an artist impression), previous studies have been sensitive only to the very rare most luminous quasars, and thus the most massive black holes. The new discoveries probe the population of SMBH with masses characteristic of the most common black holes seen in the present-day universe, and thus shed light on their origin.

The research team led by Yoshiki Matsuoka (Ehime University) used data taken with a cutting-edge instrument, Hyper Suprime-Cam (HSC), mounted on the Subaru Telescope of the National Astronomical Observatory of Japan, on the summit of Maunakea in Hawai’i. HSC is particularly powerful in that it has a gigantic field-of-view of 1.77 deg2 (seven times the area of the Full Moon), mounted on one of the largest telescopes in the world. The HSC team is carrying a survey of the sky using 300 nights of telescope time, spread over five years. The team selected distant quasar candidates from the sensitive HSC survey. They then carried out an intensive observational campaign to obtain spectra of those candidates, using the Subaru Telescope, the Gran Telescopio Canarias, and the Gemini telescope. The survey has revealed 83 previously unknown very distant quasars; together with the 17 quasars already known in the survey region, Matsuoka and collaborators found that there is roughly one supermassive black hole in each cube a billion light years on a side. Figure 3 shows images of the 100 quasars identified from the HSC data.

The discovered quasars are about 13 billion light-years away from the Earth; in other words, we are seeing them as they existed 13 billion years ago. The time elapsed since the Big Bang to that cosmic epoch is only 5 per cent of the present cosmic age (13.8 billion years), and it is remarkable that such massive dense objects were able to form so soon after the Big Bang. The most distant quasar discovered by the team is 13.05 billion light-years away, which is tied for the second most distant SMBH ever discovered.



It is widely accepted that the hydrogen in the universe was once neutral, but was “reionized” (i.e., split into its component protons and electrons) around the epoch when the first generation of stars, galaxies, and SMBHs were born, in the first few hundred million years after the Big Bang. This is a milestone of cosmic history, but it is still not clear what provided the incredible amount of energy required to cause the reionization. A compelling hypothesis suggests that there were many more quasars in the early universe than detected previously, and it is their integrated radiation that reionized the universe. However, the number density measured by the HSC team clearly indicates that this is not the case; the number of quasars seen is significantly less than needed to explain the reionization. Reionization was therefore caused by another energy source, most likely numerous galaxies that started to form in the young universe.



The present study was made possible by the world-leading survey ability of Subaru and HSC. The intensive follow-up observations by the Subaru Telescope, Gran Telescopio Canarias, and the Gemini telescope were another key to success. “The quasars we discovered will be an interesting subject for further follow-up observations with current and future facilities.”, said Matsuoka. “We will also learn about the formation and early evolution of SMBHs, by comparing the measured number density and luminosity distribution with predictions from theoretical models.” Based on the results achieved so far, the team is looking ahead to search for yet more distant SMBHs, and to reveal the epoch when the first SMBH appeared in the universe.

The research team consists of 48 astronomers around the world. Matsuoka led the team, while Nobunari Kashikawa (The University of Tokyo), Michael Strauss (Princeton University), Masafusa Onoue (Max Planck Institute for Astronomy), Kazushi Iwasawa (Universitat de Barcelona), and Tomotsugu Goto (National Tsing Hua University) have played key roles in the individual steps of the project. The results of the project are presented in the following five papers (paper [2] in particular).



[1] “Discovery of the First Low-luminosity Quasar at z > 7”, Matsuoka et al., The Astrophysical Journal Letters, 872 (2019), 2

[2] “Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). V. Quasar Luminosity Function and Contribution to Cosmic Reionization at z = 6”, Matsuoka et al. 2018, The Astrophysical Journal, 869 (2018), 150

[3] “Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). IV. Discovery of 41 Quasars and Luminous Galaxies at 5.7 ≤ z ≤ 6.9”, Matsuoka et al., The Astrophysical Journal Supplement Series, 237 (2018), 5

[4] “Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). II. Discovery of 32 quasars and luminous galaxies at 5.7 < z ≤ 6.8”, Matsuoka et al., Publications of the Astronomical Society of Japan, 70 (2018), S35

[5] “Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). I. Discovery of 15 Quasars and Bright Galaxies at 5.7 < z < 6.9”, Matsuoka et al., The Astrophysical Journal, 828 (2016), 26

The HSC collaboration includes the astronomical communities of Japan and Taiwan, and Princeton University. The HSC instrumentation and software were developed by the National Astronomical Observatory of Japan (NAOJ), the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), the University of Tokyo, the High Energy Accelerator Research Organization (KEK), the Academia Sinica Institute for Astronomy and Astrophysics in Taiwan (ASIAA), and Princeton University. Funding was contributed by the FIRST program from Japanese Cabinet Office, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Japan Society for the Promotion of Science (JSPS), Japan Science and Technology Agency (JST), the Toray Science Foundation, NAOJ, Kavli IPMU, KEK, ASIAA, and Princeton University.