Revealing the origin of supermassive black holes

Black holes with exceptionally large masses — more than a million times the mass of the Sun, known as supermassive black holes (SMBHs) — are commonly found in the universe today. However, its origins, as well as the details of when, where and how it appeared over the course of 13.8 billion years of cosmic evolution, remain mysterious.

Research over the past several decades indicates that small, extremely massive galaxies reside at the core of every galaxy, and that their mass is always one-thousandth the mass of their host galaxy.

This close relationship indicates that galaxies and ultra-massive galaxies have evolved together. Hence, revealing the origin of massive stars is crucial not only for understanding the massive planets themselves, but also for elucidating the formation processes of galaxies, key components of the observable universe.

The key to addressing this problem lies in the beginning of the universe, as time has passed since the universe appeared the great explosion (i.e. the beginning of the universe) was less than a billion years ago. Thanks to the finite speed of light, we can peer into the past by observing the distant universe. Did small and medium-sized objects really exist when the universe was only a billion years old or less?

An example of a night sky image we took with the Subaru Telescope. The small red dot in the center of the magnified image represents light coming from a distant quasar, which existed when the universe was 800 million years (13 billion light-years) old. Credit: National Astronomical Observatory of Japan

Is it possible to Black hole To gain such a large mass (more than a million solar masses and sometimes reaching billions of solar masses) in such a short time? If so, what are the underlying mechanisms and physical conditions? To approach the origin of small and medium-sized objects, one needs to observe them and compare their properties with predictions from theoretical models. To do this, one first needs to determine where they are in the sky.

The research team used the Subaru telescope located on top of Mount Maunakea in Hawaii to conduct this study. One of Subaru’s biggest advantages is its wide-area surveillance capability, and it is particularly well-suited for this purpose.

Since ultra-fine objects do not emit light, the team looked for a special class called quasars – small ultra-fine objects with bright fringes where falling material releases gravitational energy. They observed a wide area of ​​the sky equivalent to 5,000 times the full moon and successfully discovered 162 quasars residing in the early universe. In particular, 22 of these quasars existed in an epoch when the universe was less than 800 million years old, the oldest period in which quasars have been identified to date.

The large number of quasars discovered allowed them to determine a fundamental measure called the “luminosity function,” which describes the space density of quasars as a function of radiative energy. They found that quasars were forming very rapidly in the early universe, while the general shape of the luminosity function (except the amplitude) remained unchanged over time.

The luminosity function describes the space density (Φ on the vertical axis) as a function of radiative energy (M1450 on the horizontal axis). We plot the luminosity functions of quasars observed when the Universe was 0.8 (red dots), 0.9 (green diamonds), 1.2 (blue squares), and 1.5 (black triangles) billion years old. The curves represent the most appropriate functional forms. The space density of quasars has risen sharply with time, while the shape of the luminosity function has remained almost unchanged. Credit: The Astrophysical Journal Letters, 949, L42, 2023

This distinct behavior of the luminosity function provides strong constraints on theoretical models, which can eventually reproduce all observable elements and describe the origin of supermassive black holes.

On the other hand, it was known that the universe had undergone a major transition called “cosmic reionization” in its early phase. Previous observations indicate that the entire intergalactic space was ionized in this event. The source of the ionization energy is still under debate, with radiation from quasars being a promising candidate.

By incorporating the above luminosity function, we find that quasars emit 10²⁸ Photons per second in unit volume 1 Light year On one side in the early universe. This represents less than 1% of the photons needed to maintain the ionized state of intergalactic space at that time, and thus indicates that quasars made only a minor contribution to cosmic reionization. Other sources of energy are urgently needed, which, according to other recent observations, may be the built-in radiation from hot, massive stars in the formation of galaxies.

Reference: “Quasar luminosity function at z = 7” by Yoshiki Matsuoka, Masafusa Onoe, Kazushi Iwasawa, Michael A. Strauss, Nobunari Kashikawa, Takuma Izumi, Toru Nagao, Masatoshi Imanishi, Masayuki Akiyama, Jun D. Silverman, Naoko Asami, James Bush, Hisanori Furusawa, Tomotsugu Goto, James E. Gan, Yuichi Harikane, Hiroyuki Ikeda, Kohei Inayoshi, Rikako Ishimoto, Toshihiro Kawaguchi, Satoshi Kikuta, Kotaro Kohno, Yutaka Komiyama, Shin-Hsiu Lee, Robert H. Lupton, Takeo Minezaki, Satoshi Miyazaki, Hitoshi Murayama, Atsushi J. Nishizawa, Masamune Oguri, Yoshiaki Ono, Taira Oji, Masami Ochi, Paul A. Price, Hiroaki Sameshima, Naoshi Sugiyama, Philip J. Tate, Masahiro Takada, Ayumi Takahashi, Tadafumi Takata, Masayuki Tanaka, Yoshiki Toba, Xiangyu Wang and Takuji Yamashita, June 6, 2023, the Astrophysical Journal Letters.
doi: 10.3847/2041-8213/acd69f

The study was funded by the Japan Society for the Promotion of Science, the Mitsubishi Foundation, and the National Natural Science Foundation of China.