Unveiling the origin of supermassive black holes

Black holes of 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, their origins, as well as the details of when, where and how they appeared over the course of 13.8 billion years of cosmic evolution, remain a mystery.

Research over the past several decades indicates that small, supermassive galaxies reside at the heart of every galaxy, always one-thousandth the mass of their host galaxy.

This close relationship indicates that galaxies and supermassive galaxies evolved together. Hence, revealing the origin of supermassive stars is critical not only to understanding the supermassive planets themselves, but also to elucidating the processes of formation of galaxies, key components of the observable universe.

The key to addressing this problem lies at the beginning of the universe, as time has elapsed since the universe appeared the great explosion (meaning the beginning of the universe) was less than a billion years old. Thanks to the finite speed of light, we can look 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 an image of the night sky that we took with the Subaru Telescope. The small red dot in the center of the enlarged image represents light from a distant quasar, which existed when the universe was 800 million years old (13 billion light-years). 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 billions of solar masses) in such a short time? If so, what are the underlying physical mechanisms and conditions? To get closer to 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 located in the sky.

The research team used the Subaru telescope located atop Mount Mauna Kea in Hawaii to conduct this study. One of Subaru’s biggest advantages is its wide range surveillance capability, and it is particularly well suited for this purpose.

Because ultrafine objects don’t emit light, the team looked for a special class called “quasars” – small, ultrafine objects with bright tips where infalling material releases gravitational energy. They observed an expanse of sky 5,000 times the full moon and successfully detected 162 quasars residing in the early universe. In particular, 22 of these quasars existed when the universe was less than 800 million years old, the oldest period for which quasars have been identified so far.

The large number of quasars detected has allowed them to determine a basic metric called the “luminosity function,” which describes the spatial density of quasars as a function of radiative energy. They found that quasars were forming very quickly in the early universe, while the overall shape of the luminosity function (excluding amplitude) remained unchanged over time.

The luminosity function describes the density of space (Φ on the vertical axis) as a function of radiant energy (M1450 on the horizontal axis). We plot luminosity functions for 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. Curves represent the most appropriate functional shapes. The spatial density of quasars has risen sharply over time, while the shape of the luminosity function has remained virtually unchanged. Credit: The Astrophysical Journal Letters, 949, L42, 2023

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

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

By integrating the luminosity function mentioned above, we found that quasars emit 10²⁸ photons per second in unit size 1 Light year on a 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 desperately needed, which, according to other recent observations, may be the compact radiation from massive hot stars forming galaxies.

Reference: “The luminosity function of a quasar at z = 7” by Yoshiki Matsuoka, Masafusa Onoe, Kazushi Iwasawa, Michael A. Strauss, Nobunari Kashikawa, Takuma Izumi, Toru Nagao, Masatoshi Imanishi, Masayuki Akiyama, John D. Silverman, Naoko Asami, James Bush, Hisanori Furusawa, Tomotsugu Goto, James E. Gan, Yuichi Harikan, Hiroyuki Ikeda, Kohei Inayoshi, Rikako Ishimoto, Toshihiro Kawaguchi, Satoshi Kikuta, Kotaro Kohno, Yutaka Komiyama, Shin Hsu 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 Takauji Yamashita, June 6, 2023, the Astrophysical Journal Letters.
doi: 10.3847/2041-8213/acd69f

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