Probing the Genesis of Colossal Gravity Wells: The Enigma of Supermassive Black Holes

A supermassive black hole (SMBH) is the focal point at a galaxy’s core, represented by a minuscule dark point. This celestial body engulfs nearby matter, shaping it into a spiral, disk-like formation as it is pulled inward. The gravitational potential of the matter is transmuted into radiant energy that is expelled away from the disk. Such SMBHs that have luminous peripheries are termed “quasars.” Acknowledgment: Yoshiki Matsuoka

Supermassive black holes, entities with mass exceeding a million times that of the Sun, are a prevalent phenomenon in the current universe. However, questions about their origin, as well as the particularities of their development over the course of approximately 13.8 billion years of cosmic history, remain largely unresolved.

Decades of research suggest that a SMBH is invariably positioned at the heart of each galaxy, its mass constituting roughly one one-thousandth of the mass of the galaxy it occupies.

This intimate connection between galaxies and SMBHs intimates a co-evolutionary relationship between the two. Understanding the genesis of SMBHs is not only pivotal for comprehending these massive entities themselves but also for shedding light on the formative mechanisms of galaxies, which comprise the principal elements of the observable universe.

To explore this puzzle further, one must delve into the early universe—where the age of the cosmos was less than a billion years. The finite velocity of light allows us to investigate the past by examining the far reaches of the universe. Did SMBHs already exist when the universe was less than a billion years old?

Observations were made using the Subaru Telescope, capturing the night sky. A minuscule red dot at the image’s center represents the illumination from a remote quasar, dating back to when the universe was just 800 million years old (13 billion light-years distant). Courtesy: National Astronomical Observatory of Japan

Can a black hole amass such staggering mass—exceeding millions and even billions of solar masses—in a relatively brief period? To address this, one must first locate SMBHs and then match their characteristics with predictions from theoretical frameworks.

Utilizing the Subaru Telescope, situated atop Maunakea, Hawaii, the research team capitalized on its expansive field-of-view for this study. Because SMBHs are not luminescent, the researchers sought a specific category termed “quasars”—SMBHs with radiant edges resulting from gravitational energy being released from incoming matter. They scanned a celestial expanse equivalent to 5000 times the size of the full moon, ultimately discovering 162 early-universe quasars. Notably, 22 of these were from a period when the universe was less than 800 million years old, marking the oldest recognized occurrence of quasars.

This considerable dataset allowed the establishment of a foundational metric, the “luminosity function,” to describe the spatial density of quasars in terms of their radiative energy output. This study found that quasars proliferated rapidly in the early universe, although the overall configuration of the luminosity function has remained largely consistent over time.

This distinct behavior of the luminosity function puts stringent conditions on theoretical models, potentially corroborating all observable data and elucidating the origins of SMBHs.

Simultaneously, the universe underwent a significant phase change termed “cosmic reionization” during its infancy. Earlier observations suggest that this event ionized all intergalactic space, though the energy source remains a matter of debate. Quasar radiation has been considered a viable contender.

By incorporating the aforementioned luminosity function, it was determined that quasars contribute less than 1% of the requisite photons to sustain the ionized state of intergalactic space at that epoch. Thus, additional energy sources are imperative; current observations point towards the cumulative radiation emanating from enormous hot stars in emerging galaxies as a likely candidate.

The research was sponsored by the Japan Society for the Promotion of Science, the Mitsubishi Foundation, and the National Natural Science Foundation of China. The findings were published in The Astrophysical Journal Letters with the DOI: 10.3847/2041-8213/acd69f on June 6, 2023.

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More about Supermassive Black Holes

• The Astrophysical Journal Letters
• Japan Society for the Promotion of Science
• Mitsubishi Foundation
• National Natural Science Foundation of China
• Subaru Telescope
• Cosmology and Supermassive Black Holes
• Cosmic Reionization
• Galaxy Formation
• Theoretical Models of SMBHs