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Using the wide-field survey capabilities of the Subaru Telescope, astronomers discovered active supermassive black holes, or quasars, in the distant universe and then studied them with the James Webb Space Telescope (JWST). This work has revealed how galaxies and their central black holes grew 12.9 billion years ago.

From JWST observations, the team found that the host galaxies of these quasars have already become massive but now their star formation is winding down. This provides key evidence that the supermassive black holes played a major role in shaping the earliest and fastest-growing galaxies.

This discovery was made possible by combining the Subaru Telescope's wide-field survey capability with JWST's unparalleled sensitivity in the infrared wavelengths. The work is in the journal Nature Astronomy.

At the centers of galaxies lie supermassive black holes, with masses ranging from a few hundred-thousand-times to tens-of-billions-of-times the mass of the sun.

Black holes are generally classified into stellar-mass black holes and supermassive black holes. Stellar-mass black holes form at the end of a star's life, while supermassive black holes reside at the centers of galaxies. Supermassive black holes are thought to have formed during the early stages of galaxy formation and to have grown by accreting surrounding matter over cosmic timescales.

When active, these black holes emit enormous amounts of energy as they accrete the surrounding matter, making them visible as extremely bright quasars.

Observations of the nearby universe show a strong correlation between a galaxy's mass and its central black hole mass, suggesting that galaxies and black holes have grown together over cosmic time—a process known as "co-evolution." However, when and how this co-evolution began has remained unclear. To understand its early stages, astronomers need to observe distant galaxies in the early universe and disentangle the contributions by their stars from the black hole activity.

An international team including researchers from the Kavli Institute for the Â鶹ÒùÔºics and Mathematics of the Universe (Kavli IPMU, WPI) at the University of Tokyo and the National Astronomical Observatory of Japan observed two quasars, J2236+0032 and J1512+4422, located 12.9 billion light-years away (only 900 million years after the Big Bang) using JWST's Near Infrared Spectrograph (NIRSpec).

These quasars were discovered through the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP), a wide-field imaging survey at the Subaru Telescope, and were among the targets followed in JWST's first year of science operations.

The team successfully detected not only the bright light from the quasars but also, unexpectedly, absorption lines of neutral hydrogen arising from their host galaxies' stars. Detailed analyses of these absorption lines revealed that the galaxies have formed very few new stars in recent times, suggesting that a major star formation burst occurred several hundred million years earlier, after which the galaxy's growth either slowed down or nearly ceased.

J2236+0032 and J1512+4422 are among the farthest known such galaxies. This dramatic change in the galaxies' states may have been caused by the intense radiation emitted by the central black holes.

Combined analyses of JWST's near-infrared imaging data showed that the host galaxies of J2236+0032 and J1512+4422 are massive, containing the equivalent of 60 billion and 40 billion solar masses in stars, respectively.

"It was totally unexpected to find such mature galaxies in the universe less than a billion years after the Big Bang," says Dr. Masafusa Onoue (Kavli IPMU, WPI/Waseda University, Waseda Institute for Advanced Study), lead author of the study. "What is even more remarkable is that these 'dying' galaxies still host active supermassive black holes."

Previous studies have suggested that the activity of supermassive black holes can suppress the growth of their host galaxies and drive the transition from active star formation to a quiescent phase. This discovery captures that process in action, providing a new clue to understanding the complex growth history of galaxies and black holes in the early universe. It is also a uniquely Japanese achievement, made possible by combining the Subaru Telescope's powerful survey capability with JWST's exceptional sensitivity.

Building on this discovery, the research team is continuing detailed analyses of the JWST data and planning future observations to further investigate the mysterious relationship between galaxies and black holes.