Not long ago, the James Webb Space Telescope (JWST) peered into Cosmic Dawn, the cosmological period when the first galaxies formed less than one billion years after the Big Bang. In the process, it discovered something rather surprising. Not only were there more galaxies (and brighter ones, too!) than expected, but these galaxies had supermassive black holes (SMBH) much larger than cosmological models predicted. For astronomers and cosmologists, explaining how these galaxies and their SMBHs (aka. quasars) could have grown so large less than a billion years after the Big Bang has become a major challenge.
Several proposals have been made, ranging from optical illusions to Dark Matter accelerating black hole growth. In a recent study, an international team led by researchers from the National Institute for Astrophysics (INAF) analyzed a sample of 21 quasars, among the most distant ever discovered. The results suggest that the supermassive black holes at the center of these galaxies may have reached their surprising masses through very rapid accretion, providing a plausible explanation for how galaxies and their SMBHs grew and evolved during the early Universe.
The study was led by Alessia Tortosa, a researcher with the INAF’s Astronomical Observatory of Rome. She was joined by researchers from the Centre for Extragalactic Astronomy, the Centro de Astrobiología (CAB), the Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, the Institute for Fundamental Physics of the Universe, the National Institute for Nuclear Physics, the Harvard & Smithsonian Center for Astrophysics, the Italian Space Agency (ASI), the European Space Agency (ESA), the NASA Goddard Space Flight Center, and multiple observatories and universities. The paper detailing their findings was recently published in the Astronomy & Astrophysics.
Radio astronomers first observed quasars in the 1950s based on the large amounts of radiation they emitted at many frequencies. These objects, which they named “quasi-stellar objects” (quasar for short), were notable for how their cores would outshine all the stars in their disks. From the 1970s onward, astronomers learned that this phenomenon was due to the presence of SMBHs at the center of these galaxies. Since then, astronomers have been eager to get a look at the earliest galaxies in the Universe to see the “seeds” of these black holes and chart their evolution.
However, Webb’s observations revealed some surprisingly large “seeds” at the center of the early galaxies it imaged. This included galaxies like EGSY8p7, which existed just 570 million years after the Big Bang but had a central black hole roughly 9 million times the mass of the Sun. Even more surprising was UHZ1, a galaxy that existed when the Universe was about 470 million years old. At its center, Webb spotted a massive black hole (designated CEERS 1019) 40 million times the mass of our Sun – ten times the mass of Sagittarius A*, the SMBH at the center of the Milky Way.
According to the most widely accepted cosmological models, these galaxies and black holes did not have enough time to grow so large. For their study, Tortosa and her colleagues analyzed a sample of 21 quasars (including the most distant ever observed) based on X-ray data obtained by the XMM-Newton and Chandra space telescopes. This revealed a completely unexpected connection between the shape of the X-ray emissions and the speed of the winds ejecting matter from the quasars. This connection suggests that wind speeds are connected to the temperature of the gas closest to the black hole’s corona (the X-ray emitting region).
This means that the corona is connected to the powerful accretion mechanisms that allow black holes to grow. Specifically, they observed how quasars with low-energy X-ray emissions and lower temperatures have faster winds, leading to a rapid growth rate that exceeds the Eddington Limit – the theoretical limit to the mass of a star or an accretion disk. Meanwhile, quasars with higher X-ray emissions tended to exhibit slower wind speeds. As Tortosa explained in an INAF press statement:
“Our work suggests that the supermassive black holes at the center of the first quasars formed within the first billion years of the Universe’s life may have actually increased their mass very rapidly, challenging the limits of physics. The discovery of this connection between X-ray emission and winds is crucial for understanding how such large black holes could have formed in such a short time, thus providing a concrete clue to solve one of the greatest mysteries of modern astrophysics.”
Most of the XMM-Newton data was collected between 2021 and 2023 as part of a Multi-Year XMM-Newton Heritage Program known as HYPerluminous quasars at the Epoch of ReionizatION (HYPERION). This program is directed by Luca Zappacosta, an INAF researcher and the second author of the paper, and aims to study hyperluminous quasars during the cosmic dawn of the Universe. Said Zappacosta:
“In the HYPERION program, we focused on two key factors: on one hand, the careful selection of quasars to observe, choosing the titans, meaning those that had accumulated as much mass as possible, and on the other hand, the in-depth study of their properties in X-rays, something never attempted before on such a large number of objects from the cosmic dawn. We hit the jackpot! The results we’re getting are genuinely unexpected, and they all point to a super-Eddington growth mechanism of the black holes.”
This study provides valuable insights into the formation and evolution of SMBHs and their host galaxies. The team’s findings will also inform future X-ray missions, like the ESA’s Advanced Telescope for High Energy Astrophysics (ATHENA) and NASA’s Advanced X-Ray Imaging Satellite (AXIS) and Lynx X-ray Observatory, which are scheduled to launch in the next two decades. These and other next-generation instruments are expected to reveal even more about the early Universe and help resolve its deepest mysteries.
Further Reading: INAF, Astronomy & Astrophysics