Universe's oldest X-ray-spitting quasar could reveal how the biggest black holes were born
Astronomers using NASA's James Webb Space Telescope (JWST) and Chandra X-ray Observatory have discovered the oldest and most distant X-ray-spitting quasar in the known universe, and it seems to be powered by the "seed" of an ancient supermassive black hole.
Quasars are the bright hearts of active galaxies, which are fueled by active supermassive black holes that cause infalling matter to emit intense thermal radiation as they feed. Quasars can be so bright across the entire electromagnetic spectrum that they often outshine the combined light from every star in the galaxy surrounding them.
This primordial quasar, designated UHZ1, was spotted in high-energy X-ray light emitted when the cosmos was no more than 450 million years old and has thus been traveling through the universe for around 13.7 billion years to reach us. As such, this quasar could be an example of a black hole "seed" in the early universe that helps reveal how supermassive black holes reached tremendous masses of millions, or even billions, of times that of the sun.
"It's thrilling to be able to reveal the presence of a supermassive black hole, in place at the center of a galaxy a mere 450 million years after the Big Bang," study co-author Priyamvada Natarajan, a professor of astronomy and physics at Yale University, said in a statement. "NASA's Chandra space telescope detected X-rays from this distant quasar, which harbors an outsized black hole in its center."
Related: James Webb telescope reveals the universe may have far fewer active black holes than we thought
The discovery of UHZ1 is detailed in a paper published in the journal Nature Astronomy.
Understanding how supermassive black holes got so huge
Scientists theorize that supermassive black holes grew to such tremendous sizes by starting off as black hole seeds in the early universe and growing steadily by gorging on matter and merging with other black holes.
The question is, how big were these seeds to begin with? One variation of this theory suggests the early universe was packed with "light seeds" — black holes created when massive stars ran out of fuel for nuclear fusion and exploded in supernova blasts, collapsing under their own gravity.
However, this explanation doesn't give supermassive black holes enough time to reach masses equivalent to millions, let alone billions, of suns at the early times astronomers observe these behemoths in the infant universe.
One idea that would give supermassive black holes a "head start" on this process is if they started growing from "heavy seeds." Between 2006 and 2007, Natarajan developed a model suggesting that heavy black hole seeds could form in galaxies where star formation is suppressed.
These would be satellite galaxies located near the galaxies in the early universe that birthed the first stars. This model suggests that large disks of gas and dust in these satellite galaxies could collapse directly into black-hole-heavy seeds, instead of first birthing stars that eventually collapsed into black holes millions or billions of years later. These heavy-seed black hole satellite galaxies would then merge with the main star-forming galaxies nearby.
In 2017, Natarajan and her colleagues suggested that heavy black hole seed galaxies should be observable in the early universe thanks to their unique properties. In particular, the central black hole in a heavy-black-hole-seed galaxy would outweigh that galaxy's stars. This should be visible as X-ray quasars to the Chandra X-ray Observatory, as well as to the yet-to-be-launched JWST, Natarajan proposed in 2017.
Finding a heavy black hole seed
Now, six years later, the team's prediction bears fruit with the discovery of this distant X-ray quasar. UHZ1 was identified by a team led by Akos Bogdan, an astrophysicist at the Harvard and Smithsonian Center for Astrophysics, and Andy Goulding, an astrophysicist at Princeton, who combined recent data from the Chandra X-ray Observatory and JWST to peer behind galaxy Abell 2744.
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"UHZ1 is the first candidate that matches all our predicted properties for this transient class of over-massive black hole galaxies," Natarajan said. "And now we're seeing compelling first evidence. This is an exciting intersection of topics, a culmination of all the things I have been working on."
Goulding thinks there are many more heavy-seed galaxies out there just waiting to be uncovered.
"UHZ1 may only be the tip of the iceberg," he said. "The JWST has opened a new window on the early universe. It will no doubt help us find more UHZ1s and ultimately understand if over-massive black holes were commonplace."
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