RESOLVING THE LEDGER
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The most distant 'black hole star' yet found, existing in the universe less than 660 million years after the Big Bang, has provided a vital clue as to the origin of the mysterious "little red dots" that populate the James Webb Space Telescope's images of the early universe.
The consensus seems to be that these bizarre objects, which were first discovered in 2022, are growing supermassive black holes entombed within huge clouds of gas that have spectrums similar to cool, red stars, such as red giants . Recently, astronomers have been finding that some of these objects are placed within the center of young galaxies whose light has been traveling for about 12 billion years, give or take a billion, to reach us. Given their distance and faintness, black hole stars' light is difficult to disentangle from the light of a surrounding galaxy, especially when we know there is likely a galaxy there that's too faint for the JWST to resolve.
However, a newly discovered black hole star, unearthed during a JWST program called the Mirage or Miracle (MoM) survey, is acting as a kind of a missing link.
"The Mirage or Miracle survey was designed specifically to target sources considered 'risky,' meaning they could either be amazing discoveries or just interlopers, such as some cold nearby stars that look like distant galaxies," Jorryt Matthee of the Institute of Science and Technology Austria and a co-investigator on the MoM survey said in a statement. In particular, the survey focuses on objects that look like they could be very high redshift galaxies (around a redshift of 10 or more, which equates to having existed about at least 13.1 billion years ago).
The black hole star found by the survey has been named MoM-BH*-1 and is the best example yet of a "naked" black hole star, meaning astronomers are fairly certain it exists in space on its own rather than inside a burgeoning galaxy. This means all of its light is generated by accretion onto the black hole at the center of the cloud. Though we cannot see this accretion because of the obscuring gas cloud, this light energizes the surrounding cloud from within, causing it to glow just as energy generated within the sun causes its outer layers to radiate.
"The spectrum we detected is our best evidence of a cloak of gas feeding an early forming black hole," Rohan Naidu of the University of Hawaii, who led the team who discovered MoM-BH*-1 and is co-leader of the MoM survey, said in the statement.
Some of the JWST's little red dot discoveries. (Image credit: NASA, ESA, CSA, STScI, Dale Kocevski (Colby College))
Intriguingly, MoM-BH*-1 lies close in space to a young galaxy at the same redshift, and estimates suggest that it will take another 100 million years for the black hole star to collide and merge with the galaxy. Naidu and Matthee's team modeled what the spectrum of the galaxy would look like after it has merged with MoM-BH*-1, and found that it would look very much like the little red dots already known to exist within galaxies.
"Considering the black hole star as a template for the black hole component of little red dots helps clarify many of the uncertainties about them," said Matthee. "If embedded in similar host galaxies, black hole stars like MoM-BH*-1 might well serve as the central engines of baby quasars ."
In this way, MoM-BH*-1 provides strong evidence for the object at the heart of faint galaxies encompassing little red dots as being a black hole star. Recent observations of some little red dots at lower redshifts have shown the cloud of gas around the black hole begin to disintegrate, exposing the black hole and the X-rays produced by the accretion of gas onto it. These black holes appear to be growing fast, and the environments surrounding such active black holes become highly luminous, creating a quasar.
An artist's impression depicting the similarity between stars and black hole stars, except that black hole stars are at least 100,000 times larger than the sun. (Image credit: Rohan Naidu (University of Hawaii).)
"Astronomers have never lacked imagination: since the discovery of quasars, there has been no dearth of theories to explain how these black holes grew so massive so fast," said Naidu. "Something spectacular must have happened in the early universe. Now with JWST, we can directly observe this era and see for ourselves which scenarios actually occur."
Learning of the birth of supermassive black holes and whether they predate the galaxies that surround them was one of the primary science goals of the JWST before it launched. In this it has been tremendously successful so far. Despite little red dots having only been discovered four years ago, research into them and black hole stars is accelerating, with new findings coming out now almost on a weekly basis.
"These are incredibly exciting times with nearly a thousand papers and preprints on little red dots in the past two to three years," said Matthee. "Far from a mirage, this might well be a cosmic miracle."
The findings were published on Aug. 13 in the journal Nature.
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