A galaxy called HD1 appears to be about 33.4 billion light years away, making it the most distant object ever seen – and its extreme brightness is puzzling researchers
7 April 2022
A galaxy called HD1 may be the most distant object astronomers have ever spotted. Its astonishing brightness is difficult to explain and may be due to an enormous black hole at its centre or the creation of extremely massive primordial stars, both of which confound our understanding of the early universe.
Fabio Pacucci at the Harvard-Smithsonian Center for Astrophysics in Massachusetts and his colleagues found HD1 by sifting through large public data sets from several of the most powerful telescopes available. They then observed it again with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.
Those observations showed that HD1 is about 33.4 billion light years away, more than a billion light years further than the previous most distant object ever spotted, a galaxy called GN-z11. Such a distance is possible, despite the age of the universe being only about 13.8 billion years, because of the accelerating expansion of the cosmos.
The galaxy is extraordinarily bright in ultraviolet wavelengths, meaning that whatever is producing its light is probably extremely hot. There are two possible ways to make it shine so brightly: either it is undergoing a burst of star formation much bigger than we would expect for the galaxy’s relatively small size or it is home to an active supermassive black hole.
If the answer is a starburst, HD1 would have to be producing about 110 times the mass of the sun in stars every year. “This is very large. It’s a crazy number,” says Pacucci. “One explanation is this galaxy might not be forming normal stars, but these primordial stars that are much more massive and much hotter than normal nearby stars.” We have never seen such primordial stars before.
The other explanation is that HD1 could host an unexpectedly colossal supermassive black hole. “The observation of a 100-million-solar-mass black hole so early in the history of the universe would really be groundbreaking, because we really wouldn’t be sure how to form this,” says Pacucci. Black holes need time to grow, and HD1 is so distant that we are seeing it as it was just 330 million years after the big bang, so it is unclear how a black hole could have become so big so quickly.
We need more observations to be sure of HD1’s extreme distance, as well as to figure out why it is so bright, says Pacucci. “At this point, we are really stretching the capabilities of our current observatories very thin,” he says. The researchers have been awarded observation time on the James Webb Space Telescope to observe HD1, as well as two other objects called HD2 and HD3 that seem to be nearly as far away.
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