Most distant galaxy: Astronomers have found what may be the most distant object ever seen

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 has puzzled researchers


April 7, 2022

HD1, shown in red, is the most distant galaxy astronomers have ever seen

Harikane et al.

A galaxy called HD1 may be the most distant object astronomers have ever discovered. Its amazing brightness is difficult to explain and may be due to a giant black hole at its center or the formation of extremely massive primordial stars, both of which confound our understanding of the early Universe.

Fabio Pacucci of the Harvard-Smithsonian Center for Astrophysics in Massachusetts and his colleagues found HD1 by searching large public datasets from several of the most powerful telescopes available. They then observed it again using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.

These 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 discovered, a galaxy called GN-z11. Such a distance is possible despite the age of the universe being only about 13.8 billion years due to the accelerated expansion of the cosmos.

The galaxy is exceptionally bright in the ultraviolet wavelength range, meaning anything producing its light is likely extremely hot. There are two ways to make it shine so brightly: either it’s undergoing a burst of star formation much larger than we would expect given the galaxy’s relatively small size, or it’s hosting an active supermassive black hole.

If the answer is a starburst, HD1 should produce about 110 times the Sun’s mass in stars each year. “That’s very big, that’s a crazy number,” says Pacucci. “One explanation is that this galaxy may not be forming normal stars, but rather these primordial stars, which are much more massive and much hotter than normal nearby stars.” We have never seen such primordial stars.

The other explanation is that HD1 could host an unexpectedly colossal supermassive black hole. “Observing a black hole with a mass of 100 million times the mass of the Sun so early in the history of the Universe would be really groundbreaking because we really wouldn’t be sure how it could have formed,” says Pacucci. Black holes take time to grow, and HD1 is so far away that we see it as only 330 million years after the Big Bang, so it’s unclear how a black hole could grow so large so quickly.

We need more observations to determine HD1’s extreme distance and why it’s so bright, Pacucci says. “At this point, we’re really stretching the capacity of our current observatories.” The researchers were allotted observing time at the James Webb Space Telescope to observe HD1, as well as two other objects called HD2 and HD3, which appear to be almost as distant.

Magazine references: The Astrophysical JournalDOI:10.3847/1538-4357/ac53a9; MNRASin print

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