If on March 30 NASA announced with great fanfare the discovery by the 'Hubble' telescope of the most distant star, called Eärendel in homage to a JRR Tolkien poem and located 12.9 billion light years from Earth, now another astronomical object has been located even further away, at 13.5 billion light-years. With a less poetic name – they have called it HD1 – what seems to be a galaxy or a supermassive black hole would have formed only 300 million years after the Big Bang.
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The finding has been achieved by an international team of astronomers, including researchers from the Harvard-Smithsonian Center for Astrophysics. The achievement appears collected this Thursday in the Astrophysical Journal. In another accompanying article, published in the Monthly Notices of the Royal Astronomical Society Letters, scientists are speculating about the nature of this astronomical object.
The team, as reported by the Harvard-Smithsonian Center for Astrophysics in a Press release, proposes two ideas: HD1 may have been forming stars at an astonishing rate, and it may even be home to so-called Population III stars, the first stars in the universe that, until now, have never been observed. Another possibility is that HD1 contained a supermassive black hole with a mass 100 million times that of our Sun.
"Answering questions about the nature of such a distant source can be a challenge," says Fabio Pacucci, co-author of the article on the discovery and astronomer at the Center for Astrophysics, in the press release. “It's like guessing the nationality of a ship from the flag it flies, being far away on land, with the ship in the middle of a gale and thick fog. One can see perhaps some colors and shapes of the flag, but not in its entirety. In short, it is a long game of analysis and exclusion of implausible scenarios”.
The fact is that HD1 emits extremely bright ultraviolet light, something that Pacucci explains by "certain energetic processes that occurred a few billion years ago." At first, the researchers assumed that HD1 was a standard galaxy from which new stars were emerging at a high rate; but after calculating how many stars HD1 was producing, they realized that they were emerging at an "incredible" rate: more than 100 stars a year.
HD1 would represent a giant baby in the delivery room of the early universe
Avi Loeb — Astronomer at the Harvard-Smithsonian Center for Astrophysics
That's about 10 times faster, at least, than what you'd expect from a standard galaxy. That's when the team began to suspect that HD1 might be forming stars that weren't just run-of-the-mill stars.
"The first population of stars to form in the universe was more massive, more luminous and hotter than modern stars," says Pacucci. “If we assume that the stars produced in HD1 are these early, or Population III stars, then their properties could be more easily explained. In fact, Population III stars are capable of producing more ultraviolet light than normal stars, which could brighten HD1's extreme ultraviolet luminosity."
A supermassive black hole
However, the experts note in the press release, a supermassive black hole could also explain HD1's extreme luminosity. By engulfing huge amounts of gas, the region surrounding the black hole could emit high-energy photons.
If so, it would be by far the earliest supermassive black hole known to mankind, observed much closer in time to the Big Bang compared to the current record holder.
"HD1 would represent a giant baby in the delivery room of the early universe," says Avi Loeb, an astronomer at the Center for Astrophysics. HD1 was discovered after more than 1,200 hours of observation with the Subaru Telescope, the VISTA Telescope, the UK Infrared Telescope and the Spitzer Space Telescope.
It was a very hard job to find HD1 among more than 700,000 objects
Yuichi Harikane — Astronomer at the University of Tokyo
"It was a very hard job to find HD1 among more than 700,000 objects," says Yuichi Harikane, an astronomer at the University of Tokyo who discovered the galaxy. "The red color of HD1 matched the expected characteristics of a galaxy 13.5 billion light-years away surprisingly well, which gave me goosebumps when I found it."
The team then made follow-up observations from Chile with the Alma (Atacama Millimeter Radio Telescope Array) radio telescope to confirm the distance, which is 100 million light-years farther than GN-z11, the current holder of the farthest galaxy record
A mission for the 'James Webb'
With the 'James Webb' space telescope, the team of researchers will soon observe HD1 again to verify its distance from Earth. If current calculations are correct, HD1 will be the farthest – and oldest – galaxy ever recorded. The same observations will allow the team to delve deeper into the identity of HD1 and confirm whether one of their theories is correct.