They detect the most distant galaxy discovered so far

Named HD1, the galaxy is about 13.5 billion light-years away.

An international team of astronomers has discovered a galaxy that is the farthest astronomical object ever detected, researchers report in the Astrophysical Journal. Named HD1, the galaxy is about 13.5 billion light-years away.

In an accompanying article published in the Monthly Notices of the Royal Astronomical Society Letters, scientists have begun to speculate on what precisely the galaxy is.

The team proposes two ideas: HD1 may be forming stars at an astonishing rate, and it may even host Population III stars, the first stars in the universe, which, until now, have never been observed. Another possibility is that HD1 contains 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 challenging," says Fabio Pacucci, lead author of the MNRAS study, a co-author, and an astronomer at the Harvard Center for Astrophysics and the Smithsonian in the United States.

"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 dense fog," he exemplifies. "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".

HD1 is exceptionally bright in ultraviolet light. To explain this, "some energetic processes are going on there or, better yet, they happened a few billion years ago," he says.

At first, the researchers assumed that HD1 was a typical starburst galaxy, a galaxy that is creating stars at a high rate. But after calculating how many stars HD1 was producing, they got "an incredible rate: HD1 would be forming more than 100 stars every year. This is at least ten times what we expect for these galaxies." That's when the team began to suspect that HD1 might not be forming 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," explains Pacucci. "If we assume that the stars produced in HD1 are these first, 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 clarify HD1's extreme ultraviolet luminosity."

However, a supermassive black hole could also explain HD1's extreme luminosity. The black hole region could emit high-energy photons by engulfing vast amounts of gas. If so, it would be by far the earliest supermassive black hole known to humankind, 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 and a co-author of the MNRAS study. "It breaks the largest recorded quasar redshift by nearly a factor of two, one remarkable feat."

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," says Yuichi Harikane, an astronomer who discovered the galaxy at the University of Tokyo. "The red color of HD1 matched surprisingly well the expected characteristics of a galaxy located 13.5 billion light years, which gave me goosebumps when I found it".

The team then made follow-up observations with the Atacama Large Millimeter/submillimeter Array (ALMA) to confirm the distance, which is 100 million light-years farther than GN-z11, the current record holder for the farthest galaxy.

Using the James Webb Space Telescope, the team of researchers will soon re-observe HD1 to verify its distance from Earth. HD1 will be the farthest - and oldest - galaxy ever recorded if current calculations are correct.

The same observations will allow the team to delve into the identity of HD1 and confirm whether one of their theories is correct. "Forming a few hundred million years after the Big Bang, HD1's black hole must have grown from a massive seed unprecedentedly," says Loeb. "Once again, nature appears to be more imaginative than we are."