If a few days ago the discovery of the farthest star (Eärendel, whose light departed 12.9 billion years ago) was announced, now an international team of astronomers reports the farthest astronomical object observed so far: a possible galaxy named HD1 and that it is about 13.5 billion light-years away .
Details of the finding are described this week in the Astrophysical Journal and in an article published in Monthly Notices of the Royal Astronomical Society Letters , where scientists put forward the first hypotheses about what kind of galaxy it may be.
The team of astronomers, including researchers from the Harvard Center for Astrophysics and the Smithsonian (USA), propose two ideas: that HD1 is forming stars at an astonishing rate and that it even contains some of Population III – the first in the universe − ever observed, or that contains a supermassive black hole with a mass one hundred million times greater than that of our Sun.
“Answering questions about the nature of such a distant source can be challenging,” says Fabio Pacucci , lead author of the study and an astronomer at the Center for Astrophysics.
“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. It is a long game of analysis and exclusion of implausible scenarios”, points out the expert.
For now, it is known that HD1 is extremely bright in ultraviolet light , which means that “some energetic processes are happening there or, better yet, they happened a few billion years ago,” says Pacucci.
HD1 produces more than 100 stars per year
At first, the researchers assumed that HD1 was a standard galaxy that is creating stars at a high rate, but after doing the calculations, they found that it is producing stars at a rate of more than 100 stars per year, a figure “at least 10 times higher than what we expect for these galaxies”, comments the astronomer.
That’s when the team began to suspect that HD1 might not be forming run-of-the-mill stars. “The first population of stars that formed in the universe was more massive, more luminous and hotter than modern ones,” 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,” he adds.
However, another possible explanation for the extreme luminosity of HD1 could well be the existence of a supermassive black hole , since by engulfing huge amounts of gas, the region around it could emit high-energy photons .
If so, it would be by far the earliest supermassive black hole known to mankind, much closer to the Big Bang than the one that currently holds that record.
As Avi Loeb , an astronomer at the Center for Astrophysics and co-author of the study, explains , “HD1 would represent a giant baby in the delivery room of the early universe.”
But in addition, “by forming a few hundred million years after the Big Bang, a black hole in HD1 must have grown from a massive seed at an unprecedented rate. Once again, nature seems to be more imaginative than we are,” explains Loeb.
Observed with several telescopes
HD1 was discovered after more than 1,200 hours of observing with the Subaru , VISTA , UK Infrared Telescope, and Spitzer Space Telescope .
“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, author of the discovery. “HD1’s red color 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 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.
Now, using the James Webb Space Telescope, the team will re-observe HD1 to check its distance from Earth and if current calculations are correct, HD1 will be the farthest and oldest galaxy ever found. These observations will allow us to find out more about this astronomical object and confirm which of its theories is correct.
Harikane et al. (2022) “A Search for H-Dropout Lyman Break Galaxies at z ∼ 12 – 16”. The Astrophysical Journal (in press)
Pacucci et al. (2022) “Are the Newly-Discovered z∼13 Drop-out Sources Starburst Galaxies or Quasars?” Monthly Notices of the Royal Astronomical Society Letters