It’s a new record that takes us far into space and time. Harvard researchers (USA) announced on April 7, 2022 in the journal Astrophysics Journal the discovery of the most distant galaxy ever observed. “HD1” is 13.5 billion light years away from us. That’s 100 million more than “GNz11”, the current champion. And it is especially close to the beginning of the Universe, as it was only about 300 million years old when the image of this galaxy began to travel towards us … It will have made 1,200 hours of observation through four instruments, the Subaru. Telescope (Hawaii), VISTA (Atacama Desert, Chile), Infrared Telescope of the United Kingdom (Hawaii) and finally the Spitzer Infrared Space Telescope, retired since the end of 2020. The measurements were made in the infrared because from the expansion of the Universe, all distant objects are moving away from us. And the farther away they are, the faster they move away. This escape velocity is at the origin of a Doppler effect that shifts its light to the long wavelengths, corresponding to the infrared.
The mythical first stars of the Universe
However, if we were on the side of HD1, therefore without Doppler effect, it would shine brightly in the ultraviolet. And that intensity intrigues researchers. To explain this, the team is proposing two equally exciting hypotheses … According to the first, the inevitably very young galaxy HD1 would form new stars at an incredible rate: about 100 stars per year. In comparison, the Milky Way makes less than 10 a year. Better: they could be the mythical so-called stars of population III, the first to illuminate the universe, and have never been observed. “The first generation of stars to form in the universe were more massive, brighter, and hotter than today’s stars.explains on the site of Harvard University Fabio Pacucci, the lead author of the article. If we assume that those produced in HD1 are these early stars, or population III, their properties could be easily explained. Because they are capable of producing more ultraviolet light than the following, hence the extreme brightness of HD1. “ A hypothesis clarified by Hakim Atek, of the Paris Institute of Astrophysics and a member of the “Origin and Evolution of Galaxies” team, “this young galaxy can simply form more massive, and therefore brighter, stars without being in population III. HD1 is very massive for such a young galaxy. It may have hosted several generations of stars. Those in population III have a very short lifespan, a few million years.“
A black hole of one hundred million solar masses
The second hypothesis considers the presence of a supermassive hole of about 100 million solar masses in the heart of the galaxy. Ultraviolet radiation would be emitted by large amounts of gas accelerated at the speed of light before disappearing into the black hole. On the one hand, the presence of such a star would not be so surprising: most galaxies have one in their center, starting with the Milky Way, which hosts a black hole of 4 million solar masses. On the other hand, how to explain that so much matter has been added in such a short time ?! The mechanism remains to be invented …
James Webb lurking
The sequel now belongs to the James Webb Space Telescope, which will soon be up and running. “At the moment “HD1” is more a candidate for the title of “farthest galaxy”, than a proven observation. The measures taken by Fabio Pacucci are promising, but they will have to be confirmed by the JWST. “ points out Hakim Atek. Therefore, “HD1” will be added to the James Webb menu, along with GNz11. But many other distant galaxies should join this list. “The instrument will allow us to go back even further, at least 100 million light-years away from current performance. And above all, it will free us from a bias because the measurements of the HD1 were made from the ground. Only the brightest galaxies, and therefore the most massive galaxies, are accessible. However, they are still the exception: the Universe in its beginnings was populated mainly by galaxies of very low masses, less luminous. They are much better candidates for detecting the light of very early stars. “concluded Hakim Atek.