Discovery of HD1, the most distant galaxy on Earth

It is a new record that takes us far in space and time. Researchers from Harvard (USA) announced on April 7, 2022 in the journal Astrophysical 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 defending champion. And it is especially very 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 taken 1,200 hours of observation through four instruments, the Subaru Telescope (Hawaii), VISTA (Atacama Desert, Chile), United Kingdom Infrared Telescope (Hawaii) and finally Spitzer Infrared Space Telescope, retired since late 2020. The measurements took place in the infrared due to the expansion of the universe, all distant objects moving away from us. And the longer they are, the faster they move away. This velocity of flight is the starting point for a Doppler effect, which moves their light towards the long wavelengths, corresponding to the infrared.

The mythical first stars of the universe

But if we were next to HD1, therefore without Doppler effect, it would shine brightly in the ultraviolet. And this intensity fascinates researchers. To explain it, the team is considering two equally intriguing hypotheses … According to the first, the inevitably very young galaxy HD1 would form new stars at an incredible speed: about 100 stars a year. By comparison, the Milky Way earns less than 10 a year. Better: it could be the mythical so-called population III stars, the first to illuminate the universe and which have never been observed. “The very first generation of stars formed in the universe were more massive, brighter, and warmer than current starsexplains on the Harvard University website Fabio Pacucci, the lead author of the article. If we assume that those produced in HD1 are these early stars or Population III, then its properties could be easily explained. Because they are capable of producing more ultraviolet than the following, hence the extreme brightness of HD1. “ A hypothesis nuanced by Hakim Atek, of the Paris Institute of Astrophysics and member of the team “Origin and evolution of galaxies”, “this young galaxy can simply form more massive stars, and therefore bright, without being of population III. HD1 is very massive for such a young galaxy. It may have housed several generations of stars. Those of 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 the enormous amounts of gas that are accelerated to 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 at their center, starting with the Milky Way, which houses a black hole of 4 million solar masses. On the other hand, how is one to explain that such an amount of substance is collected in such a short time ?! The mechanism has yet to be invented …

James Webb looking

The successor now belongs to the soon-to-be operational James Webb Space Telescope. “Currently, “HD1” is more of a candidate for the title “the most distant galaxy” than a proven observation. The measurements made by Fabio Pacucci are promising, but they must be confirmed by JWST “, notes Hakim Atek. “HD1” will therefore 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 even further back, at least 100 million light-years compared to current performance. And above all, it will free us from a bias due to the fact that the measurements of HD1 were made from the ground. Only the brightest, and therefore the most massive, galaxies are available. However, they remain the exception: in the beginning, the universe was mainly populated by galaxies with very low masses, less luminous. They are much better candidates for detecting light from very early stars. “ends Hakim Atek.

Leave a Comment