Discovery of HD1, the most distant galaxy from 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, since 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), the United Kingdom Infrared Telescope (Hawaii) and finally the Spitzer Infrared Space Telescope, retired since late 2020. The measurements took place in the infrared, because due to the expansion of the universe, all distant objects are moving away from us. And the longer they are, the faster they move away. This escape velocity is the starting point for a Doppler effect, which shifts their light towards the long wavelengths, corresponding to the infrared.

The mythical first stars in the universe

But if we were next to HD1, therefore without the Doppler effect, it would shine brightly in the ultraviolet. And this intensity fascinates researchers. To explain it, the team considers two equally exciting hypotheses… According to the first, the necessarily very young galaxy HD1 would form new stars at an incredible rate: about 100 stars per year. In comparison, the Milky Way earns less than 10 per year. Better: it could be the mythical so-called population III stars, the first to light up the universe and which have never been observed. “The very first generation of stars to form in the universe were more massive, brighter and hotter than current starsexplains on the website of Harvard University Fabio Pacucci, the article’s lead author. 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 able to produce 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 be 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 vast amounts of gas being 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 can one explain that such a quantity of matter has been collected in such a short time?! The mechanism remains to be invented…

James Webb on the lookout

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

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