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A team of researchers from the Harvard & Smithsonian Center for Astrophysics has just identified the farthest galaxy ever observed. Baptized as HD1, is found in the early Universe and already existed just 330 million years after the Big Bang. Its presence at such an early stage in the history of the Universe is difficult to explain, since according to current theories a galaxy needs much more than that time to form and grow. The work, directed by the astrophysicist Fabio Pacucci and in which the well-known astronomer Avi Loeb has also participated, will soon be published in ‘The Astrophysical Journal’, but can already be consulted on the arXiv pre-publication server.
The light from HD1 had to travel for 13.5 billion years before reaching us and being captured by telescopes.
According to the researchers, it lies 33.4 billion light-years away, a billion light-years farther than the previous farthest galaxy, called GN-z11. Although the Universe is only 13.76 billion years old, that distance is possible because the cosmos has been expanding ever faster since its birth.
The mere existence of HD1, then, represents a real challenge for science. Therefore, its discoverers are not entirely sure what exactly this galaxy is. HD1 is extraordinarily bright at ultraviolet wavelengths, which means that whatever is producing its light is likely to be extremely hot. There are two possible ways to make a galaxy so bright: either it is undergoing a much larger ‘burst’ of star formation than we would expect for the galaxy’s relatively small size, or it is home to a supermassive black hole. active.
never seen so far
If it were the first case, HD1 would have to be producing the equivalent of about 110 stars with the mass of the Sun each year, ten times more than expected for the early Universe. According to Fabio Pacucci, “this is very big, it’s a crazy number. One possible explanation is that this galaxy might not be forming normal stars, but rather primordial stars that are much more massive and much hotter than nearby normal stars.” In any case, something never seen before.
The other explanation, however, would be even more disturbing. In fact, if HD1 were to host an unexpectedly large supermassive black hole, there would be no way to explain it with current knowledge. “Observing a black hole of 100 million or more solar masses so early in the history of the Universe would be really innovative,” says Pacucci, “because we really wouldn’t be sure how it could have formed.” Black holes, in fact, need time to grow, and HD1 is only 330 million years from the Big Bang, a time that is considered totally insufficient for a black hole to become so large.
In any case, explains Pacucci, “answering questions about the nature of such a distant source is quite a challenge. It’s like trying to guess the nationality of a ship by the flag it flies, being far away on land, with the ship in the middle of a gale and dense fog. You may be able to see some colors and shapes of the flag, but not all of it. Ultimately, it’s a long game of analysis and exclusion of implausible scenarios.”
black hole
Avi Loeb, for his part, explains that if HD1 is fed by a supermassive black hole, “it must have grown from a massive seed and at an unprecedented rate. Once again, nature seems to be more imaginative than we are.”
HD1 was discovered with the ALMA (Atacama Large Millimeter/submillimeter Array) telescope in Chile, as part of a larger study to search for galaxies in the early Universe. During it, another team of researchers from the University of Tokyo have also reported another galaxy, HD2, almost as distant as HD1. This work, to be published in the Monthly Notices of the Royal Astronomical Society, is also available on arXiv.
Now, Pacucci stresses the need for more observations. The team of researchers has already got time to use the James Webb Space Telescope, which will be able to see the first stars and galaxies in the Universe much more clearly. Papucci and his colleagues will use this time to further study HD1, and will also look at HD2 and HD3, two other galaxies that are almost at the same distance.
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