Study sheds light on mystery of giant galaxies in the early universe

When the James Webb Space Telescope began sending back its first images of the universe’s infancy two years ago, there were some surprises. Instead of tiny galaxies that had not yet had time to grow, there were clusters of gigantic stars that seemed to challenge the most widely accepted cosmological models. Earlier this year, the most luminous object in the universe was discovered 12 billion light-years away: a quasar so bright that for decades it had been mistaken for a nearby star. Instead of a sun, it was a disk of gas and dust seven light-years across orbiting a black hole with the mass of 17 billion suns.

The universe’s predilection for forming very massive objects in areas with high galaxy density was then suggested as a possible explanation. In the current universe, after billions of years of cosmic expansion, everything is further away and there is no longer enough mass to generate gigantic objects so quickly. The existence of “heavy seeds” has also been suggested, a type of object that could explain how such massive black holes formed so quickly.

However, a new study, published Monday in The Astrophysical Journal, suggests an alternative explanation. The brightness of some of these galaxies is not explained by their size or the number of stars they contain, but by the effect of the very active black holes that live inside them. These objects consume huge amounts of the gas in their vicinity and the friction of this rapidly moving material causes an extraordinary luminosity, making them appear more massive than they really are.

When the scientists eliminated these particularly bright galaxies, the mass estimates for the others fit within the predictions of the standard model of cosmology. “The bottom line is that there is no crisis in terms of the standard model of cosmology,” said Steve Finkelstein, a professor at the University of Texas at Austin and co-author of the paper. “When you have a theory that has stood the test of time like this, you have to have overwhelming evidence to rule it out, and that is not the case.”

However, the James Webb data still show more galaxies than would exist at a similar rate of formation to that seen in our region of the cosmos, and there is still no definitive interpretation of the nature of the small red dots that appear in the images from the space telescope. Teams such as the one that wrote the article in The Astrophysical Journal have observed in the spectra of these dots signs of what appears to be hydrogen moving at high speed, something typical of the accretion disks that form around black holes. If this were the case, this gas falling into the hole would explain, at least in part, the luminosity of these dots, which would not need to host so many stars. Calculating how much light comes from the moving gas and how much from the stars will help us understand what the cosmos was really like a few hundred million years after the Big Bang.

Although the new images have not caused the standard model of cosmology to collapse, it may need some fine-tuning. “We don’t understand everything very well, and that makes science fun, because it would be very boring if we could explain everything with one scientific paper and there were no more questions to answer,” said Katherine Chworowsky, the study’s first author.

In the coming years, images from the James Webb promise to offer material for rethinking the evolutionary history of the universe, including mysteries such as dark energy, the force that makes up more than 70% of the universe. The telescope’s ability to photograph distant supernovae, a type of star used to measure distances in the cosmos, will help us better estimate how quickly the universe has expanded throughout history, and its comparison of ancient and modern galaxies will also help us reconstruct how this immense, omnipresent and invisible energy pushed the universe to its present state.

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