Galactic giants of the early cosmos


Six early galaxies imaged by the James Webb Space Telescope. © NASA/ESA/CSA, I Labbe (Swinburne University of Technology); Image processing: G. Brammer (Niels Bohr Institute’s Cosmic Dawn Center at the University of Copenhagen).

It is commonly believed that the first galaxies in the cosmos were rather slender, as large galaxies like the Milky Way took longer to grow. But now, with the help of the James Webb telescope, astronomers have discovered six galaxies that were almost as massive as our Milky Way today, 500 to 700 million years after the Big Bang. If spectroscopic analyzes confirm that these reddish glowing objects are actually galaxies, this would cause problems for common cosmological models. Because such an early existence of such massive and star-rich galaxies is not planned in them.

With its first images of the distant cosmos, the James Webb Space Telescope fulfilled all expectations. Because one of its tasks is to use its high-resolution infrared optics to look back at the time of the first galaxies. According to current assumptions, the first stars in the universe could have formed around 200 to 300 million years after the Big Bang. The first galaxies then gradually developed from these star clusters. However, when exactly galaxy formation began and how quickly the early galaxies grew is unclear. However, the images taken by the Webb telescope have already shown that the first galaxies could have existed as early as around 350 million years after the Big Bang. However, these previous record holders were still quite small and low-mass, they probably only included around one billion solar masses of stars, as astronomers have determined.

Six early massive galaxies

However, a team of astronomers led by Ivo Labbé from Swinbourne University of Technology in Melbourne has now discovered six galaxies that are similar in age but significantly larger. For their study, the astronomers evaluated images from the near-infrared camera NIRCam from the James Webb Telescope's "Cosmic Evolution Early Release Science" program. They show a part of the sky near the Big Dipper, which the Hubble Space Telescope has also frequently surveyed. During their analyses, the researchers discovered a few reddish points of light whose characteristics indicated a great distance and therefore great age. Using the redshift, they determined that this light was on its way to us for more than 13 billion years and must have come from objects that existed 500 to 700 million years after the Big Bang. This suggested that these were early galaxies.

"We expected to see only tiny baby galaxies at this early time," explains co-author Joel Leja of Pennsylvania State University. “Instead, we found galaxies that existed at the dawn of the cosmos, but were as advanced as our own. These objects were far more massive than anyone expected.” According to the brightness data, all six early galaxies could contain more than 10 billion solar masses of stars, and one of them could even have a stellar mass of more than 100 billion solar masses. "My first thought was that we had made a mistake and would surely find it soon," says Leja. "But so far we haven't been able to find a fault despite all our efforts."

Contradiction to current models

This means that these six early objects could be by far the most massive galaxies from this early era ever discovered - if they really are galaxies. “The data do indicate that these are galaxies. However, there is also the possibility that at least some of these objects could be dust-shrouded black holes,” explains Leja. "But even then, the masses we have detected for these objects mean that the universe at that time could have contained up to a hundred times more stellar mass than previously thought. Even if we halve our result, it's still an amazing amount.” According to current cosmological models, there wasn't enough normal matter so shortly after the Big Bang to form so many stars in such a short time. "Even if only one of these galaxies is real, it pushes our understanding of cosmology to the limit," explains co-author Erica Nelson of the University of Colorado at Boulder.

In order to explain the existence of such massive galaxies at this early point in time, either the matter density in the young universe would have to have been two to five times greater than that stated by current models, the team writes. Or the galaxies must have grown in a different way than previously assumed. In both cases, however, this requires a fundamental change in our previous understanding of cosmic development, explains Leja. "We found something here that is so unexpected that it poses a real problem for science," says the researcher. He and his colleagues now hope to also be able to target these six objects with the James Webb Space Telescope's high-resolution near-infrared spectrometer (NIRSpec) in the near future. Because the exact light spectra of these objects could clarify whether they are actually galaxies and what elements they contain. "A spectrum will immediately tell us whether these objects are real," says Leja.

Source: Ivo Labbé (Swinbourne University of Technology, Melbourne) et al.; nature, doi: 10.1038/s41586-023-05786-2

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