In an article published today in Astronomical JournalAstronomers analyzed the evolution of massive galaxies at redshifts 4-8, selected from the JWST Cosmic Evolution Early Release Survey (CEERS).
Color composite image of the extremely red quasar-like object A2744-QSO1. Image credit: Furtak et al., doi: 10.1038/s41586-024-07184-8.
“We still see more galaxies than predicted, although none of them are massive enough to ‘destroy’ the universe,” said Katherine Chworowsky, a doctoral student at the University of Texas at Austin.
According to the new study, the galaxies that appeared overly massive likely host black holes that rapidly consume gas.
Friction in the fast-moving gas releases heat and light, making these galaxies much brighter than if the light came only from stars.
This extra light can make it appear that the galaxies contain many more stars and are therefore more massive than we would otherwise estimate.
When scientists remove these galaxies, called small red dots, from the analysis, the remaining early galaxies are not too massive to fit the Standard Model’s predictions.
“The bottom line is that there is no crisis with regard to the standard model of cosmology,” said Professor Steven Finkelstein of St. Petersburg University.
“When you have a theory that has held up for so long, you have to have overwhelming evidence to really dismiss it. And that’s just not the case.”
This solves the main dilemma, but a less thorny problem remains: in the Webb data of the early universe, there are still about twice as many massive galaxies as expected by the Standard Model.
One possible reason for this could be that stars formed faster in the early universe than they do today.
“Perhaps galaxies in the early universe were better at converting gas into stars,” Chworowsky said.
Star formation occurs when hot gas cools to the point where it gives in to gravity and condenses into one or more stars.
But when the gas contracts, it heats up and creates outward pressure.
In our region of the universe, the balance of these opposing forces causes the star formation process to be very slow.
Some theories suggest that this may be because the early universe was denser than it is today, making it harder to blow out gas during star formation, causing the process to be faster.
At the same time, astronomers have been analyzing the spectra of small red dots discovered with Webb. Researchers from both the CEERS team and other teams found evidence of fast-moving hydrogen gas, a hallmark of black hole accretion disks.
This supports the idea that at least some of the light emitted by these compact red objects comes from gas swirling around black holes, rather than from stars. And it reinforces the conclusion of Chworowsky and his colleagues that they are probably not as massive as astronomers first thought.
However, further observations of these fascinating objects are still to come and should help solve the mystery of how much light comes from stars and how much from gas around black holes.
In science, answering one question often leads to new questions.
While the authors have shown that the standard model of cosmology probably works, their work suggests that new ideas about star formation are needed.
“And so this fascination remains. Not everything is fully understood. That is what makes this type of science so exciting, because it would be a terribly boring field if you could find out everything with a single paper or if there were no more questions to answer,” said Chworowsky.
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Katharina Chworowsky et al. 2024. Evidence for a flat evolution of the volume density of massive galaxies at z = 4-8 from CEERS. AJ 168, 113; doi: 10.3847/1538-3881/ad57c1
