In a paper published today in the Astronomical Journal, astronomers analyzed the evolution of massive galaxies at redshifts of 4-8 selected from the JWST Cosmic Evolution Early Release Survey (CEERS).
Composite-color image of the extremely red, quasar-like object A2744-QSO1. Image credit: Furtak et al., doi: 10.1038/s41586-024-07184-8.
“We are still seeing more galaxies than predicted, although none of them are so massive that they ‘break’ the Universe,” said University of Texas at Austin graduate student Katherine Chworowsky.
According to the new study, the galaxies that appeared overly massive likely host black holes rapidly consuming gas.
Friction in the fast-moving gas emits heat and light, making these galaxies much brighter than they would be if that light emanated just from stars.
This extra light can make it appear that the galaxies contain many more stars, and hence are more massive, than we would otherwise estimate.
When scientists remove these galaxies, dubbed little red dots, from the analysis, the remaining early galaxies are not too massive to fit within predictions of the Standard Model.
“So, the bottom line is there is no crisis in terms of the Standard Model of cosmology,” said Steven Finkelstein’s Professor Steven Finkelstein.
“Any time you have a theory that has stood the test of time for so long, you have to have overwhelming evidence to really throw it out. And that’s simply not the case.”
Although they’ve settled the main dilemma, a less thorny problem remains: there are still roughly twice as many massive galaxies in the Webb data of the early Universe than expected from the Standard Model.
One possible reason might be that stars formed more quickly in the early universe than they do today.
“Maybe in the early Universe, galaxies were better at turning gas into stars,” Chworowsky said.
Star formation happens when hot gas cools enough to succumb to gravity and condense into one or more stars.
But as the gas contracts, it heats up, generating outward pressure.
In our region of the Universe, the balance of these opposing forces tends to make the star formation process very slow.
But perhaps, according to some theories, because the early Universe was denser than today, it was harder to blow gas out during star formation, allowing the process to go faster.
Concurrently, astronomers have been analyzing the spectra of little red dots discovered with Webb, with researchers in both the CEERS team and others finding evidence of fast-moving hydrogen gas, a signature of black hole accretion disks.
This supports the idea that at least some of the light coming from these compact, red objects comes from gas swirling around black holes, rather than stars — reinforcing Chworowsky and colleagues’ conclusion that they are probably not as massive as astronomers initially thought.
However, further observations of these intriguing objects are incoming, and should help solve the puzzle about how much light comes from stars versus gas around black holes.
Often in science, when you answer one question, that leads to new questions.
While the authors have shown that the Standard Model of cosmology likely isn’t broken, their work points to the need for new ideas in star formation.
“And so there is still that sense of intrigue. Not everything is fully understood. That’s what makes doing this kind of science fun, because it’d be a terribly boring field if one paper figured everything out, or there were no more questions to answer,” Chworowsky said.
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Katherine Chworowsky et al. 2024. Evidence for a Shallow Evolution in the Volume Densities of Massive Galaxies at z = 4-8 from CEERS. AJ 168, 113; doi: 10.3847/1538-3881/ad57c1
