- Some early galaxies appeared larger and brighter in observations by the Webb Space Telescope than predicted by cosmological theory.
- Because the cosmological standard model of the early universe did not agree with this new data, the scientists wondered where the error lay.
- A new study says hot accretion disks around black holes in these early galaxies can make them brighter, which could explain why they appear more massive than they are.
The Webb Space Telescope issued this press release on August 26, 2024. Subject to modification by EarthSky.
Were early galaxies too massive?
When astronomers first caught a glimpse of galaxies in the early universe using the Webb Space Telescope, they expected to find tiny galaxies. But instead, some galaxies seemed to have grown so massive so quickly that simulations couldn’t explain them. Some researchers suspected that there might be something wrong with the theory that explains our universe as a whole, known as the Standard Model of Cosmology.
A new study – published in Astronomical Journal and led by Katherine Chworowsky, a doctoral student at the University of Texas at Austin, suggests that some early galaxies are much less massive than they first appeared. It could be that black holes in the early universe formed these galaxies see big and bright. Chworowsky said:
We still observe more galaxies than predicted, although none of them are massive enough to “destroy” the universe (that is, make the standard model of cosmology untenable or improbable).
Webb’s Cosmic Evolution Early Release Science (CEERS) survey provided the evidence. The survey is led by Steven Finkelstein, a professor of astronomy at UT Austin and co-author of the study.
Black holes increase brightness
According to this latest study, the galaxies that appeared overly massive likely contain black holes that are rapidly consuming gas. Friction in the fast-flowing gas emits heat and light, making these galaxies much brighter than they would be if that light came only from stars. This extra light can make it seem like the galaxies contain many more stars, so they appear more massive than we would otherwise appreciate. When scientists remove these galaxies, called “little red dots” because of their red color and small size, from the analysis, the remaining early galaxies are not too massive to fit the Standard Model’s predictions. Finkelstein said:
So the bottom line is that there is no crisis with the Standard Model of cosmology. Any time you have a theory that has held up for so long, you have to have overwhelming evidence to really throw it out. And that’s just not the case.
Early galaxies are efficient star factories
Although they have solved the main dilemma, a less thorny problem remains: In Webb’s data of the early universe, there are still about twice as many massive galaxies as the Standard Model would predict. One possible reason for this could be that stars formed faster in the early universe than they do today. Chworowsky said:
Perhaps galaxies in the early universe were better at converting gas into stars.
Stars form when hot gas cools enough to succumb to gravity and condense into one or more stars. But as 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. But perhaps some theories suggest that the higher density of the early universe than today made it harder to blow gas out during star formation, making the process faster.
Further evidence of black holes
At the same time, astronomers have been analyzing the spectra of the “little red dots” discovered with Webb. Researchers from both the CEERS team and others have found evidence of fast-moving hydrogen gas, a sign 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 stars. And that bolsters Chworowsky and his team’s conclusion that they’re probably not as massive as astronomers first thought. But more observations of these intriguing objects are ahead. And that should help solve the mystery of how much light comes from stars versus gas around black holes.
In science, answering one question often leads to new questions. While Chworowsky and his colleagues have shown that the standard model of cosmology probably works, their work suggests that new ideas about star formation are needed. Chworowsky said:
And so the fascination remains. Not everything is fully understood. That’s what makes this kind of science so exciting, because it would be a terribly boring field if you figured everything out in one paper or there were no more questions to answer.
Conclusion: Some early galaxies appear too large and bright to fit into current cosmological theories. According to a new analysis of observations made with the Webb Space Telescope, the galaxies’ brightness may come from the active black holes at their centers.
Source: Evidence for a flat evolution of the volume density of massive galaxies at z = 4–8 from CEERS
About the Webb Space Telescope