Did black holes make early galaxies blaze really brightly?

• Some early galaxies appeared bigger and brighter when observed by the Webb space telescope than cosmological theory predicted.
• Because the standard cosmological model of the early universe didn’t match this new data, scientists wondered where the error was.
• A new study says hot accretion disks around black holes in those early galaxies can make them brighter. That, therefore, can explain why they appear more massive than they are.

The Webb Space Telescope published this news release August 26, 2024. Edits by EarthSky.

Early galaxies too massive?

When astronomers got their first glimpses from the Webb space telescope of galaxies in the early universe, they were expecting to find galactic pipsqueaks. But instead some galaxies appeared to have grown so massive, so quickly, that simulations couldn’t account for them. Some researchers suggested something might be wrong with the theory that explains our universe as a whole. The theory is known as the standard model of cosmology.

Now a new study – published in the Astronomical Journal and led by University of Texas at Austin graduate student Katherine Chworowsky – suggests some early galaxies are much less massive than they first appeared. It could be that black holes in the early universe make these galaxies look big and bright. Chworowsky said:

We are still seeing more galaxies than predicted, although none of them are so massive that they ‘break’ the universe [that is, make the standard model of cosmology untenable or unlikely].

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 study co-author.

Black holes add to brightness

According to this latest 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. Hence, they appear more massive than we would otherwise estimate. When scientists remove these galaxies, dubbed “little red dots” (based on their red color and small size), from the analysis, the remaining early galaxies are not too massive to fit within predictions of the standard model. Finkelstein said:

So, the bottom line is there is no crisis in terms of the standard model of cosmology. 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.

Early galaxies are efficient star factories

Although they’ve settled the main dilemma, a less thorny problem remains: There are still roughly twice as many massive galaxies in Webb’s 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. Chworowsky said:

Maybe in the early universe, galaxies were better at turning gas into stars.

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.

More evidence of black holes

Concurrently, astronomers have been analyzing the spectra of “little red dots” discovered with Webb. Researchers in both the CEERS team and others have found 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. And that reinforces Chworowsky and team’s conclusion that they are probably not as massive as astronomers initially thought. However, further observations of these intriguing objects are incoming. And that 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, it leads to new questions. While Chworowsky and colleagues have shown the standard model of cosmology likely isn’t broken, their work points to the need for new ideas in star formation. Chworowsky said:

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.

Bottom line: Some early galaxies appear too big and bright to fit current cosmological theories. According to new analysis of observations with the Webb space telescope, the galaxies’ brightness may come from the active black holes at their centers.

Source: Evidence for a Shallow Evolution in the Volume Densities of Massive Galaxies at z = 4–8 from CEERS

Via Webb Space Telescope