Cosmic Evolution: Early Galaxies—'Not So Massive After All' | Webb Telescope
This image shows a small portion of the field observed by NASA’s James Webb Space Telescope’s NIRCam (Near-Infrared Camera) for the Cosmic Evolution Early Release Science (CEERS) survey. It is filled with galaxies. The light from some of them has traveled for over 13 billion years to reach the telescope. Hundreds of small galaxies against the black background of space. Several white spiral galaxies are near image center. Most of the galaxies are various shades of orange and red, and many are too tiny to discern a shape. A handful of foreground stars show Webb's six diffraction spikes.
There were galaxies that appeared to have grown so massive, so quickly, that simulations could not account for them. However, a new study finds that a great percentage of those early galaxies are in fact much less massive than they first appeared. Black holes in those galaxies make them appear much brighter and bigger than they really were.
It was called a crisis in cosmology. However, now astronomers can explain a number of surprising recent discoveries. Not long after the James Webb Space Telescope began science operations in July 2022, headlines proclaimed that observations of distant galaxies were “breaking theories of cosmic evolution.” Astronomers had found galaxies that appeared much brighter than expected. If all of that light came from stars, then those galaxies would have formed so many stars, so quickly, that the leading theory for the universe’s formation and evolution could not explain them.
New research finds that several of those early galaxies are in fact much less massive than they first appeared. Much of their light came, not from stars, but from a hot accretion disk surrounding a supermassive black hole.
When astronomers gained their first glimpses of galaxies in the early universe from NASA’s James Webb Space Telescope, they were expecting to find galactic pipsqueaks, but instead they found what appeared to be a bevy of Olympic 'bodybuilders'. Galaxies appeared to have grown so massive, so quickly, that simulations could not account for them. Researchers suggested this meant that something might be wrong with the theory that explains what the universe is made of and how it has evolved since the Big Bang, known as the standard model of cosmology.
According to a new study in the Astronomical Journal led by University of Texas at Austin graduate student Katherine Chworowsky, a significant portion of those early galaxies are in fact much less massive than they first appeared. Black holes in these galaxies make them appear much brighter and larger than they really were.
"Evidence for a Shallow Evolution in the Volume Densities of Massive Galaxies at z = 4–8 from CEERS"
“We are still seeing more galaxies than predicted, although none of them are so massive that they ‘break’ the universe,” Chworowsky said.
The evidence was provided by Webb’s Cosmic Evolution Early Release Science (CEERS) Survey, 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, and hence are 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.
“So, the bottom line is there is no crisis in terms of the standard model of cosmology,” Finkelstein said. “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.”
Efficient Star Factories
Although they have 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.
“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.
More Evidence of Black Holes
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 part of the light coming from these compact, red objects comes from gas swirling around black holes, rather than stars—reinforcing Chworowsky and their team’s 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, this leads to new questions. While Chworowsky and their colleagues have shown that the standard model of cosmology likely is not broken, their work points to the need for new ideas in star formation.
“And so there is still that sense of intrigue,” Chworowsky said. “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.”
The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, the European Space Agency (ESA) and the Canadian Space Agency (CSA).
Image Credit: NASA, ESA, CSA, Steve Finkelstein (UT Austin)
Release Date: Aug. 26, 2024
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