Images of early galaxies taken by the James Webb Space Telescope reveal unexpected brightnesses that call into question our understanding of cosmology. Northwestern University simulations show that the brightness of these galaxies is due to sporadic star formation rather than their massive size, which is consistent with current cosmological models.
Intense flash, not mass, solves impossible mystery of brightness. Scientists were shocked when they saw the first images of the universe's earliest galaxies taken by the James Webb Space Telescope (JWST). These young galaxies appear to be too bright, too large, and too mature to have formed so soon after the Big Bang. It's like a baby growing into an adult in just a few years.
The startling discovery even caused some physicists to question the Standard Model of Cosmology, wondering whether it should be overturned.
Galaxy brightness and mass
A team of astrophysicists led by Northwestern University used new simulation methods to find that these galaxies may not be that massive after all. While a galaxy's brightness is typically determined by its mass, new findings suggest that less massive galaxies can shine just as brightly from irregular, brilliant bursts of star formation.
The discovery not only explains why young galaxies appear to have deceptive masses, but also fits the standard model of cosmology.
The research was published in the Astrophysical Journal Letters on October 3.
Artist's conception of an early starburst galaxy. This image, drawn from FIRE simulation data used for this study, could explain recent JWST findings. Stars and galaxies appear as bright white points of light, while more diffuse dark matter and gas appear as purple and red. Source: Aaron M. Geller, Northwestern University, CIERA+IT-RCDS
Claude-André Faucher-Giguère of Northwestern University, senior author of the study, said: "The discovery of these galaxies was a huge surprise because they are much brighter than expected. Normally, a galaxy is bright because it is large. But because these galaxies were formed at the dawn of the universe, not long enough after the Big Bang. How could these huge galaxies come together so quickly? Our simulations show that there is no problem for galaxies to form at this brightness at the dawn of the universe."
Sun Guochao, who led the study, added: "The key is to reproduce a sufficient amount of light in a system in a short time. This occurs either because the system is very massive or because it has the ability to produce large amounts of light quickly. In the latter case, the system does not need to be that massive. If the star formation occurs in a burst, it will emit a flash. This is why we see several very bright galaxies."
Faucher-Giguère is an associate professor of physics and astronomy in Northwestern's Weinberg College of Arts and Sciences and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Sun is a postdoctoral fellow at CIERA at Northwestern University.
Understanding the dawn of the universe
The cosmic dawn is a period of time approximately 100 million to 1 billion years after the Big Bang, marked by the formation of the first stars and galaxies in the universe. Before JWST went into space, astronomers knew little about this ancient period.
"JWST has taught us a lot about the dawn of the universe. Before JWST, most of what we knew about the early universe was extrapolated from data from a very small number of sources. With dramatically improved observational capabilities, we can see the physical details of galaxies and use this hard observational evidence to study physics to understand what was going on."
In the new study, Sun, Faucher-Giguère and their team used advanced computer simulations to model how galaxies formed after the Big Bang. The simulated Cosmic Dawn galaxies are as bright as those observed by JWST. These simulations are part of the Feedback in Relativistic Environments (FIRE) project, which Faucher-Giguère co-founded with collaborators at Caltech, Princeton University, and the University of California, San Diego. Collaborators on the new study include researchers from the Flatiron Institute's Center for Computational Astrophysics, MIT and the University of California, Davis.
FIRE simulations combine astrophysics theory and advanced algorithms to simulate galaxy formation. These models allow researchers to explore how galaxies form, grow and change shape, taking into account the energy, mass, momentum and chemical elements returned by stars.
When Sun, Faucher-Giguère and their team ran simulations of early galaxies that formed at the dawn of the universe, they found that stars were formed in bursts -- a concept known as "explosive star formation." In massive galaxies like the Milky Way, stars form at a steady rate, with the number of stars gradually increasing over time. But what's called burst star formation occurs when stars form in an alternating pattern -- many stars form all at once, then very few new stars over millions of years, and then many more stars.
"Explosive star formation is particularly common in low-mass galaxies," Faucher-Giguère said. "The details of why this happens are still the subject of ongoing research. But what we think happens is that there is a burst of star formation, and then a few million years later, those stars explode as supernovae. The gas is kicked out and then falls back to form new stars, driving the cycle of star formation. But when galaxies are massive enough, their gravity is stronger. When supernovae explode, their gravity is not strong enough to throw gas out of the galaxy. Gravity holds the galaxy together, bringing it into a stable state."
Bright galaxies and universe models
The simulation was also able to produce the same number of bright galaxies as revealed by JWST. In other words, the number of bright galaxies predicted by the simulations matches the number of bright galaxies observed.
Although other astrophysicists have hypothesized that explosive star formation could be responsible for the unusual brightness of galaxies at the dawn of the universe, Northwestern University researchers are the first to use detailed computer simulations to show that this is possible. And they were able to do this without adding new factors that are inconsistent with the Standard Model of our universe.
"Most of the light in a galaxy comes from the most massive stars," said Faucher-Giguère. "Because more massive stars burn faster, they live shorter lives. They quickly use up their fuel in nuclear reactions. Therefore, the brightness of a galaxy is more directly related to the number of stars it has formed over the past few million years, rather than to the mass of the entire galaxy."