When researchers glimpsed the first images and data from the James Webb Space Telescope (JWST), humanity’s largest and most powerful space telescope, they noticed something peculiar. A large number of bright galaxies deep in the universe formed during a period called “Cosmic Dawn,” when the first stars and galaxies formed within 500 million years after the Big Bang. 

The findings were unexpected. Many cosmological models of galaxy formation indicated that such luminous galaxies could not form so early in the universe. But new research published in The Astrophysical Journal Letters shows that a theoretical model produced roughly five years ago predicted these very observations.  

“When we analyze these galaxies in our simulation as the JWSTwould have seen them, they look quite similar to what JWST observed,” said theoretical astrophysicist Andrew Wetzel, an associate professor of physics and astronomy at UC Davis and a co-author on the paper. “Our predictions held true. This is about as good as it gets for theorists like myself: to make a prediction and then to see it realized.” 

The paper, which was led by researchers from Northwestern University, credits the large number of bright galaxies observed at Cosmic Dawn to bursty star formation, a natural occurrence in the universe that was an emergent property of the team’s cosmological model.   

Fluctuation of starlight

According to Wetzel, many previous models concerning galaxy formation during Cosmic Dawn characterized the brightness of these galaxies as steady through time. Wetzel compared the brightness from those models to a flashlight. When turned on, the beam is steady and constant. 

But that’s not how the first galaxies behaved in the early universe. Their brightness was much more variable over time.

“The key is that these faint galaxies are undergoing very rapid fluctuations in their brightness,” Wetzel said. “So for a small amount of time, those normally faint galaxies temporarily get very bright.” 

It just so happens that the JWST collected images and data at a time when these galaxies exhibited burstiness, according to Wetzel. During the Cosmic Dawn period, large amounts of stars formed at once, burning brightly before exploding as supernovae and dying off.

“One of the most important ingredients of our model is modeling how stars feedback on the galaxy as a whole,” Wetzel said. 

A FIRE model

Wetzel and his colleagues’ model uses cosmological simulations from the Feedback in Realistic Environments (FIRE) project, which aims to “develop and explore cosmological simulations of galaxy formation that directly resolve the interstellar medium of individual galaxies while capturing their cosmological environment,” according to the FIRE collaboration’s website. 

“We have to run these simulation models using national supercomputing facilities, given their high dynamic range and how physics-rich they are,” Wetzel said. “Almost all areas of astrophysics come together.”

In essence, the team inputs the basic laws of physics underlying galaxy formation, like gravity, fluid dynamics and radiation, among other physical processes. They then run the simulation on the project’s advanced supercomputers.  

The burstiness of the ancient galaxies observed by the JWST was an emergent property of the team’s FIRE simulation, bolstering the idea that bursty star formation naturally explains the abundance of bright galaxies at Cosmic Dawn.

“There was no specific parameter that says, ‘Here’s how bursty the galaxies are,’” Wetzel said. “This burstiness, with rapidly varying brightness of these early galaxies, arose naturally from our computational models.”

Currently, Wetzel and his students are working on a project to adapt their FIRE simulation to questions concerning how the Milky Way galaxy formed. The project harnesses current Milky Way data to reconstruct formation histories of individual stars. 

“We sometimes describe this field as galactic archaeology,” Wetzel said. “We’re taking the present-day record of the Milky Way and we’re using it to reverse engineer how it appeared throughout history, including during Cosmic Dawn.” 

The research will help us further understand the nascent days of the galaxy we call home.