CosmicAI Researcher Julian Muñoz, alongside CosmicAI postdoctoral scholar Jenna Samuel and CosmicAI alum Mahdi Qezlou explored how galaxies built their stellar mass in the first billion years of cosmic history, finding that they go through “bursts” of star formation with pauses of inactivity in between.
The team collaborated with Guochao Sun (Northwestern), Jordan Mirocha (Caltech/JPL), Alessandra Venditti (UT Austin), and Charlotte Simmonds (Universidad de Chile & Cambridge), John Chisholm (UT Austin), Emily Bregou (UT Austin) and Ryan Endsley (UT Austin).
What the team did
Galaxies build their stars over the entire age of the universe. Albeit long, this process is not smooth. Exploding stars and stellar winds blow gas out of galaxies, so that star formation stalls; eventually gas falls back, and the cycle repeats. Astronomers call this "burstiness," and the patterns of those bursts encodes the physics of how galaxies form.
The team built a novel computational framework that treats each galaxy's star formation like a flickering signal, described by just two numbers: how big the swings are, and how long each burst lasts. They then compared it against three kinds of Hubble and JWST measurements of galaxies seen in the first billion years of cosmic history: how many galaxies there are, how they cluster spatially, and how much hydrogen glow they emit compared to ultraviolet light. This ratio is key, since the two kinds of light are produced by stars of very different ages, tracing star formation over roughly 10 and 100 million years respectively, so it acts as a speedometer. Comparing the two tells you whether a galaxy is bursting right now or taking a nap. The model runs in under a second, which let the team search millions of possibilities against the data.
What researchers found
We found that early galaxies are really bursty. A typical galaxy swings by a factor of about 4 in its star-formation rate. Imagine changing how much you eat from one day to another by that much! The striking result is that burstiness depends on size: galaxies in the smallest dark-matter halos swing by a factor of 10 or more. Smaller galaxies have weaker gravity, so a single generation of exploding stars can shut them down entirely, then let them roar back.
We also measured how long each burst lasts: roughly 20 million years. That number is a fingerprint. It's too slow for stellar winds and too fast for galaxy mergers, but it matches almost exactly what's expected from supernovae, so this is strong evidence that exploding stars are the main engine regulating the first galaxies.
Why the work matters
One of JWST's biggest surprises was finding far more bright galaxies in the early universe than models predicted, prompting speculation about “breaking cosmology”. We offer a simpler explanation: because the earliest galaxies were small, they were also burstier. A flickering galaxy caught mid-flash looks much brighter than its average, so our model reproduces what JWST sees all the way back to 200 million years after the Big Bang, with no new ingredients required.
The paper was published in the Monthly Notices of the Royal Astronomical Society, Volume 547, Issue 4, April 2026, stag415.
arXiv