FIU study challenges leading explanation for why galaxies stop forming stars

MIAMI, July 21, 2026 /PRNewswire/ -- For decades, astronomers have cast galaxy mergers as the universe's ultimate relationship drama: two galaxies collide, chaos erupts, black holes awaken and star formation comes to a screeching halt. Upending this long-held belief, new research from Florida International University suggests most galaxies don't "die" in spectacular cosmic pileups after all.

Understanding why galaxies stop forming stars is one of the biggest questions in galaxy evolution because it helps scientists explain how galaxies change over billions of years. The researchers found galaxy mergers are neither necessary nor sufficient to stop galaxies from forming new stars — a process astronomers call quenching. Instead, the findings suggest slower internal processes play a much larger role than galaxy mergers in shutting down star formation, according to Camilo Casimiro, an FIU Physics graduate student and lead author of the study that was published recently in Monthly Notices of the Royal Astronomical Society.

"I've always been fascinated by galaxy evolution and the question of why some galaxies continue forming stars while others stop completely," Casimiro said.

For decades, astronomers have viewed galaxy mergers as one of the leading explanations for why otherwise active galaxies eventually stop forming new stars. The researchers set out to test a longstanding idea in astronomy known as the "merger-quasar-quench" paradigm. The theory proposes that when galaxies collide, the interaction funnels huge amounts of gas toward the galaxies' centers, fueling supermassive black holes. Those black holes then unleash enormous amounts of energy, heating or expelling the gas needed to form stars.

It's a dramatic theory — and one that has shaped astrophysics research for decades. Traditionally, astronomers have looked for super bright quasars, spectacular black hole outbursts and dramatic events inside a galaxy's center, according to Asa Bluck, assistant professor of Physics, Casimiro's advisor and coauthor of the study. However, he says what really matters is the total energy a black hole releases over billions of years — even if it's not flashy or particularly spectacular. 

The research team analyzed more than 11,000 simulated galaxies using IllustrisTNG, one of the world's most advanced cosmological simulations. The simulation allows scientists to track billions of years of cosmic history, following how galaxies grow, collide, feed their central black holes and eventually stop forming stars. If galaxy mergers truly drove quenching, researchers expected to see galaxies stopping star formation soon after those collisions. Instead, they found that rarely happened. Only about 3% of major galaxy mergers were followed by quenching within a billion years, according to the research.

Even when researchers included all mergers — large and small — only about 12% were associated with galaxies shutting down star formation. Perhaps even more surprising, most galaxies that stopped forming stars showed no evidence of a recent merger at all.

The research is a bold endeavor for Casimiro's first-ever peer-reviewed paper.

"Challenging a long-standing idea in galaxy evolution is something I never expected to be doing so early in my career," he said. 

"At the same time, I have a great deal of trust in my advisor and in the scientific process itself. One of the most important parts of science is being willing to test even widely accepted ideas against new data and better simulations."

The findings don't answer what ultimately causes galaxies to stop forming stars, but they suggest astronomers may need to look beyond dramatic collisions to explain one of galaxy evolution's biggest mysteries. 

Instead of asking what dramatic event ended a galaxy's life, Casimiro says scientists may need to focus on what quietly keeps it from reigniting. It's a shift in thinking that shows change, even on the largest scales imaginable, doesn't usually come from a single catastrophic event — but rather from long, gradual shifts that build over time. The findings suggest that one of astronomy's biggest questions may have a less dramatic answer than scientists have long believed.

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SOURCE Florida International University