---
title: Galaxy Mergers Do Not Drive Star Formation Shutdown, Cosmological Simulation Study Finds
description: FIU researchers used the IllustrisTNG simulation to show galaxy mergers are neither necessary nor sufficient for quenching, pointing to secular processes instead.
author: Darie Nani (Editor-in-Chief)
updated: 2026-07-23T19:29:55.633Z
canonical: https://www.sovereignmagazine.com/article/galaxy-mergers-do-not-drive-star-formation-shutdown-cosmological-simulation-study-finds
image: https://cdn.nanimediahouse.com/fiu-galaxy-quenching-study-19699.webp
categories: Science & Tech
content_type: News
region: United States
publication: Sovereign Magazine
schema_type: Article
---

Most galaxies that stopped forming stars showed no evidence of a recent collision with another galaxy, according to a study published in Monthly Notices of the Royal Astronomical Society. The finding, drawn from one of the largest systematic tests of the so-called merger-quasar-quench paradigm to date, challenges a framework that has shaped observational astronomy for decades.

The research, led by Camilo Casimiro, a Physics graduate student at Florida International University, used IllustrisTNG, a high-resolution cosmological simulation that models billions of years of galaxy evolution across a synthetic universe of more than 11,000 galaxies. The scale of the data set allowed the team to move past anecdotal cases and run a controlled statistical test: if mergers truly caused quenching, the signature should be consistent and measurable across the full population.

That expectation did not hold: only about 3% of major galaxy mergers were followed by star formation shutdown within one billion years. When all mergers, large and small, were included, that figure rose to roughly 12%. After accounting for random coincidence using a stellar-mass-matched control sample, no statistically meaningful merger excess was found in quenched galaxies at all.

## What Is the Merger-Quasar-Quench Paradigm

The paradigm that the study set out to test has been a cornerstone of galaxy-evolution theory since at least the 1990s. Its logic is mechanistically tidy: two galaxies collide, gravitational forces channel enormous quantities of gas toward their shared center, the central supermassive black holes consume that gas and emit intense radiation as quasars, and the energy released heats or ejects the surrounding gas that would otherwise have condensed into new stars, leaving the galaxy without the raw material it needs to sustain star formation and bringing that process to a halt.

The appeal of the model was partly aesthetic. Mergers are visible, dramatic events that leave unmistakable structural signatures. Quasars are among the most luminous objects in the universe. Linking the two to produce a single explanatory chain made observational sense, and for years the literature accumulated supporting cases.

What was harder to test was the counterfactual: whether mergers were genuinely necessary, or whether the observed correlation between merger signatures and quenched galaxies could survive rigorous statistical controls. Asa Bluck, assistant professor of Physics at FIU and Casimiro's co-author and advisor, has argued that astronomers have historically focused on the brightest, most spectacular black hole outbursts, when what actually matters for quenching may be the cumulative energy a black hole releases over billions of years, through slow and largely unremarkable accretion.

## What the IllustrisTNG Results Say About How Paradigms Get Revised

The study's broader significance is methodological as much as astronomical. Paradigms in observational science typically accumulate supporting cases faster than they accumulate controlled tests, because the controlled test requires a population large enough to separate signal from coincidence. IllustrisTNG provided that population. [The paper finds](https://arxiv.org/abs/2603.12651) that once a stellar-mass-matched control is applied, no merger excess is observed, and concludes that secular processes dominate the growth of supermassive black holes and the quenching of central galaxies in this simulation.

Secular processes, in this context, means the slow internal evolution of a galaxy driven by its own dynamics rather than external collisions: gradual black hole growth through ordinary accretion, internal gravitational instabilities, and the steady accumulation of feedback energy over long timescales. These mechanisms lack the narrative clarity of a merger, but they now appear to carry far more explanatory weight.

The study does not settle what ultimately causes quenching. It eliminates one candidate as the primary driver and redirects the question. As Casimiro puts it, the field may need to shift from asking what dramatic event ended a galaxy's life to asking what quietly keeps it from reigniting.

> "Challenging a long-standing idea in galaxy evolution is something I never expected to be doing so early in my career. 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."
> — Camilo Casimiro, Physics graduate student, Florida International University

## What Comes Next for Galaxy Quenching Research

The IllustrisTNG simulation, while sophisticated, is still a model. Its conclusions about secular dominance hold within the assumptions built into the simulation, including how it models black hole feedback and star formation thresholds. Whether the same statistical picture holds in observational data, across the actual universe rather than a simulated one, remains an open question.

What the study does establish is that the merger-quasar-quench paradigm cannot be assumed. Future observational programmes looking to understand why galaxies cease star formation will need to design tests that can distinguish merger-driven quenching from secular quenching in real data, a considerably harder task than identifying merger signatures in isolation. The methodology the FIU team applied, a mass-matched control population across a large sample, offers a template.

The study is available to read at [news.fiu.edu](https://news.fiu.edu).

## FAQ

**Q: Why do galaxies stop forming stars?**
Galaxy quenching, the cessation of star formation, is one of the central unsolved problems in galaxy evolution. Proposed mechanisms include feedback from supermassive black holes, which can heat or expel the gas needed to form stars, as well as ram-pressure stripping in galaxy clusters and slower internal processes sometimes called secular evolution. The new FIU study adds weight to secular processes as the dominant driver, at least within the IllustrisTNG simulation.

**Q: What is the IllustrisTNG simulation?**
IllustrisTNG is a suite of large-scale cosmological simulations that model the formation and evolution of galaxies across billions of years of cosmic history. It tracks the behavior of dark matter, gas, stars, and supermassive black holes in a synthetic universe, allowing researchers to run statistical tests on galaxy populations at scales impossible to achieve through direct observation alone. It is widely used in theoretical astrophysics as a benchmark environment for testing galaxy-evolution models.

**Q: What did the FIU galaxy quenching study find?**
Analyzing more than 11,000 simulated galaxies in IllustrisTNG, Casimiro and Bluck found that only about 3% of major galaxy mergers were followed by star formation quenching within one billion years, and that most quenched galaxies showed no evidence of a recent merger. After applying a stellar-mass-matched control sample to account for random coincidence, no statistically significant merger excess was found among quenched galaxies. The study concludes that mergers are neither necessary nor sufficient to cause quenching in the simulation, and that secular internal processes are the dominant mechanism.