# Webb finds hidden merger scars in galaxies that suddenly stopped making stars

> A study in Monthly Notices used deep Webb observations to reveal faint structural scars in galaxies that had recently stopped forming stars. The work points to violent galactic encounters as a likely trigger for the sudden shutdown of star birth in some...

Canonical URL: https://www.argo.net/webb-finds-hidden-merger-scars-in-galaxies-that-suddenly-stopped-making-stars/
Byline: University of Nottingham
Published: 2026-07-13T15:00:50+00:00
Categories: News, Space

![Distant galaxies and star clusters in deep space](https://www.argo.net/wp-content/uploads/2026/07/distant_galaxies_deep_space.jpg)

A study in [Monthly Notices](https://academic.oup.com/mnras/article/550/1/stag987/8722177) used deep Webb observations to reveal faint structural scars in galaxies that had recently stopped forming stars. The work points to violent galactic encounters as a likely trigger for the sudden shutdown of star birth in some of the early Universe's most massive systems.

By examining about 120 recently quenched galaxies across eight infrared wavelengths, researchers led by **David T. Maltby** at the **University of Nottingham** found a quiet-looking population with a turbulent past. Their smooth light hides subtle distortions that earlier observations could miss.

The galaxies appear as they were billions of years ago, during a time when the cosmos was building stars at an extraordinary rate. Webb's sharp infrared view gives astronomers a new way to test why some of the biggest galaxies stopped growing so abruptly.

## Webb sees beneath the calm glow

The galaxies in the study belong to a class called **post-starburst galaxies**. These systems recently experienced intense star formation, then rapidly shut down. That makes them valuable targets for astronomers who want to catch a galaxy soon after its star-making engine goes quiet.

At first glance, many of these galaxies look settled. Their light fades outward from bright centers in a smooth pattern. That appearance suggests a mature and stable structure, especially when seen in shallower images.

Webb changed the view by looking deeper. Using **James Webb Space Telescope** data from the **PRIMER programme**, the team studied each galaxy across multiple wavelengths. This allowed them to compare shapes in the rest-frame optical and near-infrared light.

The researchers also compared the post-starburst sample with roughly 3,000 passive and star-forming galaxies. That larger reference group helped show which features were unusual and which simply matched normal galaxy structure at similar cosmic times.

## Why giant galaxies went quiet

Galaxies grow when cold gas collapses into new stars. During the early Universe, massive galaxies had rich gas supplies and formed stars rapidly. Then a subset of them stopped in a short span of cosmic time.

Astronomers call this shutdown **quenching**. The term describes the end of major star formation, though the cause can vary from galaxy to galaxy. Gas can be used up, heated, blown away, or disrupted by powerful internal and external events.

The mystery is especially sharp for the largest early galaxies. Many were active during **cosmic noon**, a period about 9 to 11 billion years ago when star formation across the Universe reached its peak. Their rapid halt shaped the population of massive galaxies that astronomers see today.

Post-starburst galaxies are useful because they preserve a short-lived record of that transition. Their stars show that a burst happened recently, while their low star formation shows that the burst ended quickly. Their shapes can reveal what happened just before the shutdown.

## Faint scars point to violent collisions

The Nottingham-led team looked for signs of disturbance in the galaxies' structure. Some tests measured how lumpy or asymmetric the visible light appeared. On those simple measures, many post-starburst galaxies looked much like older passive galaxies.

A more sensitive test told a different story. The researchers subtracted a smooth model of each galaxy, then studied the faint leftover light. In the most massive early systems, that residual light showed stronger asymmetry.

Those faint leftovers are the **residual disturbances**. They act like scars from a recent upheaval. A galaxy can look calm overall while still carrying subtle evidence of a violent event in its outskirts or inner structure.

That pattern fits a merger scenario. When two gas-rich galaxies collide, gas can rush inward, trigger a burst of star formation and help build a dense central remnant. Feedback from young stars or an active black hole can then help clear or heat the gas.

The study treats this as evidence for a rapid and disruptive route to quenching. The finding is strongest in massive galaxies at earlier epochs, which supports the idea that **gas-rich mergers** helped shut down star formation in some of the Universe's young giants.

## Compact "red nuggets" carry the strongest clues

The most massive quenched galaxies in the Webb sample are unusually compact. They pack enormous stellar mass into relatively small regions. Astronomers often describe dense early quiescent systems like these as **red nuggets**.

Their compactness matters because major mergers can drive gas toward the center of a galaxy. That inflow can create a dense burst of star formation near the core. After the burst fades, the galaxy may be left with a small and heavy stellar body.

Webb's multiwavelength coverage helped test whether the compact shapes were real. Dust and young stars can distort a galaxy's apparent structure. By measuring the galaxies across eight infrared bands, the researchers found that the main structural picture stayed consistent.

This consistency strengthens the case that the compactness reflects true galaxy structure. The massive post-starburst galaxies appear dense, rounded and recently disturbed. Together, those traits match a formation path involving rapid collapse or a disruptive merger event.

The finding also gives astronomers a clearer target for future work. If red nuggets preserve merger scars for only a limited time, deep Webb imaging can help catch them before those signals fade into the smooth glow of old stars.

## Smaller galaxies took a gentler path

The study also found a split between massive early systems and smaller galaxies at later cosmic times. Lower-mass post-starburst galaxies showed less evidence of hidden disturbance. Many remained more disc-like in shape.

That difference suggests that quenching follows more than one route. In smaller galaxies, star formation may fade after slower environmental or internal changes. Their structures can remain orderly because the shutdown involves less violent rearrangement.

This result helps explain why galaxies with the same recent star-forming history can look different. A post-starburst signature tells astronomers that star formation stopped quickly. The galaxy's shape then adds clues about how that shutdown happened.

For the smaller systems, the absence of strong merger scars points toward milder processes. Gas supplies can decline, feedback can regulate star formation, or surroundings can gradually strip or heat gas. The Webb study shows that structure can separate these possible pathways.

## What this means for today's giant ellipticals

Today's largest elliptical galaxies are old, massive and mostly quiet. Many are thought to have grown from dense early systems that formed their stars quickly, then shut down. The Webb results connect those modern giants to dramatic events in the young Universe.

If early massive post-starburst galaxies became the seeds of **giant elliptical galaxies**, their hidden scars matter. They suggest that some of the calmest-looking galaxies may have passed through a short and violent phase. That phase may have built dense stellar cores and ended major star formation.

The work also shows why Webb is so powerful for galaxy evolution studies. Its infrared vision reaches light stretched by cosmic expansion. Its sensitivity reveals faint structure that can sit below the smooth main glow of a distant galaxy.

Future observations can push this test closer to the moment when quenching occurred. Larger samples could show how often mergers trigger shutdowns and how often gentler processes dominate. Spectroscopy could also help connect shape, star age, gas content and black hole activity.

For now, the message from Webb is striking. Some galaxies that look peaceful in the early Universe carry the marks of a recent collision. Those marks help explain how the cosmos built massive galaxies, then brought their star formation to an abrupt end.