Stephan’s Quintet has long fascinated astronomers. Discovered in the nineteenth century, this compact gathering of galaxies lies roughly 280 million light-years away in the constellation Pegasus, and four of its members are locked in a slow-motion gravitational dance. Repeated close encounters have shredded tidal tails, driven shock waves through intergalactic gas, and created one of the best local laboratories for studying the kind of galaxy interactions that were far more common in the early universe. Because the group is relatively close by cosmic standards, telescopes can resolve individual star-forming regions within it, making it an ideal target for testing theories about how galaxies convert gas into stars.

The connection between molecular gas and star formation is one of the foundational relationships in modern astrophysics. Stars are born when clouds of molecular hydrogen, traced by carbon monoxide at the cold temperatures where hydrogen itself cannot be observed, become dense enough for gravity to overwhelm internal pressure and collapse. In the distant past, when galaxies collided more frequently, these encounters compressed molecular gas, triggering bursts of star formation that astronomers still observe in ancient galaxies. Yet observations have revealed a curious contradiction: some regions packed with abundant molecular gas produce surprisingly few new stars. Understanding why a galactic collision can ignite star formation in one place while suppressing it in another has intrigued scientists for decades.

To tackle the problem, the Osaka Metropolitan University team turned to the Atacama Compact Array, a network of radio telescopes perched on the high, dry Chajnantor Plateau in Chile. The instrument is designed to excel at mapping extended structures of molecular gas, and the researchers used it to observe emission from carbon monoxide molecules at a wavelength that traces the bulk of the molecular gas throughout Stephan’s Quintet. The resulting map is the first of its kind: a comprehensive, high-fidelity census of where the group’s molecular gas resides and how it moves, covering the interacting galaxies and the turbulent regions between them.

The map itself is striking. Contour lines of carbon monoxide emission, arranged much like the elevation lines on a topographic map, reveal reservoirs of molecular gas distributed across the group, with higher contour levels marking regions richer in gas. But the true breakthrough came when the researchers compared the amount and motion of gas in each region with the efficiency with which that region was forming stars. The pattern that emerged was clear and consistent: regions where the molecular gas was moving violently — where internal motions were broad, chaotic, and turbulent — formed stars far less efficiently than expected, even when those regions contained plentiful gas.

Interactions between galaxies, the team concluded, can both compress and disperse molecular gas, creating dramatic differences in star formation activity across a single galaxy group. In some places, gravitational encounters squeeze gas clouds together, raising their density and igniting star formation. In others, the same encounters inject enormous amounts of kinetic energy, stirring the gas into turbulent motion. The findings pointed to turbulence as an important factor in regulating where stars can form, a conclusion that helps reconcile the long-standing paradox of gas-rich regions that stubbornly refuse to produce stars.

The physical mechanism the researchers propose is elegant in its simplicity. For a star to form, a parcel of gas must settle, become concentrated, and eventually collapse under its own gravity. Turbulence sabotages each of these steps. If the gas is highly turbulent, its chaotic motions spread the gas out, preventing individual parts of a cloud from settling into dense, quiescent clumps. Instead of fragmenting into the dense cores that seed stars, the cloud remains diffuse and agitated, its material constantly churned and redistributed. The result is a region brimming with star-forming fuel but starved of the calm conditions needed to use it — fewer opportunities to form stars, despite an abundance of raw material.

This turbulence-regulated picture has implications that reach far beyond Stephan’s Quintet. Star formation is one of the most fundamental processes in galaxy evolution, setting how galaxies grow, how their stellar populations age, and how their light changes over billions of years. Galaxy collisions and interactions were rampant in the early universe, when galaxies were smaller, denser, and more frequently merging. If turbulence injected by those interactions could suppress star formation in gas-rich environments, then the history of cosmic star formation is not simply a story of gas supply — it is also a story of gas dynamics. As Associate Professor Kazuyuki Muraoka of the research team noted, understanding how galaxy collisions and interactions in the early universe enhance or suppress star formation will give researchers a better tool to trace the history of galaxy evolution across cosmic time.

Studies like this one help refine astronomers’ picture of the universe and, as Muraoka reflected, encourage us to reflect on our own place within it. The Milky Way has absorbed smaller galaxies throughout its history and will merge with the Andromeda Galaxy in the distant future, and the same physics mapped in Stephan’s Quintet will govern how that encounter unfolds. By connecting the fine-scale motions of molecular gas to the grand narrative of galaxy evolution, the Osaka Metropolitan University team has shown that the fate of entire stellar populations can hinge on something as seemingly intangible as the stirring of gas — a reminder that in the cosmos, chaos and creation are two sides of the same coin.

Subject of Research: Molecular gas turbulence and suppressed star formation in the interacting galaxies of Stephan's Quintet

Article Title: Turbulent times for star formation in Stephan’s Quintet

Article References: Turbulent times for star formation in Stephan’s Quintet. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Stephan's Quintet, star formation, molecular gas, turbulence, galaxy interactions, carbon monoxide, Atacama Compact Array, radio astronomy, galaxy evolution, Osaka Metropolitan University, The Astrophysical Journal, interacting galaxies