Stephan's Quintet Is Full of Gas and Barely Making Stars. Osaka Astronomers Say Turbulence Is Why.

Stephan's Quintet has been one of astronomy's showpieces since it was catalogued in 1877, and it has always carried a puzzle. Four of its five galaxies are physically tangled together, crashing through one another's gas at enormous speed. Collisions like that are supposed to trigger bursts of star formation. This one has plenty of gas and not nearly enough new stars.

A team at Osaka Metropolitan University has now produced the first detailed map of molecular gas across the entire group, using the Atacama Compact Array in Chile — the short-baseline component of ALMA, built specifically to capture the diffuse, extended structures that the main array's higher-resolution antennas tend to miss. Molecular hydrogen is the fuel that becomes stars, but it does not radiate usefully at these temperatures, so astronomers trace it with carbon monoxide emission instead, and mapping it across a whole interacting group rather than a single galaxy is the part that had not been done.

What the map shows is a clean inverse relationship. The regions where the gas is moving most violently — where the line widths are broadest, meaning the gas within a single beam is churning at a wide spread of velocities — are the regions producing the fewest stars per unit of available gas. Abundant fuel, poor conversion.

"Interactions between galaxies can both compress and disperse molecular gas, creating dramatic differences in star formation activity," said Misaki Yamamoto of the Graduate School of Science, who led the work with associate professor Kazuyuki Muraoka. The team's reading is that turbulence generated by the ongoing collisions keeps the gas stirred, and stirred gas will not sit still long enough to collapse under its own gravity into a star.

That resolves the tension in the Quintet's appearance. A galaxy collision is not a single kind of event with a single outcome. It compresses gas in some places, which helps, and it injects kinetic energy into gas everywhere, which hurts, and the balance between the two decides whether a given patch lights up or stays dark. The Quintet appears to be a system where the stirring is winning across most of its volume.

The result was published in The Astrophysical Journal, DOI 10.3847/1538-4357/ae7b30.

The implication reaches past one group of galaxies. Interactions and mergers were far more common in the early universe than they are now, and models of how galaxies built their stellar mass generally treat a merger as a star formation trigger. If turbulence routinely suppresses star formation in the same systems that mergers are supposed to ignite, then the amount of gas a galaxy has is a weaker predictor of how many stars it makes than the field has assumed. The question becomes not how much fuel is present, but how quietly it is sitting.