Universe Today
The movement of star-forming gas in galaxies is a fundamental research question in astronomy. Galaxies are huge interacting systems where molecular hydrogen is subjected to multiple forces. It's stretched out into streams between galaxies, and compressed in shock fronts elsewhere. It's diffuse and spread out in some places, and densely packed in others. It's dark and cold, or lit up with radiation.
Unfortunately, molecular hydrogen is not easy to observe, even though it's by far the most abundant molecule in giant molecular clouds. Instead, astronomers map carbon monoxide (CO), which is only a trace species in star-forming clouds. Despite making up only a tiny fraction of the volume, CO is widespread in clouds, and responds more readily to energy changes, making it a reliable proxy for molecular hydrogen.
Astronomers refine their understanding of gas movement by observing it in more types of environments, and in more complex environments. Researchers in Japan recently used CO observations to map gas in an especially complex and unique environment: Stephan's Quintet.
The galaxy in the upper left (NGC 7320) is not one of the members of the compact group in Stephan's Quintet. This Hubble image from 2009 shows how three of the members of the compact group are distorted, with stretched and misshapen spiral arms, and long tidal tails of gas, all lit up by countless star clusters. Image Credit: NASA, ESA, and the Hubble SM4 ERO Team. Public Domain.
Stephan's Quintet (SQ) is a well-known visual grouping of five galaxies that has the distinction of being one of the JWST's first official images. Despite being named a quintet, and also appearing as one, only four of the member galaxies are actually in a group together. The fifth is a foreground galaxy that's about 7 times closer to Earth than the other galaxies. The quintet was the first compact galaxy group ever discovered, and is also the most well-studied compact group.
Japanese researchers used the Atacama Compact Array (ACA) to observe the gas in the four members of the compact group. The Atacama Compact Array is a subset of the Atacama Large Millimeter/submillimeter Array (ALMA). ACA uses a smaller set of ALMA's radio antennae to better observe objects like molecular clouds.
Their results are in a paper published in The Astrophysical Journal titled "Molecular Gas Structure and Star Formation Diversity in Stephan’s Quintet Revealed by ACA CO(1–0) Mapping". The lead author is Misaki Yamamoto from the Graduate School of Science at Osaka Metropolitan University.
Since Stephan's Quintet is the most well-studied compact group, many other research groups have mapped its CO. But this work, according to the authors, is much more detailed than many other efforts.
"We present 12CO(1–0) mapping across the entire system of Stephan’s Quintet, a well-known compact galaxy group, observed by Atacama Compact Array (7 m array + Total Power) of the Atacama Large Millimeter/submillimeter Array," the authors write. "These observations provide the first large-scale (137 kpc × 119 kpc), spatially resolved (∼5.5 kpc) molecular gas map of a compact group."
The map of SQ not only showed where most gas was concentrated, it also showed how turbulent different regions were. While star formation is generally more profuse where more gas is concentrated, this study highlights how turbulence affects star formation.
Most of the molecular gas is in the disk of NGC 7319 and in the regions between the galaxies. That includes the shocked filament and the tidal tail from NGC 7319.
The overall image is from the Digital Sky Survey 2, and the magenta box shows the region observed with the Atacama Compact Array. The CO mapping of the region shows that there's little gas inside the member galaxies. "A large amount of H i gas is distributed outside the galaxies, forming prominent tidal tails extending eastward from NGC 7319," the authors explain. Image Credit: Yamamoto et al. 2026. ApJ
The gas distribution in SQ indicates that it's an evolved galaxy group where neutral hydrogen has been stripped from the galaxies and is now concentrated outside of their disks. As a result, the galaxies are hydrogen deficient.
But the CO map also showed where turbulence is most active, and how that affects star formation. In this work, velocity dispersion in clumps of gas indicates turbulence. The wider the dispersion, the greater the turbulence.
"Along the tidal tail and its surroundings, we found not only an extended molecular gas component but also four discrete CO clumps, with velocity dispersions of ∼10–30 km s−1 and molecular gas masses of order 107–108 M⊙," the researchers explain.
The researchers found that star formation effieciency is negatively correlated with CO velocity dispersion.
"While regions with small velocity dispersion exhibit SFEs comparable to those of nearby disk galaxies, those with large velocity dispersion (∼50–150 km s−1) around the shocked filament show strongly suppressed star formation," the authors write.
This figure is an integrated intensity map of SQ divided into three velocity components. Image Credit: Yamamoto et al. 2026. ApJ
“Interactions between galaxies can both compress and disperse molecular gas, creating dramatic differences in star formation activity,” lead author Yamamoto said in a press release. “The findings pointed to turbulence as an important factor in regulating where stars can form.”
Interacting galaxies can generate turbulence. This study shows that, in that turbulent setting, star forming gas struggles to concentrate and settle down enough to form stars.
“Star formation is one of the most fundamental processes in galaxy evolution. Studies like ours help refine our picture of the universe and encourage us to reflect on our place within it,” said co-atuhor Kazuyuki Muraoka. “Understanding how galaxy collisions and interactions in the early universe enhance or suppress star formation will allow researchers a better tool to trace the history of galaxy evolution across cosmic time.”
"In conclusion, our results suggest that turbulence plays a significant role in regulating star formation in interacting systems," the authors write.
