Astronomers using Green Bank Telescope (GBT) have created large-scale maps of carbon monoxide (CO), the dark molecular gas of a star-forming complex. Swan X.
These images show the location of the dark molecular gas CO in Cygnus X. Image credit: NSF/AUI/NRAO NSF/P. Vostin.
For decades, scientists have known that most new stars are born inside clouds of cold molecular hydrogen.
Most of this molecular hydrogen is invisible to most telescopes—it doesn't emit light that can be easily detected.
Traditionally, astronomers have hunted these clouds by looking for carbon monoxide (CO), a molecule that acts as a blinking sign of star-forming regions.
However, it turns out that there is a lot of star-forming gas that does not “ignite” in CO.
This hidden material—the dark molecular gas CO—was one of astronomy's biggest blind spots.
In a new study, NRAO astronomer Kimberly Emigh and colleagues mapped this hidden gas across a huge swath of the sky by observing radio spectral lines of atomic recombination known as carbon radio recombination lines (CRRLs).
The team's map covers Cygnus X, a star-forming region about 5,000 light-years away in the constellation Cygnus.
“It's like suddenly turning on a light in a room and seeing all sorts of structures that we didn't even know existed,” Dr. Emigh said.
The new map reveals a vast network of arcs, ridges and networks of dark gas that permeate Cygnus X.
These shapes show where star-forming material is collected and grown before it becomes visible, just as before it becomes visible in CO as molecular clouds.
The authors demonstrated that these faint carbon signals, detected at very low radio frequencies, are an incredibly powerful tool for detecting hidden gas that directly links ordinary matter to the formation of new stars.
They discovered that this dark gas doesn't just sit there; it flows, shifts and moves at speeds much higher than previously thought. These turbulent flows can affect the rate of star formation.
They also found that the brightness of these carbon lines is directly related to the intense starlight bathing the region, highlighting the important role that radiation plays in galactic recycling.
“By making the invisible visible, we will finally be able to trace how the raw materials in our Galaxy are transformed from simple atoms into complex molecular structures that will one day become stars, planets and perhaps life,” Dr Emig said.
“And this is just the beginning of understanding these previously unseen forces.”
results were published on October 17 in the magazine Astrophysical Journal.
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Kimberly L. Emigh etc.. 2025. Cold dark gas in Cygnus X: the first large-scale mapping of low-frequency carbon recombination lines. APJ 992, 216; two: 10.3847/1538-4357/adfa17
