Astronomers expose CO-dark molecular gas, previously invisible to telescopes

An international team of astronomers has created the first-ever large-scale maps of a mysterious form of matter, known as the dark molecular gas CO, in one of the most active star-forming districts of our Milky Way, Cygnus X. Their findings, made using the Green Bank Telescope (GBT), provide crucial new clues about how stars formed in the Milky Way.

For decades, scientists have known that most new stars are born inside clouds of cold molecular hydrogen. Much of this molecular hydrogen is invisible to most telescopes: it does not emit easily detectable light.

Traditionally, astronomers searched for these clouds by looking for carbon monoxide (CO), a molecule that acts as a flashing signal to star-forming regions. However, it turns out that there is a lot of star-forming gas that doesn’t “light up” in CO. This dark, hidden material (called dark molecular CO gas) has been one of the biggest blind spots in astronomy.

Now, for the first time, astronomers have mapped this hidden gas across a huge swath of sky – more than 100 times the area covered by the full moon – by observing the radio spectral lines of recombining atoms, known as carbon recombination radio lines (CRRL). The team’s map covers the bustling region of Cygnus

“It’s like suddenly turning on the lights in a room and seeing all kinds of structures we didn’t know existed,” says Kimberly Emig, an associate scientist at the National Radio Astronomy Observatory (NRAO) and lead author of the new study published in The Astrophysics Journal.

The new map reveals a vast network of arcs, ridges and webs of dark gas crisscrossing Cygnus X. These shapes show where stellar matter is gathered and grown, before it becomes visible as CO in molecular clouds. Research demonstrates that these faint carbon signals, detected at very low radio frequencies, provide an incredibly powerful tool for discovering the hidden gas that directly links ordinary matter to the formation of new stars.

The researchers discovered that this dark gas isn’t just still; it flows and shifts, and moves at much higher speeds than previously thought. These turbulent flows can determine how quickly stars can form. The team also discovered that the brightness of these carbon lines is directly linked to the intense light from stars bathing the region, highlighting the powerful role radiation plays in galactic recycling.

“By making the invisible visible, we can finally track how the raw material in our galaxy is transformed from simple atoms into complex molecular structures that will one day become stars, planets and perhaps life,” explains Emig, “And this is only the beginning of understanding these never-before-seen forces.”

The GBT has become the world’s premier tool for this type of research, and even larger CRRL studies (such as the GBT survey of diffuse ionized gas at low frequencies) are underway to explore other star-forming regions of the Milky Way. The information gleaned here will help astronomers around the world model how our galaxy – and potentially others – builds massive clouds in which stars can form.