Lauren Fuge
Astrophysicists have just discovered a vast molecular cloud right under our noses. Only 300 light-years away, this crescent-shaped cloud of gas and dust is invisible to the naked eye but is enormous – if we could see it in the night sky, it would take up the space of 40 Moons. It’s one of the largest single structures in the sky.
Molecular clouds like these are composed of molecules such as hydrogen and carbon monoxide. They are typically detected using radio or infrared observations, which can pick up the chemical signature for carbon monoxide (CO). But this cloud is “CO-dark”. It doesn’t emit the characteristic signature that conventional detection techniques see, and it has long eluded scientists, despite being in our galactic neighbourhood.
Instead, the cloud is primarily composed of molecular hydrogen. This isn’t easy to directly observe – but in this case, that’s exactly what astrophysicists achieved, by detecting the light emitted by the hydrogen itself.
The discovery is reported in Nature Astronomy.
“This is the first-ever molecular cloud discovered by looking for far ultraviolet emission of molecular hydrogen directly,” says Blakesley Burkhart, astrophysicist at Rutgers University-New Brunswick and leader of the team. “The data showed glowing hydrogen molecules detected via fluorescence in the far ultraviolet. This cloud is literally glowing in the dark.”
Burkhart and team discovered the cloud in a dataset taken by a far-ultraviolet spectrograph on the Korean satellite STSAT-1. The data was released publicly in 2023, when the team searched it using this innovative technique.
“It’s kind of wild that we can see this cloud in data that we didn’t think we would see,” Burkhart says. “This opens up new possibilities for studying the molecular universe.”
Co-author Thavisha Dharmawardena, a NASA Hubble Fellow at New York University, agrees: “The use of the far ultraviolet fluorescence emission technique could rewrite our understanding of the interstellar medium, uncovering hidden clouds across the galaxy and even out to the furthest detectable limits of cosmic dawn.”
Fittingly, the team dubbed the cloud “Eos”, after the Greek goddess of dawn.
Because of its relative proximity to Earth, Eos could help us better understand the interstellar medium – the gas and dust in the space between stars that serves as the raw material for stars and planets to form.
“When we look through our telescopes, we catch whole solar systems in the act of forming, but we don’t know in detail how that happens,” Burkhart says. “Our discovery of Eos is exciting because we can now directly measure how molecular clouds are forming and dissociating, and how a galaxy begins to transform interstellar gas and dust into stars and planets.”
The team is now scouring existing data for other molecular hydrogen clouds that have eluded us until now. They recently posted another pre-print study to arXiv, tentatively reporting finding the most distant molecular gas yet in data from the James Webb Space Telescope (JWST).
“Using JWST, we may have found the very furthest hydrogen molecules from the sun,” Burkhart says. “So, we have found both some of the closest and farthest using far-ultraviolet emission.”