Eos is among the largest single sky structures discovered and is believed to be a potential cradle for future star formation. The cloud's presence had gone unnoticed because it is rich in molecular hydrogen but contains little carbon monoxide, the gas typically used in radio and infrared surveys to detect such structures. Scientists describe Eos as "CO-dark," making conventional detection techniques ineffective.
The team, led by astrophysicist Blakesley Burkhart of Rutgers University-New Brunswick and the Center for Computational Astrophysics at the Flatiron Institute, used data from the FIMS-SPEAR far-ultraviolet spectrograph aboard Korea's STSAT-1 satellite. The dataset had been publicly released in 2023.
"This is the first-ever molecular cloud discovered by looking for far ultraviolet emission of molecular hydrogen directly," said Burkhart. "The data showed glowing hydrogen molecules detected via fluorescence in the far ultraviolet. This cloud is literally glowing in the dark."
The interstellar medium, which comprises gas and dust between stars, provides the raw material for forming new stellar systems. Eos, situated on the edge of the Local Bubble that surrounds our solar system, offers a rare and nearby laboratory for studying how molecular clouds develop, dissolve, and contribute to galactic evolution.
Eos is estimated to have a mass about 3,400 times that of the Sun and is expected to evaporate over the next 6 million years. The study suggests this detection technique could revolutionize the search for hidden molecular clouds throughout the Milky Way and into the early universe.
"The use of the far ultraviolet fluorescence emission technique could rewrite our understanding of the interstellar medium," said Thavisha Dharmawardena, a NASA Hubble Fellow at New York University and co-lead author on the study. "We could uncover hidden clouds across the galaxy and even out to the furthest detectable limits of cosmic dawn."
The cloud was named both for the mythological Greek goddess of dawn and for a proposed NASA mission aimed at expanding this detection approach across the galaxy. Meanwhile, Burkhart's team continues to analyze data from multiple sources, including the James Webb Space Telescope, and has tentatively identified some of the most distant molecular gas yet seen.
"Using JWST, we may have found the very furthest hydrogen molecules from the sun," Burkhart said. "So, we have found both some of the closest and farthest using far-ultraviolet emission."
Research Report:A nearby dark molecular cloud in the Local Bubble revealed via H2 fluorescence
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