An international team of scientists including University of Arizona astronomer Erika Hamden has discovered a molecular cloud that is one of the largest single structures in the sky, and among the closest molecular clouds to Earth ever detected. 

Previously invisible to telescopes, the discovery hints at the presence even more vast clouds in the near universe, visible only in ultraviolet wavelengths and more luminous and energetic than ever imagined.

Image

"Looking at the universe in the ultraviolet really changes how we see these molecular clouds. With infrared instrumentation, you kind of see through them. But in the UV, we can begin to make out the actual surface and structure," said Hamden, an associate astronomy professor at Steward Observatory and director of the U of A Space Institute.

The scientists have named the molecular hydrogen cloud "Eos," after the Greek goddess of mythology who is the personification of dawn. The team's discovery is outlined in a study published in Nature Astronomy.

Image

Molecular clouds are composed of gas and dust, with the most common molecule being hydrogen – the fundamental building block of stars and planets and essential for life. They also contain other molecules such as carbon monoxide. Molecular clouds are often detected using conventional methods, such as radio and infrared observations that easily pick up the chemical signature for carbon monoxide.

For this work, the scientists employed a different approach.

"This is the first-ever molecular cloud discovered by looking for far ultraviolet emission of molecular hydrogen directly," said lead study author Blakesley Burkhart, an associate professor in the Department of Physics and Astronomy in the Rutgers School of Arts and Sciences. "The data showed glowing hydrogen molecules detected via fluorescence in the far ultraviolet. This cloud is literally glowing in the dark."

 Because of its proximity, the gas cloud presents a unique opportunity to study the properties of a structure within the interstellar medium – the gas and dust that fills the space between stars and serves as raw material for new stars.

 The research team is excited about the discovery of Eos because it allows them to directly measure how molecular clouds form and dissociate, and how interstellar gas and dust turn into stars and planets.

 Located about 300 light-years from Earth, the crescent-shaped gas cloud sits on the edge of the Local Bubble, a large gas-filled cavity in space that encompasses the solar system. Scientists estimate that Eos' mass is about 3,400 times that of the sun. The team used models to show it is expected to evaporate in 6 million years.

Eos was revealed to the team in data collected by a far-ultraviolet spectrograph called FIMS-SPEAR (an acronym for fluorescent imaging spectrograph) that operated as an instrument on the Korean satellite STSAT-1. A far-ultraviolet spectrograph breaks down far-ultraviolet light emitted by a material into its component wavelengths, just as a prism does with visible light, creating a spectrum that scientists can analyze.

The findings highlight the importance of innovative observational techniques in advancing the understanding of the cosmos, Burkhart said. She noted that Eos is dominated by molecular hydrogen gas but is mostly "CO-dark," meaning it doesn't contain much of the material and doesn't emit the characteristic signature detected by conventional approaches. That explains how Eos eluded being identified for so long, researchers said.

"The story of the cosmos is a story of the rearrangement of atoms over billions of years," Burkhart said. "The hydrogen that is currently in the Eos cloud existed at the time of the Big Bang and eventually fell onto our galaxy and coalesced nearby the sun. So, it's been a long journey of 13.6 billion years for these hydrogen atoms."

While the FIMS-SPEAR spectrograph surprised the team in its ability to pick up traces of molecular hydrogen emissions, the technology to explore the universe in the UV spectrum has evolved in the 22 years since the STSAT-1 satellite launch.  One of the world's largest teams of UV instrumentalists is based at Steward Observatory and led by Hamden, and the team has its sights set on exploring molecular cloud formation with the latest technology.

Hamden is gearing up to submit a proposal for a space mission to NASA this summer. The mission, also be dubbed Eos, aims to broaden the approach of detecting molecular hydrogen to greater swaths of the Milky Way, investigating the origins of stars by studying the evolution of molecular clouds, and other science investigations.

If NASA funds the Eos mission, astronomers will be able to observe these clouds and star-forming regions in a way that is not possible now, said Hamden. 

"We'd be seeing much finer details with much higher resolution, getting a multidimensional view of these really important structures," she said.

"There's so much unexplored space in the ultraviolet," she added. "I wonder how many more things like the Eos cloud are just waiting for us to look at them in the right way."