Nearby Gas Clouds Actually Form Behemoth Structure That Might Be The Milky Way’s Arm, Study Finds

Nearby Gas Clouds Actually Form Behemoth Structure That Might Be The Milky Way’s Arm, Study Finds
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Astronomers have discovered that many of the star-forming regions we see in the sky actually seem to form an undulating, 8,800-light-year-long wave containing 3 million solar masses’ worth of gas that could make up our local arm of the Milky Way galaxy.

Astronomers have long thought that a large, expanding ring of young stars, gas, and dust surrounded our solar system, forming a region called “Gould’s Belt.” But when a team of scientists in the United States, Austria, and the United Kingdom actually measured the distances to the objects supposedly comprising this belt, they found that the belt didn’t exist—and instead, these objects formed a far stranger structure. They’re calling it the Radcliffe Wave.

“We think we’ve found the largest coherent star-forming structure [yet observed], and it’s right under our noses,” Catherine Zucker, graduate student at the Harvard Smithsonian Centre for Astrophysics, told Gizmodo.

Back in April 2018, the European Space Agency released the second batch of data from the Gaia spacecraft, including the positions of 1.7 billion stars and the motions of 1.3 billion. That fall, University of Vienna astrophysicist João Alves began a fellowship at the Radcliffe Institute for Advanced Study at Harvard University. Alves hoped to use the trove of Gaia data in order to create a map of the young stars, gas, and dust in the local interstellar neighbourhood. He collaborated with Zucker and the other study authors, who had developed a simpler method to measure the distances to nearby dust clouds. The method combines the Gaia light plus the distances and colours of distant stars to see how their light has been scattered by intervening dust clouds—the very dust clouds they were interested in mapping.

As the scientists performed the analysis, they slowly realised that many of the famous nebulas and gas clouds in the sky, such as the Orion Nebula, didn’t form a ring as they expected. Instead, these dust clouds aligned, ultimately into a 8,800-light-year-long stripe with a wave in it as if it had been plucked like a string, according to the paper published today in Nature. Not entirely convinced, the researchers organised a red team meeting, calling on their colleagues to try to debunk the results, said Zucker. But the findings held up, no matter how they tried to poke holes in them.

Given the size and shape of the structure, the team had reason to believe that they’d just pieced together a part of the local arm of our spiral-shaped Milky Way galaxy.

“Beautiful spirals [in other galaxies] are made of segments,” Alves told Gizmodo. He suspected his team had found such a segment. “We’re so close to it that we didn’t recognise it before.” The structure doesn’t quite align with past measurements of the galactic arm, however. You can look at an interactive 3D map of the Radcliffe Wave structure, the location where Gould’s Belt would have been, and the arm of the galaxy here.

“The relationship of structures like the Radcliffe Wave to the formation of larger spiral arms in the Milky Way remains uncertain, but certainly intriguing,” Mark Reid, a senior radio astronomer not involved in this work who studies the structure of the local galaxy at Smithsonian Astrophysical Observatory, told Gizmodo in an email. But Reid said the measurement of the structure “checks out well against highly accurate distances using radio astronomy techniques.”

Now that researchers have found such a structure, they want to figure out how it was created and what in the universe could have produced a giant waveform that causes gas to bulge 500 light-years out of the galactic plane in either direction. That’s still a mystery—some sort of massive object, like a dwarf galaxy, must have perturbed all of the gas, Alves said, but he couldn’t think of any candidates that would have passed by and caused the wave. Perhaps the culprit is dark matter, the mysterious mass that appears to form the universe’s scaffolding and much of the mass in galaxies. The researchers hope to run computer simulations to see if they can recreate the structure and potentially find similar structures in other galaxies.

The Radcliffe Wave is not too far away—at its closest, it’s 1,000 light-years from the Sun. It’s possible that the Sun passed through it around 13 million years ago, Zucker told Gizmodo.

One researcher not involved in the study, professor Andreas Brukert from the Ludwig-Maximilians University of Munich, wondered why whatever created the wave didn’t move the Sun as well, or whether such a thing could happen. Continuing to study these objects will hopefully improve our knowledge of galactic dynamics more generally, he said; it reiterates that galaxies are more than just flat disks with stars; they are full of other behaviours and waves that make them complex structures.

Gaia’s third data release is scheduled for the second half of this year, and there’s no telling what other incredible discoveries it might yield.