Giant wave in the Milky Way

Radcliffe wave

This illustration shows the location and extent of the Radcliffe wave in the Milky Way. (Image: Alyssa Goodman / Harvard University)

Although the Milky Way is our galactic home, it still holds surprises – especially in terms of its large-scale structures. Astronomers have now discovered one of them “on our doorstep”: it is a gigantic, wavy band of interconnected star formation regions. The surprising thing about it: For around 150 years, astronomers have believed that these rockets surround our solar system roughly in a ring. Only new observation data from the Gaia satellite have now revealed the 9000 light-year band of this “Radcliffe wave” structure.

More than 150 years ago, astronomers Benjamin Gould and John Herschel noticed that bright, young stars, interstellar gas clouds and star formation regions seemed to be concentrating along an arch in the night sky. They concluded that the sun and its galactic environment are surrounded by an expanding ring of stars, gas and dust – the so-called Gouldian belt. This ring is believed to be inclined 20 degrees from the main plane of the Milky Way. However, the three-dimensional shape and size of this large collection of star cradles has remained open to this day. Because the astronomers lacked the technical possibilities to determine their distance more precisely. “To estimate the mass and size of such gas clouds, you need to know how far away they are,” explains Harvard University co-author Alyssa Goodman.

Huge band instead of ring-shaped belt

The European Gaia satellite has now supplied the astronomers with precisely this data. This space telescope has been surveying our Milky Way with unprecedented precision since 2013 and has since determined the positions and distances of almost 1.7 billion stars and has also mapped the movements of these stars. Using this data, researchers led by first author João Alves from Harvard University and the University of Vienna have for the first time determined the exact distance of the star cradles in the Gould Belt – with a surprising result. Because the mapping revealed that the components of this supposed belt do not form a ring, but a huge, wavy band. This “Radcliffe wave” structure is approximately 9000 light years long and 400 light years wide – the largest structure of its kind in the Milky Way, as the researchers report. The gigantic “wave” protrudes 500 light years up and down from the main plane of the galaxy. At its next point, the Radcliffe wave is only about 500 light years away from the sun.

“No astronomer expected us to live next to such a gigantic, wave-like accumulation of gas,” says Goodman. This band makes up about 20 percent of the width of the local Orion arm of the Milky Way and extends over 40 percent of its length. “We were shocked when we realized how long and straight the Radcliffe wave is when viewed three-dimensionally and from above. The existence of this wave forces us to rethink our ideas about the three-dimensional structure of the Milky Way. ”Because the straight shape of this huge band refutes the concept of the Gouldian belt. Co-author Stefan Meingast from the University of Vienna adds: “Our new findings mean the end for the Gould Belt. It’s a little sensation that this structure was just a projection effect. ”

Sun crosses giant ribbon

The discovery of the huge band of star cradles and gas clouds demonstrates how distorted large-scale astronomical structures can appear from Earth. “This structure was right in front of our eyes all the time, but we couldn’t really see it – until now,” says Alves. What previously appeared to the astronomers as an arc and thus part of a ring is really just a small section of a wavy band. In the new results, the supposed 20-degree incline of the Gouldian belt proves to be a part of the Radcliffe wave that extends from the wave trough in Orion to a wave crest in Cepheus. There are also indications that this band of star cradles and gas clouds changes over time: It is likely that it oscillates around the main plane of the Milky Way, as the researchers report. Our sun always crosses the path of this gigantic band: “We know that our sun interacts with this structure. It crossed the Orion arm and a whole series of supernovae 13 million years ago and in another 13 million years it will pass through this structure again, ”explains Alves. “It’s a little bit like riding this wave.”

Astronomers don’t know what caused the Radcliffe wave. “The structure is too big and straight to have been created by the effects of an earlier generation of massive stars,” report Alves and his team. It is therefore more likely that this narrow band is the result of a large-scale galactic process – either a shock front in a spiral arm or the gravitational changes in the main plane of the Milky Way. The researchers hope that further studies, in particular on the dynamics of this structure, will provide more information about their mechanism of origin.

Source: João Alves (University of Vienna) et al., Nature, doi: 10.1038 / s41586-019-1874-z

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