Europe’s Gaia spacecraft has spotted a ‘bridge of stars’ between two dwarf galaxies. The bridge—a halo of faint light arching between two of the Milky Way’s nearest neighbors—is 43,000 light-years long.
Gaia’s main mission is to map the motions of a billion nearby stars in an effort to understand how our galaxy formed. Unlike Hubble, which only observes a relatively small portion of the sky, Gaia’s telescopes can sweep over the entire firmament in about a month. That allowed scientists to detect the bridge of stars between the Large and Small Magellanic Clouds, which aren’t typically close together. The results are published in Monthly Notices of the Royal Astronomical Society.
About 200 million years ago, as the two dwarf passed close to each other, the gravity of the Large Magellanic Cloud (LMC) may have started pulling stars and hydrogen gas from the smaller galaxy. Meanwhile, the Milky Way’s gravity could be stripping stars away from the LMC. These purloined stars form the fuzzy bridge between galaxies, which is difficult to see with the naked eye.
Although structures like this were predicted, they’ve never been observed until now.
“The bridge is extremely faint, and cannot be seen by eye on the images, paper coauthor Vasily Belokurov told Popular Science. “Instead, we counted the stars between the Clouds and found an excess in star count arranged in a narrow bridge-like structure.”
The stream of stolen stars leads to clues about galaxy formation. They reveal that the LMC must be significantly bigger than scientists thought, with perhaps a tenth of the mass of the Milky Way. And the stellar pathway also reveals where the two galaxies have been before. Galactic orbits are often very difficult to detect, but the star bridge is like a brilliant trail of breadcrumbs.
Together, the information from this study and the rest of the Gaia survey shows that “the current model of galaxy formation is holding up well to the increased scrutiny,” said co-author Denis Erkal.
Update 2/9/2017 at 4pm EST: This post was updated with a quote from the study author.