A chain of the Milky Way linked to a galactic collision

When most of us imagine the shape of the Milky Way, the galaxy that contains our own sun and hundreds of billions of other stars, we think of a central mass surrounded by a flat disk of stars spinning around. of her. However, astronomers know that instead of being symmetrical, the structure of the disk is distorted, more like the edge of a fedora, and that the distorted edges are constantly moving around the outer edge of the galaxy.

“If you’ve ever seen the audience wave in a stadium, it’s very similar to this concept,” said Xinlun Cheng, a graduate student in astronomy at the College and Graduate School of Arts & Sciences at the University of Virginia. . “Each member of the audience gets up, then sits at the right time and in the right order to create the wave as they circle the stadium. This is exactly what the stars in our galaxy do. Only in this case, as the wave circles the galaxy disk, the galaxy disk also revolves around the center of the galaxy. In terms of analogy with sports fans, it is as if the stadium itself is also spinning. “

What caused this distortion has been the subject of debate. Some researchers suggest that the phenomenon is the result of the instability of the galaxy itself, while others claim that it is the vestige of a collision with another galaxy in the distant past.

A recent article published in the Astrophysics Journal by Cheng, who studies the movements of stars, and his colleagues, Borja Anguiano, associate postdoctoral researcher at AVU, and Steve Majewski, professor in the college’s department of astronomy, can finally put an end to this debate.

Using data from the Gaia Space Observatory, a satellite launched in 2013 by the European Space Agency to measure the positions, distances and motions of billions of stars and information from APOGEE, an infrared spectrograph developed by UVA to examine the chemical composition and movements of stars, astronomers now have the tools to observe the movements of stars in the Milky Way with unprecedented precision.

“By combining information from the APOGEE instrument with information from the Gaia satellite, we are starting to understand how the different components of the galaxy move,” said Anguiano, who is interested in both the movements of these components and the phenomena likely to occur. originally caused these movements.

“It is now possible to characterize these movements with unprecedented precision thanks to the precision and the statistical robustness of the immense catalog of stars which was probed by the Gaia satellite,” explained Majewski. “Meanwhile, our own large database of stellar chemistries generated by APOGEE gives us the unique ability to infer stellar ages. This allows us to explore how stars of different ages participate in the chain and allows us to focus on the date of its creation. , then, gives us an idea of ​​why it was created. “

Using this data, Cheng and his colleagues developed a model that characterizes the parameters of the galactic chain, where it begins in the outer disk, how fast the chain moves, and the shape of the chain. The model helped them determine that the distortion, which does not affect our own sun, but now passes through our solar system at speeds that allow it to make a full rotation around the galaxy every 450 million years, is not the result of the Milky Way’s own internal mass. Instead, it is the relic of the gravitational pull on the Milky Way’s disk by the nearby passage of a satellite galaxy, possibly the dwarf spheroidal galaxy of Sagittarius, about 3 billion ago. years.

“We can still see our galaxy’s disk shaking as a result,” Anguiano said.

The data the team has collected from the new tools available to astronomers may just be the start of a new wave of discovery about our universe and how it came about.

“We are entering an era of astronomy, especially galactic astronomy, in which we measure the movement of stars to such a level of precision that we can map their past orbital paths and begin to understand how they may have been. affected in older times and how other galaxies approaching our own interacted with the stars at the time of their birth, ”Anguiano said. “This level of precision opened a new door to understanding our galaxy’s past and how it was assembled.”

The article, “Exploring Galactic Distortion Through Asymmetries in Galactic Disc Kinematics,” by Cheng and colleagues, was published in the December issue of Astrophysics Journal.