Mysterious fossil spiral arms discovered in the Milky Way

An international team of astronomers led by researcher Chervin Laporte of the Institute of Cosmos Sciences of the University of Barcelona, ​​analyzed the motion data of Gaia, available from December 2020, to identify coherent structures. Their resulting map revealed the existence of many previously unknown spinning filamentous structures at the edge of the disc. It also gave a sharper overview of previously known structures.

Numerical simulations predict that such filamentous structures form in the outer disk from past satellite interactions, but the enormous amount of substructure revealed by this map was not foreseen and remains a mystery.

What could these structures be? One possibility is that they are the remnants of tidal arms of the Milky Way's disk that have been excited at different times by various satellite galaxies. Our galaxy is now surrounded by 50 of these satellites and has engulfed numerous other galaxies in its past. At present, the Milky Way is thought to be perturbed by the dwarf galaxy of Sagittarius. But in its more distant past, it interacted with another intruder who has now scattered its debris in the periphery of our galaxy.

In a previous study, the same team showed that one of the filamentous structures in the outer disk , the Anticenter Stream, had stars that were predominantly over 8 billion years old. This potentially makes it too old to have been excited by Sagittarius alone and instead points to the Gaia sausage.

Another possibility is that not all of these structures are true fossil spiral arms, but instead form crests. of large-scale vertical distortions in the disk of the Milky Way. "We believe the discs respond to satellite impacts that create vertical waves that propagate like ripples on a pond," says Laporte.

To try to differentiate between the two explanations, the team has now ensured a dedicated follow-up program with the William Herschel Telescope over the Canary Islands in order to study the properties of stellar populations in each substructure. Future investigations will help shed light on the nature and origin of these celestial structures.

“Typically this region of the Milky Way has remained poorly explored due to intervening dust severely obscuring most of the galactic central plane ", Commented Laporte, who then added:" while dust affects the brightness of a star, its motion remains unchanged. We were certainly very excited to see that the data on Gaia's motions helped us discover these filamentous structures! Now the challenge remains to understand what exactly these things are, how they came about, why in such large numbers and what they can tell us about the Milky Way, its formation and evolution ".