Structures in our galaxy's outer disk have been identified as likely fossil spiral arms, remnants disrupted by encounters with the gravity of nearby dwarf galaxies.
The Gaia Space Observatory has revealed the motion of millions of stars in exquisite detail, allowing astronomers to identify patterns, with stars' similar movements revealing shared origins or histories. This has created a science of galactic archaeology, which has identified smaller galaxies absorbed into the Milky Way so thoroughly their identities were previously invisible.
Dr Chervin Laporte of the University of Barcelona has used Gaia's observations to study an often neglected part of the galaxy, the area opposite the galactic center as seen from Earth. In Monthly Notices of the Royal Astronomical Society: Letters Laporte and co-authors describe structures they propose were part of the Milky Way from the beginning, but have been so distorted by external influences they can hardly be identified.
Known as the galactic anticenter, the region they studied is centered on the constellation Auriga. There the authors found numerous filaments that had never been reported before and brought some previously known structures into sharper view.
With so many structures identified, Laporte and co-authors don't think they all have the same causes. Some are likely to be the legacies of ancient spiral arms disturbed by external forces, while others may be the crests of waves produced by forces within the galaxy itself. The team has now booked telescope time to examine each structure in more detail to distinguish between the two causes.
“Typically this region of the Milky Way has remained poorly explored due to the intervening dust which severely obscures most of the Galactic midplane,” Laporte said in a statement.
Even prior to Gaia, the discovery of the Sagittarius dwarf galaxy (SDG) revealed to astronomers the Milky Way is not quite the serene place we once imagined. Although it has just one three-thousandths of the Milky Way's mass, the SDG is concentrated enough its gravity can perturb stars in nearby parts of our galaxy's outer rim. Laporte previously modeled the way a galaxy like the SDG would produce corrugations in the galactic disk with the motion of its galactic field.
Since then numerous other small galaxies have been discovered, initially thought to be orbiting the Milky Way, but now identified as new arrivals. Some of these are close and dense enough to have substantial influences of their own.
One of the structures identified prior to this work is called the anticenter stream. Its stars are mostly around 8 billion years old, making it likely the stream is a legacy, not of an external galaxy, but of one that was incorporated into the Milky Way long ago.
Laporte co-authored a recent study that found evidence of a spiral feature indicating a perturbation in the galactic disk from an encounter with an unknown dwarf galaxy a few hundred million years ago.
“We were certainly very excited to see that the Gaia motions data helped us uncover these filamentary structures!” Laporte said. “Now the challenge remains to figure what these things exactly are, how they came to be, why in such large numbers, and what they can tell us about the Milky Way, its formation and evolution."