Astronomers Find Fossil Spiral Arms in Milky Way Galaxy

Using data from ESA’s Gaia satellite and archival spectroscopic surveys, astronomers have mapped the Milky Way disk substructure at distances over 10,000 parsecs (32,616 light-years). The resulting map reveals the existence of many previously unknown spinning filaments at the edge of the Milky Way’s disk.

All-sky map of the Milky Way in motion using the Gaia data. Areas with significant motion are shown in black/purple and those with relatively low motion in yellow. A number of large scale filamentary disc structures are evident about the midplane. The map also shows the Magellanic Clouds and their connecting stellar bridge to left, while the Sagittarius dwarf galaxy currently being torn apart can be seen on the right (main body). Image credit: Laporte et al., doi: 10.1093/mnrasl/slab109.

“Numerical simulations predict filamentary structures to form in the outer disk of the Milky Way Galaxy from past satellite interactions, but the sheer quantity of substructure revealed by the new map was not expected and remains a mystery,” said Dr. Chervin Laporte, an astronomer with the Institute of Cosmos Sciences at the University of Barcelona, and colleagues.

“What could these structures possibly be? One possibility is that they are the remains of tidal arms from the Milky Way disk which were excited at different times by various satellite galaxies.”

The Milky Way is now surrounded by nearly 50 satellite galaxies and has engulfed numerous such galaxies in its past.

At present, our Galaxy is thought to be being perturbed by the Sagittarius dwarf galaxy, but in its more distant past it interacted with another intruder, the Gaia Sausage, which has now dispersed its debris into the Milky Way’s outskirts.

In an earlier study, Dr. Laporte and co-authors showed that one of the filamentary structures in the outer disk, the Anticenter Stream, had stars which were predominantly more than 8 billion years old.

This makes it potentially too old to have been excited by Sagittarius alone and instead points to the Gaia Sausage.

Another possibility is that not all these structures are actual genuine fossil spiral arms but instead form the crests of large scale vertical distortions in the Milky Way disk.

“We believe that disks respond to satellite impacts which set up vertical waves that propagate like ripples on a pond,” Dr. Laporte said.

To try to distinguish between the two explanations, the authors now plan to study the properties of the stellar populations in each substructure using the William Herschel Telescope.

“Typically this region of the Milky Way has remained poorly explored due to the intervening dust which severely obscures most of the Galactic midplane,” Dr. Laporte said.

“While dust affects the luminosity of a star, its motion remains unaffected.”

“We were certainly very excited to see that the Gaia motions data helped us uncover these filamentary structures!”

“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.”

A paper on the findings was published in the Monthly Notices of the Royal Astronomical Society: Letters.

_____

Chervin F.P. Laporte et al. 2022. Kinematics beats dust: unveiling nested substructure in the perturbed outer disc of the Milky Way. MNRASL 510 (1): L13-L17; doi: 10.1093/mnrasl/slab109