Unveiling substructures at the edge of the Galaxy

"Typically, this region of the Milky Way has remained poorly explored due to the intervening dust which severely obscures most of the Galactic midplane," says Chervin Laporte, first signer of the article. "While dust affects the luminosity of stars, it has no effect on their motion. As a result, one can use the stars' motion to perform a tomography of the Galaxy's outermost regions," adds the ICCUB researcher. The team analysed the Gaia motion data, available from December 2020, to identify coherent structures.

The map reveals the existence of many previously unknown coherently rotating filamentary structures at the edge of the disc. It also gives a sharper global view of previously known structures. Numerical simulations predict such filamentary structures to form in the outer disc from past satellite interactions, however the sheer quantity of substructure revealed by this map was not expected and it remains a mystery.

What could these filamentary structures be?

Our Galaxy is surrounded by fifty satellite galaxies and has engulfed numerous galaxies in its past. At present, the Milky Way is thought to have been perturbed by the Sagittarius dwarf galaxy, a fact that confirmed Laporte's earlier theoretical models. However, in its more distant past it interacted with another intruder, the Gaia Sausage, which has now dispersed its debris into the stellar halo. The researchers formulated the hypothesis that states that these filamentary structures are remains of tidal arms from the Milky Way disc, which were excited at different times by various satellite galaxies.

Laporte notes that in an earlier study, they already showed that one of the thread-like structures in the outer disc, called the Anticenter Stream, had stars which were predominantly older than eight billion years, "making it potentially too old to have been caused by Sagittarius alone, but more in line with a Sausage origin."

"Another possibility," adds the researcher, "would be that not all these structures are actually genuine disc substructures, but instead form the crests of vertical density waves in the disc seen in projection, forming an optical illusion that the disc is highly substructured."

The team has secured a dedicated follow-up programme with the WEAVE spectrograph to study the similarities and differences in stellar populations in each substructure. Through the study of radial velocities, chemical abundances and potentially stellar ages, the upcoming surveys WEAVE, SDSS-V and PFS will also shed light into the origins of the substructures.