What is deforming the Milky Way?

The Milky Way is not like other barred spiral galaxies; instead of showing a flat disc, it is curved upwards on one side and downwards on the other.

Many of the stars in the Milky Way are distributed in the thin disk, the region of space occupied by the spiral arms of our galaxy. Recently, scientists discovered that the disk is slightly deformed at the edges.

Now, thanks to data from ESA’s Gaia star mapping satellite, they’ve discovered that the deformed disc of our galaxy, the Milky Way, is wobbling, similar to the motion of a spinning top, confirming earlier indications that this deformation is not static. They call it precession. The reason? It’s the result of a collision with a smaller galaxy sometime in the past of the Milky Way.

Thanks to ESA data from Gaia

Astronomers tracked the position and motion of 12 million giant stars to measure how fast the deformation rotates around the galaxy. They discovered that it changes orientation over time, staggers like a top, and completes a full rotation much faster than expected.

“We measure the speed of deformation by comparing the data with our models. Depending on the speed obtained, the deformation would complete a rotation around the centre of the Milky Way in 600 to 700 million years,” Eloisa Poggio of the Astrophysical Observatory of Turin, Italy, explained to the magazine Nature Astronomy that published the study. “That is much faster than we expected based on predictions from other models, such as those observing the effects of the spherical halo". As an example, the Sun orbits the galactic centre every 220 million years.

Something hit the Milky Way?

The direction and magnitude of the rate of precession of galactic deformation favour the scenario that this is the result of a recent or ongoing encounter with a satellite galaxy, rather than the relic of the ancient history of our galaxy.

What is not clear is when it was or what galaxy was involved, but it seems that such a deformation could only have been caused by a relatively recent or even ongoing cosmic dispute with one of the Milky Way’s satellite galaxies. Astronomers believe that it could be the Sagittarius Dwarf Spheroidal Galaxy that, is in a fairly close orbit with our galaxy and historically has proven to be quite harmful to it. However, there is nothing to worry about: The Milky Way will absorb Sagittarius and incorporate its globular clusters, although this is still a long way off.

"With Gaia, for the first time, we have a lot of data on numerous stars, whose motion is measured so precisely that we can try to understand the large-scale movements of the galaxy and shape its history of formation", said ESA Project Associate Scientist Gaia Jos de Bruijne. "This is something unique. This is really Gaia’s revolution".

Reference: E. Poggio et al. Evidence of a dynamically evolving Galactic warp, Nature Astronomy (2020). DOI: 10.1038/s41550-020-1017-3