The satellite Gaia has detected substructures in the Milky Way that has been caused by a gravitational disturbance several hundred million years ago.
According to scientists from the University of Barcelona and University of Groningen, the Milky Way almost collided with a smaller galaxy, causing motions of millions of stars in its galactic disk.
The findings were published in the journal Nature.
Like Ripples On Water
"We have observed shapes with different morphologies, such as a spiral similar to a snail's shell," explained researcher Teresa Antoja. "These substructures allow us to conclude that the disk of our galaxy suffered an important gravitational disturbance about 300 and 900 million years ago."
The researchers theorized that this disturbance was caused by a passing galaxy, particularly, the Sagittarius Dwarf, that nearly collided with the disk of the Milky Way a long time ago. They made the conclusion after they had compared the structure and level of twisting of the spiral to existing models of the dynamic of the galaxy.
Antoja described this movement as similar to ripples of water on a pond or metals repelling from a magnet. When the Sagittarius Dwarf passed, the stars were reordered in a certain way. Eventually, these stars maintained the effects of the near-collision and now, a spiral can be seen.
Antoja added that the study implies the Milky Way Galaxy is still sensitive to disturbances and still can change over time. The Saggitarius Dwarf is currently being "cannibalized" by the Milky Way Galaxy. Albeit small, the galaxy, which is home to tens of millions of stars can still make a "notable gravitational impact."
The Hunt Continues
The findings of Gaia changes the way astronomers look at the Milky Way Galaxy. Although there was a couple of collisions in the past, the Milky Way Galaxy looked unperturbed, suggesting that these activities happened a long time ago.
"We tend to think of the galaxy as having a relatively quiet history," explained Kathryn Johnston, an astrophysicist who was not involved in the study. "What these results are showing us more about its history of interactions, which may help us understand more about galaxies as a whole."
Gaia is a space observatory that is run by the European Space Agency. Its mission is to observe the stars and create a three-dimensional map of the Milky Way Galaxy, charting composition, formation, and evolution.
According to the study, the discovery is its first great piece of "galactic archaeology" that could give researchers a peek at the history of the Milky Way Galaxy.