Gaia is a star mapping mission that the ESA is conducting, and it recently discovered something very interesting. Gaia has found that the Milky Way galaxy is still facing the effects of a near collision that set millions of stars to rippling like water in a lake. ESA scientists believe that the near collision happened in the past 300-900 million years.
Gaia made the discovery because of a pattern of movement that has given the Milky Way its disc shape. The pattern of movement was revealed thanks to the capability Gaia has to accurately measure the positions of more than a billion stars and their velocities across the plane of the sky.
For a few million of those stars, Gaia was able to estimate the full 3-dimensional velocity. That allowed scientists to study stellar motion using a combination of position and velocity known as “phase space.” Steller motions in phase space revealed the unexpected pattern when the positions of the stars were plotted against the velocities.
The computer used to crunch the data showed a shell-like pattern in the graph that plotted the altitude of the star above or below the plane of the Galaxy against the velocity in the same direction. The shapes were so clear in the data that at first, the scientists believed there was some sort of error. Data error was ruled out because the data had been validated by multiple tests.
These patterns hadn’t been seen before because the quality of data gathered by Gaia is a massive step up from what was available to scientists before. The scientists say that no one had investigated this phase mixing in the disc of our galaxy before despite the fact that other studies had looked at phase mixing in other astrophysical settings. The theory is that the Sagittarius dwarf galaxy’s close pass by the Milky Way in the last 200 to 1000 million years set the motion off.