Snail shell pattern in the velocity of stars

This graph shows the altitude of stars in our Galaxy above or below the plane of the Milky Way against their velocity in the same direction, based on a simulation of a near collision that set millions of stars moving like ripples on a pond.

The snail shell-like shape of the pattern reproduces a feature that was first seen in the movement of stars in the Milky Way disc using data from the second release of ESA’s Gaia mission, and interpreted as an imprint of a galactic encounter. 

The close encounter revealed by the Gaia data likely took place sometime in the past 300–900 million years, and the culprit could be the Sagittarius dwarf galaxy, a small galaxy containing a few tens of millions of stars that is currently in the process of being cannibalised by the Milky Way.

Full story: Gaia hints at our Galaxy’s turbulent life