The movements of over six million stars in the Milky Way have been tracked, revealing that groups of stars follow different courses as they orbit the Galactic centre, reports a paper published this week in Nature. This non-uniform rotation is believed to be the result of a smaller, satellite galaxy passing close to the Milky Way hundreds of millions of years ago.
The majority of stars in the Milky Way are located in its disk, the flat region surrounding the Galaxy’s central bulge. The internal structure of the disk has been shaped by varied influences. The Milky Way’s central bar and spiral arms induce radial migration, for example, and the influence of satellite galaxies can alter stellar movements. However, when modelling galaxies it is generally assumed that the movement of the disk’s stars is largely in dynamic equilibrium and symmetric about the Galactic plane.
Teresa Antoja and colleagues analysed the movements and positions of stars in the Milky Way’s disk, using data from the Gaia space observatory. Generating a special position–velocity diagram that allows astronomers to sort star motions into categories, the authors find various spiral patterns in the data. This does not mean that the stars move along spirals but rather that there are populations of stars that move on different paths through the Milky Way while still participating in the overall rotation.
The authors suggest that this movement was formed by the nearby passage of the Sagittarius dwarf galaxy between 300 and 900 million years ago. These different motions had not been resolved in previous studies, which had limited precision and included fewer stars.