The Milky Way has grown over time as other galaxies have collided with and been consumed by our galaxy. Each collision triggered wrinkles that still ripple through different families of stars, affecting how they move and behave in space. ESA’s Gaia space telescope aims to unravel the history of our galaxy by studying these wrinkles – and new research from the mission now reveals that the most recent of these galactic collisions took place billions of years later than we thought.
This image visualises the Milky Way and its surrounding ‘halo’ of stars. Most stars in the Milky Way lie in the disc (like the Sun, for example), but stars from past collisions end up in the halo, a large ‘cloud’ of stars that extends outwards in all directions. These halo stars have been enhanced in this image, but in reality would be very dim compared to the disc. The halo appears smooth and uniform here, this would indicate that a merger has occurred in the ancient past (as opposed to in recent history, which would produce a ‘wrinklier’ halo).
The new Gaia finding reveals that the wrinkles we see in the Milky Way were likely caused by a dwarf galaxy colliding with the Milky Way around 2.7 billion years ago – in other words, a large portion of the Milky Way only joined us within the last few billion years.
Our galaxy’s two prominent satellite galaxies (the Large and Small Magellanic Clouds) are visible to the lower right.
See this image in a slider, compared to a ‘wrinkly’ halo: <LINK>