Galaxies collide with each other on a pretty regular basis. Our own Milky Way, for instance, has gobbled up dozens of smaller galaxies in the past, and Andromeda is currently hurtling towards us at 109 km (68 mi) per second. An international team of astronomers has now found evidence of a celestial smash-up between the Milky Way and an unknown dwarf galaxy that took place around eight to 10 billion years ago, and forever changed the face of our home galaxy.

According to the researchers, the evidence for this cosmic collision is all around us, from the bulge at the center of the Milky Way to the spread-out halo at the very fringes. The now-defunct dwarf galaxy has been dubbed the "Gaia Sausage," after the ESA's Gaia satellite used to plot out the trajectories of its stars, and the apparent shape those measurements revealed.

"We plotted the velocities of the stars, and the sausage shape just jumped out at us," says Wyn Evans, co-author of the study. "As the smaller galaxy broke up, its stars were thrown out on very radial orbits. These Sausage stars are what's left of the last major merger of the Milky Way."

Minor mergers happen all the time, but the researchers say the Sausage galaxy would have been the largest to ever hit the Milky Way, containing the mass of about 10 billion Suns. According to simulations, the impact would have caused the Milky Way's galactic disk to puff up or even fracture, sending Sausage stars piling into the center – which we now see as the bulge – and flicking others out into the spherical halo, on very stretched orbits.

"The collision ripped the dwarf to shreds, leaving its stars moving on very radial orbits, like needles," says Vasily Belokurov, co-author of the study. "These stars' paths take them very close to the center of our galaxy. This is a tell-tale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed."

The team also noticed that the Sausage galaxy seems to have brought in at least eight globular clusters with it – another clue that the extinct dwarf was much larger than others.

The research was published in the Monthly Notices of the Royal Astronomical Society, as well as four related papers in other journals.

Source: University of Cambridge