A giant sausage once crashed into the Milky Way

Around about 10 billion years ago the Milky Way collided with a small sausage-shaped galaxy in a titanic event that determined the futures of both.

In a series of five papers published in the journals Monthly Notices of the Royal Astronomical Society and The Astrophysical Journal Letters, as well as the preprint site arXiv.org, astronomers from the Simons Foundation Centre for Computational Astrophysics in the US lay out evidence that suggests the shape and size of our home galaxy was substantially affected by a violent encounter with a dwarf one they dub “The Sausage”.

“The collision ripped the dwarf to shreds, leaving its stars moving in very radial orbits,” explains one of the researchers, Vasily Belokurov. 

The stars, freed from the gravitational forces of their own galaxy, continued speeding along, following trajectories that took them very close to the centre of the Milky Way. Their journey, adds Belokurov, “is a telltale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed”.

A question might reasonably be posed concerning how the astronomers behind the suite of journal papers knew the dwarf galaxy resembled – albeit on a cosmic scale – a speeding kransky.

To reach the conclusion, the researchers used data obtained from the European Space Agency’s Gaia satellite. The craft, launched in 2013, is collecting information to create a three-dimensional map of the Milky Way.

It is tasked with taking measurements of about one billion stars – less than 1% of the Milky Way’s full complement – and among those, it turns out, are some of the ones that took up residence after the collision.

“We plotted the velocities of the stars, and the sausage shape just jumped out at us,” explains one of the astronomers, Wyn Evans. 

“As the smaller galaxy broke up, its stars were thrown onto very radial orbits. These Sausage stars are what’s left of the last major merger of the Milky Way.”

Despite being classified as a dwarf, the incoming banger was in fact a huge entity, with a total mass (including dark matter) more than 10 billion times that of the sun. 

The collision, the astronomers suggest, would have severely damaged the Milky Way’s disc – which would then have had to regrow. Sausage debris was scattered all around the galaxy, but especially close to the centre, where it created its trademark “bulge”.

Evidence suggests the existence of at least eight globular clusters – collections of stars – now in the Milky Way that originated in the Sausage galaxy and survived intact.

“While there have been many dwarf satellites falling onto the Milky Way over its life, this was the largest of them all,” says researcher Sergey Koposov.

The papers can be found here, here, here, here and here.