A “sausage” in the Milky Way

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An impression of the encounter between the Milky Way galaxy and the smaller Sausage galaxy. Photo: V. Belokurov (Cambridge, U.K.); based on image by ESO/Juan Carlos Munoz

The Milky Way and a smaller “Sausage” galaxy collided around 10 billion years ago, according to an international team of astronomers which has discovered this ancient and dramatic head-on collision by studying data from the Gaia satellite of the European Space Agency (ESA). The finding has been described in a series of three papers published in the “Monthly Notices of the Royal Astronomical Society” (MNRAS), “The Astrophysical Journal Letters” (ApJL) and the online preprint repository

The cosmic crash was a defining event as it reshaped the structure of our galaxy in its early history, creating both its inner bulge and its outer halo, the scientists say. The astronomers hypothesise that an unknown dwarf galaxy smashed into the Milky Way eight to 10 billion years ago. The dwarf did not survive the impact and its debris is now part of our galaxy.

“The collision ripped the dwarf to shreds, leaving its stars moving in very radial orbits that are long and narrow like needles,” says Vasily Belokurov of the University of Cambridge and the Center for Computational Astrophysics at the Flatiron Institute, New York. The stars’ paths take them “very close to the centre of our galaxy. This is a telltale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed,” he adds.

The Gaia spacecraft has been mapping the stellar content of our galaxy, recording the journeys of stars as they travel through the Milky Way. As a result, astronomers now know the positions and trajectories of our celestial neighbours with unprecedented accuracy. Because of the pattern of paths of the stars from the galactic merger, they have now been nicknamed the Gaia Sausage. “We plotted the velocities of the stars, and the sausage shape just jumped out at us. These Sausage stars are what’s left of the last major merger of the Milky Way,” says Wyn Evans of Cambridge.

Though the Milky Way continues to collide with other galaxies, such as the small Sagittarius dwarf galaxy, the Sausage galaxy was much more massive. Its total mass in gas, stars and dark matter, according to the scientists, was more than 10 billion times the mass of our sun, and the crash caused a lot of mayhem because of its piercing trajectory. The Milky Way’s disk was probably puffed up or even fractured following the impact and would have needed to regrow, they say. It is that strewn wreckage of the Sausage galaxy that has created the “bulge” at the Milky Way’s centre and the surrounding “stellar halo”, the team has argued, based on numerical simulations of the galactic mashup.

In simulations run by Denis Erkal and his colleagues of the University of Surrey, stars from the Sausage galaxy enter stretched-out orbits. The orbits are further elongated by the growing Milky Way disc, which has got swollen and become thicker following the collision. Evidence of this galactic remodelling is seen in the paths of stars inherited from the dwarf galaxy, says Alis Deason of Durham University. “The Sausage stars are all turning around at about the same distance from the centre of the galaxy.”

The new research has also revealed at least eight large, spherical clumps of stars called globular clusters, which, the researchers say, were probably brought into our galaxy by the Sausage galaxy. Small galaxies generally do not have globular clusters of their own, so the Sausage galaxy must have been big enough to host a collection of clusters.

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