Flowing stars in Milky Way’s halo point to galactic cannibalisation

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The Milky Way galaxy sits in a halo of old stars. A simulation where the Milky Way merged with another, smaller galaxy shows stars, in purple, moving in ‘flows’ around the Milky Way. Their direction is indicated with arrows, and bigger arrows mean they’re moving faster.
Amina Helmi / Jovan Veljanoski / Maarten Breddels / University of Groningen

Bunches of stars travelling together around and through our galaxy, the Milky Way, were debris thrown out as smaller galaxies were cannibalised, new observations suggest.

Astrophysicists in the Netherlands and the US, led by the University of Groningen’s Amina Helmi, tracked the speed and motion of more than 200,000 stars in the Milky Way’s halo – a roughly spherical conglomerate of old stars and clusters surrounding the galaxy. 

They found the stars tended to move and flow in groups, which is expected if they arrived on smaller galaxies and were tossed out during a galactic merger.

The work was published in Astronomy and Astrophysics. 

Astronomers think the Milky Way grew, in part, via mergers with many smaller counterparts, but tracing when and how these collisions took place remains a mystery.

What we do know is when two galaxies merge, stars and gas and other bits and pieces are flung out into the space surrounding the new, combined galaxy – its halo.

So Helmi and her crew traced the movements of stars in our halo using data from the European Space Agency’s space-based Gaia mission, which launched in December 2013, with information from the ground-based Radial Velocity Experiment (RAVE) survey.

The RAVE survey has collected velocity information for more than 400,000 stars, while Gaia has done so for more than two million.

The researchers cross-matched RAVE and Gaia data and found 210,263 stars in common. And when they traced these stars’ movements, they found they mainly flowed in rivers around and through the Milky Way. 

See video below for an illustration of the stars’ movements.

That a large fraction of the stars travel like flocks of birds suggests they originated from smaller galaxies which merged with the Milky Way long ago, Helmi says.

“We believe there might be tens or even hundreds such flocks. At the moment, we only see small groups with just a few stars, but that is probably because we do not yet have all the necessary data.”

Indeed, the Gaia mission still has another two years of data collection ahead, and will examine targets farther away, so the motion of more stars will soon be added to the mix. The current work, the researchers write, “may be considered an appetiser of what can be expected”.

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