Our Galaxy's cannibal past revealed

The Milky Way grew to its current size by snacking on other galaxies according to a recent study of data taken by the Gaia satellite.

The Gaia satellite has mapped out the position and velocity of over a billion stars
Published: June 13, 2018 at 12:00 pm
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The remains of the Milky Way’s cannibalistic past have been discovered by a new study using the latest data from ESA's Gaia satellite.


The results show that in the past our Galaxy merged with several others, the remnants of which can still be seen today.

Though astronomers know that galaxies like the Milky Way grow by devouring each other it’s not certain whether this happens by repeatedly snacking on smaller galaxies or by feasting on one large galaxy.

“We discovered five small clusters [in the Milky Way] which we believe are remnants of five merger events,” says Helmer Koppelman from the University of Groningen.

There were also signs that the Milky Way had consumed another, larger galaxy as well.

Koppelman used the Gaia data to analyse the trajectories of stars, finding which ones might have originated outside the Galaxy.

“We collected information from stars within 3,000 lightyears of the Sun, as the accuracy of the position and movement is highest for stars that are near us,” says Koppelman.

First, he filtered out the stars that were originally part of the Milky Way, which are easily identified as they move around the centre of the disk.

This left around 6,000 stars that could have originated in other galaxies that were been consumed by the Milky Way.

Some of these were in five small clusters, but the remainder all seemed to have a shared history.

“These stars form a huge ‘blob’ with retrograde movement compared to the disk.

This suggests that they are the result of a merger with a large galaxy.

In fact, we believe that this merger event must have remodelled the disk in our Milky Way,” says Koppelman.

Koppleman will now extend his search further out into the galaxy, taking advantage of Gaia’s massive catalogue of stars.

The Gaia mission started in 2013 with the aim of measuring the position and radial velocity of over one billion stars in the Milky Way and Local Group, creating the largest 3D map of the Galaxy ever created.


The second batch of data from Gaia, which this study was based on, was released on 25 April 2018.


Ezzy Pearson is the News Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.

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