Six million stars in the Milky Way are moving “like ripples on a pond” after a close encounter with another galaxy way before the age of the dinosaurs.
The near collision happened between 300 to 900 million years ago when a small ‘satellite’ galaxy came close to the Milky Way.
It set the stars spinning in a “non-uniform” rotation – revealing that groups can follow different courses as they orbit the centre of their galaxy.
Most stars in the Milky Way are located in its disk – the flat region surrounding the central bulge.
It was assumed their movement is largely in dynamic equilibrium – and symmetric about the galactic plane.
But using data from the Gaia satellite’s star mapping mission the European Space Agency (ESA) team discovered a variety of spiral patterns.
This means there are populations of stars that move on different paths through the Milky Way – while still participating in the overall rotation.
Dr Teresa Antoja and colleagues say it was caused by the gravity of the Sagittarius dwarf – one of dozens of mini-galaxies that surround the Milky Way.
Over the age of the universe it has made several loops around our galaxy – which is still enduring the effects.
The Gaia space observatory not only precisely measures the positions of more than a billion stars – but also their velocities on the plane of the sky.
This enabled the first study of stellar motion of its kind using the combination of position and velocity – known scientifically as ‘phase space’.
This uncovered an interesting and totally unexpected pattern when the star’s positions were plotted against their velocities.
Corresponding author Dr Antoja, of Barcelona University, said she couldn’t quite believe her eyes when she first saw it on her computer screen.
In particular she spotted a snail shell-like pattern in the graph that had never been seen before.
Dr Antoja said: “At the beginning the features were very weird to us. I was a bit shocked.
“I thought there could be a problem with the data because the shapes are so clear.”
But her results published in Nature were confirmed by multiple validation tests carried out by experts all over Europe.
The reason they had not been seen before was the quality of the Gaia data was a huge step up from what had come before.
Dr Antoja said: “It looks like suddenly you have put the right glasses on and you see all the things that were not possible to see before.”
With the reality of the structure confirmed it came time to investigate why it was there.
Dr Antoja said: “It’s a bit like throwing a stone in a pond – which displaces the water as ripples and waves.”
Unlike water molecules which settle again the stars retain a ‘memory’ that they were perturbed. This memory is found in their motions.
After some time the ripples may no longer be easily visible in the distribution of stars – but they are still there when you look in their velocities.
The Milky Way is a ‘cannibal’ – growing by eating smaller galaxies and clusters of stars. But that didn’t seem to be the case here.
Sagittarius contains a few tens of millions of stars and is currently in the process of being cannibalised by the Milky Way
Its last close pass to our galaxy was not a direct hit – it passed close by. This would have been enough so that its gravity perturbed some stars in our like a stone dropping into water
The clincher was estimates of Sagittarius’s last close encounter with the Milky Way place it sometime between 200 million and 1 billion years ago.
This is almost exactly what the researchers calculated as an origin for the beginning of the snail shell-like pattern.
Co-author Professor Amina Helmi, of Groningen University in The Netherlands, said: “The discovery was easy – the interpretations harder.
“And the full understanding of its meaning and implications might take several years.”
Dr Timo Prusti, Gaia project scientist at ESA, said: “This is exactly the kind of discovery we hoped would come from the Gaia data.
“The Milky Way has a rich history to tell – and we are starting to read that story.”
