Milky Way still in thrall of a 900 million-year-old cosmic collision

The Milky Way galaxy is still enduring the effects of a near collision that took place sometime in the past 300-900 million years and set millions of stars moving like ripples on a pond, scientists say.

The European Space Agency (ESA)'s star mapping mission, Gaia, discovered the pattern of movement in stars in the Milky Way disc -- one of the major components of our galaxy.

The pattern was revealed because Gaia not only accurately measures the positions of more than a billion stars but also precisely measures their velocities on the plane of the sky.

For a subset of a few million stars, Gaia provided an estimate of the full three-dimensional velocities, allowing a study of stellar motion using the combination of position and velocity, which is known as 'phase space'.

In phase space, the stellar motions revealed an interesting and totally unexpected pattern when the star's positions were plotted against their velocities.

Teresa Antoja from Universitat de Barcelona in Spain said that she was surprised to see a snail shell-like pattern in the graph that plotted the stars' altitude above or below the plane of the Galaxy against their velocity in the same direction.

Researchers performed many tests on the data to look for errors that could be forcing such shapes on the data. Yet no matter what they checked, the only conclusion they could draw was that these features do indeed exist in reality.

The reason they had not been seen before was because the quality of the Gaia data was a huge step up from what had come before.

"It looks like suddenly you have put the right glasses on and you see all the things that were not possible to see before," said Teresa.

"It is a bit like throwing a stone in a pond, which displaces the water as ripples and waves," she said.

Unlike the water molecules, which settle again, the stars retain a 'memory' that they were perturbed. This memory is found in their motions.

After some time, although the ripples may no longer be easily visible in the distribution of stars, they are still there when you look in their velocities.

From computer simulations, researchers believe that the ripples may have been caused by the Sagittarius dwarf galaxy -- which contains a few tens of millions of stars and is currently in the process of being cannibalised by the Milky Way.

Estimates of Sagittarius's last close encounter with the Milky Way place it sometime between 200 and 1000 million years ago, which is almost exactly what researchers calculated as an origin for the beginning of the snail shell-like pattern.

The scientists plan to investigate this galactic encounter as well as the distribution of matter in the Milky Way by using the information contained in the snail shell shape.