Between 300 million and 900 million years ago, our Milky Way galaxy nearly collided with the Sagittarius dwarf galaxy. Data from the ESA’s Gaia mission shows the ongoing effect of this event, with stars moving like ripples on the surface of a pond. The galactic collision is part of an ongoing cannibalization of the dwarf galaxy by the much-larger Milky Way.

“At the beginning the features were very weird to us.” – Teresa Antoja, Lead Researcher, Universität de Barcelona, Spain.

Gaia’s main science objective is to survey one billion stars in our galactic neighborhood. It’s creating a precision 3D map of the Milky Way, and answering questions about the origin and evolution of our galaxy. The ESA launched Gaia in 2013 on a planned five-year mission.

Gaia discovered the encounter between the Milky Way and the Sagittarius dwarf galaxy by mapping the movement of stars in the Milky Way’s galactic disc. Gaia accurately measures the positions and precise velocities of a billion stars. The spacecraft worked out what’s known as ‘phase space’ for a subset of a few million stars, by estimating the three-dimensional velocities for those stars.  This phase space, or combination of position and velocity, allowed scientists to study the stellar motion of stars in the disc.

“I was a bit shocked and I thought there could be a problem with the data because the shapes are so clear.” – Teresa Antoja, Universitat de Barcelona, Spain.

The stellar motions in phase space revealed something shocking and unexpected. Lead researcher Teresa Antoja, from the Universität de Barcelona in Spain, was shocked by what the data revealed.  She thought there must be a problem in the data. A snail-like pattern that had never been seen before stood out. The pattern was in the graph of the stars’ altitude above or below the plane of the Galaxy, plotted against their velocity in the same direction. “At the beginning the features were very weird to us,” says Teresa. “I was a bit shocked and I thought there could be a problem with the data because the shapes are so clear.”

But the Gaia mission is a huge enterprise. It has Data Processing and Analysis teams all over Europe, and those teams tested all the data before releasing it. Antoja and her colleagues also tested the data to see if errors had caused the shape to appear. But their hunt for errors turned up nothing. They had to accept that the unusual and unexpected shape was actually there.

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

Antoja recognized that the shape was revealed because the data from Gaia was of such high quality. It was like putting on the right glasses. The next step was to figure out what caused this shape.

The ripples in the stars of the Milky Way disc were like ripples on a pond. “It is a bit like throwing a stone in a pond, which displaces the water as ripples and waves,” explains Teresa. However, a pond’s surface calms down pretty quickly. The motion of the stars has been ongoing for hundreds of millions of years.

Once the team knew that they were seeing something real, they began to investigate the cause of these ‘ripples.’ What had happened to the Milky Way to produce them?

The Milky Way, like many large galaxies, is a bit of a cannibal. One way it’s grown so large is by absorbing stars from other star clusters and other galaxies. Once absorbed, these stars mix with the rest of the galaxy. But in this case, the new members of the Milky Way didn’t mix in.

Amina Helmi works at the University of Groningen, in the Netherlands. She’s a collaborator on the project and the second author on the resulting paper. Helmi recalled studies by herself and other researchers on the Sagittarius dwarf galaxy. Sagittarius contains only a few tens of millions of stars, far fewer than the 100 to 400 billion stars in the Milky Way. Helmi knew that the Milky Way was eating the Sagittarius galaxy.

The last time the two galaxies came close to each other they didn’t collide. But the gravity from that close encounter was enough to disturb some of the stars in the Milky Way’s disc. Just like a thrown stone rippling the surface of a pond.

“This is exactly the kind of discovery we hoped would come from the Gaia data.” – Timo Prusti, Gaia Project Scientist at ESA.

The estimate for the last encounter between Sagittarius and the Milky Way was between 200 million and 1 billion years ago. And that time frame matches the time frame established by the Gaia data.

It sounds very convincing. “The study implies, definitely, that our galaxy’s disk is dynamically young, sensitive to disturbances and changing over time”, said Antoja. But there is still the need for caution. The correlation between the motion of the stars in the Milky Way’s disc and the Sagittarius galaxy is based on simple computer models and analyses.

The Gaia data has a lot more to teach us about the evolution of the Milky Way. The team behind the study intends to examine the whole phenomenon more fully. There’s a lot of work to do, and an enormous amount of data to comb through.

“The discovery was easy; the interpretation’s harder. And the full understanding of its meaning and implications might take several years.” said Amina.

“This is exactly the kind of discovery we hoped would come from the Gaia data,” added Timo Prusti, Gaia Project Scientist at ESA.

Sources

• ESA Press Release: GAIA Hints at our Galaxy’s Turbulent Life
• Universität de Barcelona Press Release: GAIA Detects a Shake in the Milky Way
• GAIA Mission Home
• Research Paper: A Dynamically Young and Perturbed Milky Way Disk