A galactic near-miss set stars on an unexpected path around the Milky Way

Between 300 and 900-million years ago the Sagittarius dwarf galaxy made a close pass by the Milky Way, setting millions of stars in motion, like ripples on a pond. The after-effects of that galactic near miss are still visible today, according to newly published findings.

The unique pattern of stars left over from the event was detected by the European Space Agency’s star mapping mission, Gaia. The details are contained in a paper written by Teresa Antoja and colleagues from the Universitat de Barcelona in Spain, and published in the journal Nature. 

The movements of over six million stars in the Milky Way were tracked by Gaia to reveal that groups of them follow different courses as they orbit the galactic centre.

In particular, the researchers found a pattern that resembled a snail shell in a graph that plotted star altitudes above or below the plane of the galaxy, measured against their velocity in the same direction. This is not to say that the stars themselves are moving in a spiral, but rather that the roughly circular orbits correlate with up-and-down motion in a pattern that has never been seen before.{%recommended 4642%}

While some perturbations in densities and velocities had been seen previously, it was generally assumed that the movement of the disk’s stars is largely in dynamic equilibrium and symmetry about the galactic plane. Instead, Antoja’s team discovered something had knocked the disk askew.

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

Whereas water will eventually settle out after being disturbed, a star’s motion carries signatures from the change in movement. While the ripples in the distribution caused by Sagittarius passing by has evened out, the motion of the stars themselves still carry the pattern.

“At the beginning the features were very weird to us,” says Antoja. “I was a bit shocked and I thought there could be a problem with the data because the shapes are so clear.”

The new revelations came about because of a huge increase in quality of the Gaia data, compared to what had been captured previously. The new information provided, for the first time, a measurement of three-dimensional speeds for the stars. This allowed the study of stellar motion using the combination of position and velocity, known as “phase space”.

“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 Antoja.

Computer models suggest the disturbance occurred between 300 and 900 million years ago – a point in time when it’s known the Sagittarius galaxy came near ours.

In cosmic terms, that’s not very long ago, which also came as a surprise. It was known that the Milky Way had endured some much earlier collisions – smashing into a dwarf galaxy some 10 billion years ago, for instance – but until now more recent events had not been suspected. The Gaia results have changed that view.