A satellite galaxy or other massive object that passed through the Milky Way 100 million years ago was such a shock to the system that its many millions of stars are still showing the effects of the collision today, a team of Canadian and U.S. physicists has found.

Astrophysicist Lawrence Widrow of Queen's University in Kingston, Ont., and his colleagues looked at data for hundreds of thousands of stars in the Milky Way and found that the way they were moving within what is known as the disk of the galaxy indicated they had been disturbed in some way.

'What we've seen suggests that perhaps the galaxy isn't quite in equilibrium.'— Larry Widrow, Queen's University

"The stars are moving up and down through the disk, and we expect them to have a distribution of positions and velocities in something close to an equilibrium distribution," Widrow said.

"What we've seen suggests that perhaps the galaxy isn't quite in equilibrium, and the hypothesis is that something has come through and perturbed the disk in this way."

That "something" could have been a smaller satellite galaxy, such as the Sagittarius dwarf galaxy, or a massive dark matter object, Widrow said, and most likely was only one of many such satellites to pass through the Milky Way and disrupt its stars.

"The idea of satellites coming through the disk and perturbing the disk is an old idea; the idea that we're seeing a particular signature of this, a smoking gun, is what's new," Widrow said.

'Ripples in a pond'

The Milky Way "disk" is the mass of stars orbiting around the centre of the spiral galaxy held together by gravity. The disk contains tens of billions of stars moving at a speed of around 200 kilometres per second around the galaxy centre as well as vertically above and below the so-called galactic midplane in a complex motion Widrow compares to a dance.

"Each star has an ideal circular path around the centre of the galaxy, but they're also bobbing up and down in and out of the plane of the disk and they're also not quite on circular paths, they're on slightly elliptical orbits," he said.

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An illustration of the expected distribution of stars around the midplane of the galaxy, left, and the actual observed distribution, suggesting an encounter that disrupted the stars and distributed them more unequally around the midplane. (Courtesy of Lawrence Widrow/Queen's University)

"When we studied these motions … we noticed what appeared to be perturbations or oscillations or waves in the positions and velocities."

By assuming that the undisturbed state of the stars should be a more even distribution of motion, the researchers speculated that the oscillations were a sign that something had passed through the galaxy and disrupted the equilibrium.

"In physics, if there is some force trying to bring the system back into equilibrium, the system tends to overshoot, and that gives rise to oscillations, ripples in a pond," Widrow said.

"You almost think of these stars as behaving like a fluid in the disk of the galaxy held together by gravity."

Sloan sky survey provided data

Widrow and his fellow physicists at the Fermi National Accelerator Laboratory in Illinois, the University of Chicago and the University of Kentucky focused on a cluster of several hundred thousand stars in what astronomers call the solar neighbourhood — the region of the galaxy disk near the sun.

They used measurements from the Sloan Digital Sky Survey, which compiles data from a telescope at the Apache Point Observatory in New Mexico and makes it openly available to scientists for their own use. 

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A NASA illustration of the Milky Way, a spiral galaxy with two arms wrapping off the ends of a central bar of stars. (NASA/JPL-Caltech)

By analyzing everything from the stars' brightness, colour, velocity and different kinds of motion and by setting up a computer simulation of how a galaxy might respond were it perturbed by one of the satellite galaxies known to surround the Milky Way, they were able to determine how long ago the disruption might have happened.

"One has to set up a galaxy that has been perturbed and then evolve it and see how long it takes for those perturbations to die away, and they seem to die away on the time scale of hundreds of millions of years, so if we're seeing the perturbations now, they must have occurred within the last 100 or 200 million years," Widrow said.

Satellite galaxy interactions likely common

Most astronomers suspect that the Milky Way is never quite entirely in equilibrium but is continually being disturbed by one satellite or another passing through it, and Widrow admits that his team's explanation is only one hypothetical scenario of one kind of disruption that could cause the kind of motion detected by the Sloan telescope.

'It's an open question what might have caused this effect and when exactly it happened and how big the dwarf was. These are things we hope to study," he said.

Widrow anticipates that the study of the motion of stars and the effect of satellite galaxies on the Milky Way disk will only get better as data from upcoming space missions accumulates over the next two years.

"There's quite a lot of discussion in the astronomical community about how many satellites there actually are and how they interact with the disk of the galaxy and what effect they're having on the disk," Widrow said. "What we'd like to think is that this is perhaps one piece of a broader discussion of how the disk galaxies interact with these satellites."

The findings are published in the Astrophysical Journal Letters.