The Milky Way has a ‘kink’ or warp in its disc that ‘wobbles like a spinning top’ and it could have been created by an ongoing collision with a nearby dwarf galaxy.
Observations from the European Space Agency Gaia mission helped scientists from the Turin Astrophysical Observatory track the warp as it orbits the galactic core.
It was first discovered in the late 1950s but astronomers haven’t been able to determine exactly what caused the misshape in the galactic disc until now.
Data from Gaia helped scientists discover the warp changes its orientation over time and also orbits the galactic centre much faster than expected.
The ESA team now believe it had to have been caused by a powerful collision with another galaxy – possibly the nearby dwarf galaxy Sagittarius.
Gaia was launched in December 2013 to create a 3D map of the Milky Way Galaxy and get a better picture of how it formed, what it is made of and how it is changing.
The latest release of Gaia data helped scientists properly map the warp as it orbits the centre of the galaxy and found it does so every 600-700 million years.
Previous theories on the cause of the warp in the disc included the influence of the intergalactic magnetic field or the effects of a dark matter halo.
A dark matter halo is a large amount of unseen matter scientists expect to find orbiting galaxies – if it had an irregular shape its force could bend the galactic disc.
The warp or kink is in the outer edge of the solar system disc and orbits the galactic centre once every 600 to 700 million years
New Gaia data showing the faster than expected speed the warp is orbiting the galactic core shows it can’t have been caused by dark matter – it had to have been caused by something much more powerful.
‘We measured the speed of the warp by comparing the data with our models,’ said lead author Eloisa Poggio of the Turin Astrophysical Observatory in Italy.
‘Based on the obtained velocity, the warp would complete one rotation around the centre of the Milky Way in 600 to 700 million years,’ Poggio said.
‘That’s much faster than what we expected based on predictions from other models, such as those looking at the effects of the non-spherical halo.’
While the warp is orbiting faster than expected, it is still slower than a star orbits the galactic centre – the Sun orbits every 220 million years.
The flying saucer-like telescope also measures the velocities at which individual stars move in the sky over long periods of time.
This allows astronomers to ‘play’ the movie of the Milky Way’s history back- and forward in time over millions of years.
‘It’s like having a car and trying to measure the velocity and direction of travel of this car over a very short period of time and then, based on those values, trying to model the past and future trajectory of the car,’ says astronomer Ronald Drimmel.
‘If we make such measurements for many cars, we could model the flow of traffic,’ said Drimmel, who co-authored the paper into the phenomenon.
‘Similarly, by measuring the apparent motions of millions of stars across the sky we can model large scale processes such as the motion of the warp.’
The galaxy at the front of the queue to take the blame for the warp is the Sagittarius Dwarf Spheroidal Galaxy – a close neighbour responsible for a number of collisions
The galaxy at the front of the queue to take the blame for the warp is the Sagittarius Dwarf Spheroidal Galaxy.
This is a close neighbour responsible for a number of collisions with the Milky Way.
It is about 10,000 light years in diameter and has an elliptical loop shape. It orbits over the galactic poles of the Milky Way at about 50,000 light years from the core.
Sagittarius is believed to have burst through the Milky Way’s galactic disc several times in the past.
It came close enough to trigger rippling movements in stars near the core of our galaxy as close as 900 million years ago.
The warp was first discovered in the late 1950s but astronomers haven’t been able to determine exactly what caused the misshape in the galactic disc until now
Astronomers think that Sagittarius will be gradually completely absorbed by the Milky Way – a process which is already underway.
‘With Gaia, for the first time, we have a large amount of data on a vast amount stars,’ according to Jos de Bruijne from ESA.
Gaia has already helped scientists discover other major collisions between the Milky Way and neighbouring galaxies over its long history.
About 8 to 11 billion years ago another galaxy collided with the Milky Way and ‘puffed up the thick disc, filling it with stars’.
It also encountered a ghost galaxy that left ripples in its hydrogen.
A major discovery of Gaia was the galaxy named ‘Gaia Sausage’ that left stars moving about in odd orbits after it smashed into the Milky Way.
‘The motion of these stars is measured so precisely that we can try to understand the large scale motions of the galaxy and model its formation history,” he said.
‘This is something unique. This really is the Gaia revolution.’
The research has been published in Nature Astronomy.