Kink in the Milky Way’s disc could have been created by an ongoing collision


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 – 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

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.

WHAT IS THE EUROPEAN SPACE AGENCY’S GAIA PROBE AND WHAT IS DESIGNED TO DO?

Gaia is an ambitious mission to chart a three-dimensional map of our galaxy, the Milky Way, and in the process reveal its composition, formation and evolution.

Gaia has been circling the sun nearly a million miles beyond Earth’s orbit since its launch by the European Space Agency (ESA) in December 2013. 

On its journey, the probe has been discreetly snapping pictures of the Milky Way, identifying stars from smaller galaxies long ago swallowed up by our own.

Tens of thousands of previously undetected objects are expected to be discovered by Gaia, including asteroids that may one day threaten Earth, planets circling nearby stars, and exploding supernovas. 

Artist's impression of Gaia mapping the stars of the Milky Way. Gaia's mapping effort is already unprecedented in scale, but it still has several years left to run. Gaia maps the position of the Milky Way's stars in a couple of ways. It pinpoints the location of the stars but the probe can also plot their movement, by scanning each star about 70 times

Artist’s impression of Gaia mapping the stars of the Milky Way. Gaia maps the position of the Milky Way’s stars in a couple of ways. It pinpoints the location of the stars but the probe can also plot their movement, by scanning each star about 70 times

Astrophysicists also hope to learn more about the distribution of dark matter, the invisible substance thought to hold the observable universe together.

They also plan to test Albert Einstein’s general theory of relativity by watching how light is deflected by the sun and its planets.

The satellite’s billion-pixel camera, the largest ever in space, is so powerful it would be able to gauge the diameter of a human hair at a distance of 621 miles (1,000 km).

This means nearby stars have been located with unprecedented accuracy.

Gaia maps the position of the Milky Way’s stars in a couple of ways.

Gaia’s all-sky view of our Milky Way Galaxy and neighbouring galaxies, based on measurements of nearly 1.7 billion stars. The map shows the total brightness and colour of stars observed by the ESA satellite in each portion of the sky between July 2014 and May 2016. Brighter regions indicate denser concentrations of especially bright stars, while darker regions correspond to patches of the sky where fewer bright stars are observed. The colour representation is obtained by combining the total amount of light with the amount of blue and red light recorded by Gaia in each patch of the sky.

Gaia’s all-sky view of our Milky Way Galaxy and neighbouring galaxies, based on measurements of nearly 1.7 billion stars. The map shows the total brightness and colour of stars observed by the ESA satellite in each portion of the sky between July 2014 and May 2016. Brighter regions indicate denser concentrations of especially bright stars, while darker regions correspond to patches of the sky where fewer bright stars are observed. The colour representation is obtained by combining the total amount of light with the amount of blue and red light recorded by Gaia in each patch of the sky.

It pinpoints the location of the stars but the probe can also plot their movement, by scanning each star about 70 times.

This is what allows scientists to calculate the distance between Earth and each star, which is a crucial measure.

In September 2016, ESA released the first batch of data collected by Gaia, which included information on the brightness and position of over a billion stars.

In April 2018, this was expanded to high-precision measurements of almost 1.7 billion stars.