spaceSpace and Physics

The Milky Way’s Warp Might Be Due To An Ongoing Interaction With A Nearby Galaxy


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockMar 3 2020, 15:15 UTC

The galactic disc of the Milky Way, our galaxy, is not flat but warped upwards on one side and downwards on the other. The two smaller galaxies in the lower right corner are the Large and Small Magellanic Clouds, two satellite galaxies of the Milky Way.  Stefan Payne-Wardenaar; Magellanic Clouds: Robert Gendler/ESO

A lot of the stars in the Milky Way are distributed in the thin disk, the region of space occupied by our galaxy's spiral arms. Recently researchers found that the disk is slightly warped at the edges, and new data suggest that this twist might be due to a recent or even ongoing interaction with a nearby galaxy.

Using the European Space Agency's Gaia Observatory data release from 2018, astronomers tracked the position and motion of 12 million giant stars to measure how fast the warp goes around the galaxy. They discovered it changes orientation over time, wobbling like a spinning top, and completes a full rotation much more quickly than expected. Their work is published in Nature Astronomy.


“We measured the speed of the warp by comparing the data with our models. Based on the obtained velocity, the warp would complete one rotation around the centre of the Milky Way in 600 to 700 million years,” lead author Eloisa Poggio of the Turin Astrophysical Observatory, Italy, said in a statement. “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.”

Both the direction in which the warp moves and how quickly it is happening have offered important clues on what might have caused it. The speed it's going is faster than the intergalactic magnetic field would allow, suggesting it must be something powerful like a collision. The culprit has long been considered an interaction with one of the Milky Way's satellite galaxies, and most likely a recent or ongoing encounter rather than an ancient one.

The researchers are pointing the finger in particular at the Sagittarius dwarf galaxy. This little galaxy has already burst through the disk several times in the past and is being slowly but steadily unspooled by the Milky Way.

This is not a photograph but a density map of stars from Gaia where the presence of the stream of stars from the Sagittarius dwarf is clearly visible. ESA/Gaia/DPAC, CC BY-SA 3.0 IGO

While the warp is significant, it's also twice as far from the galactic center than the Milky Way so there is no need to worry about what it might mean for our Solar System.

“The Sun is at the distance of 26 000 light-years from the galactic centre where the amplitude of the warp is very small,” Poggio said. “Our measurements were mostly dedicated to the outer parts of the galactic disc, out to 52 000 light-years from the galactic centre and beyond.”

The Gaia spacecraft is currently preparing the most accurate map of the Milky Way, measuring the precise position and velocity of over a billion stars. After its first data release back in 2016, and the second one in 2018, a new data release is expected at some point this year.

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