Milky Way's Strange Warped Shape Could Be Caused by an Ongoing Collision With Another Galaxy

The Milky Way has a strange warped shape, and scientists now believe this may be the result of a relatively recent or ongoing collision with another galaxy.

While you may think of the Milky Way as a flat disk, it is actually shaped almost like an "S." Its outer edges appear distorted compared to the inner flat disk, curving slightly upwards at one end, while the other end curves downwards.

The warp is visible in the distribution of stars, dust and gas in the galaxy. The orientation of this warp—or its "precession" as astronomers call it—also changes over time.

Researchers have long debated what the cause of this warping is, and several hypotheses have been proposed to explain it. This includes the influence of a ring of dark matter surrounding the galaxy, or the intergalactic magnetic field.

However, analysis of data collected by the European Space Agency's (ESA) Gaia space observatory has revealed the precession rate is much faster than what we would expect to see if dark matter or the intergalactic magnetic field were causing it. Findings are published in the journal Nature Astronomy.

"We measured the speed of the warp using precise measurements from Gaia satellite for millions of stars. Based on our results, the warp is moving very fast, completing one rotation around the Galactic Center in 600 to 700 million years," lead author Eloisa Poggio, from the Turin Astrophysical Observatory, Italy, told Newsweek. "For this reason, something more powerful—like an interaction with a satellite—would be needed to explain our results."

In light of this finding, the researchers say that the most likely explanation for such a high warp speed is the Milky Way colliding with another galaxy—an event that would involve incredibly powerful gravitational forces capable of distorting the shape of the galactic disk.

It is not clear which galaxy could be behind the distorted shape, or when such a collision could have taken place, according to the scientists. A potential candidate is the dwarf galaxy Sagittarius, which orbits the Milky Way and is located around 65,000 light-years from Earth.

This galaxy is thought to have had encounters with the Milky Way in the past. For example, a study published in the journal Nature in 2018 suggested that Sagittarius and the Milky Way nearly collided between 300 and 900 million years ago.

Astronomers predict that the Milky Way will eventually consume Sagittarius entirely.

According to the authors of the latest study, our solar system is relatively unaffected by the warping of the Milky Way because of its location, away from the edges. "The sun is at the distance of 26,000 light-years from the Galactic Center 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 center and beyond."

Milky Way warped shape
This illustration shows the warped structure of the Milky Way. Stefan Payne-Wardenaar; Inset: NASA/JPL-Caltech; Layout: ESA

The researchers say the latest findings were only made possible by the unprecedented data that has been collected by Gaia. The goal of the Gaia mission—which began six years ago—is to create the largest, most detailed three-dimensional map of the Milky Way by surveying around one percent of its estimated 100 billion stars. For each of these stars, the space observatory will accurately measure their location and motion around the Milky Way's Galactic Center.

"With Gaia, for the first time, we have a large amount of data on a vast amount stars, the motion of which is measured so precisely that we can try to understand the large scale motions of the galaxy and model its formation history," Jos de Bruijne, Gaia deputy project scientist, said in a statement. "This is something unique. This really is the Gaia revolution."

Milky Way
Stock photo: The Milky Way as seen from the Earth. iStock