Our galaxy is rippling thanks to a mysterious force — we might finally know what it is

An analysis of over 20 million stars shed new light on our galaxy's cannibalistic past.
Chris Young
An illustration of Milky Way
An illustration of Milky Way


A nearby mini-galaxy, the Sagittarius dwarf galaxy, slowly crashed through the Milky Way and ripped stars out of their regular orbits on more than one occasion, according to a new paper in the Royal Astronomical Society journal.

Researchers used data from the European Space Agency's Gaia space observatory to compare the movements of over 20 million stars located throughout our galaxy, as per a LiveScience report.

Their analysis shed new light on the galaxy's violent past — one in which galaxies tear into each other, shifting their structures for eons to come.

Ripples through space

The researchers focused primarily on the outer stretches of the Milky Way's disc, revealing a mysterious ripple that seems to have interfered with stars' orbital trajectories.

"We can see that these stars wobble and move up and down at different speeds," study author Paul McMillan, an astronomer at Lund University in Sweden, said in a translated statement.

The team modeled a wave pattern that may explain the way the ripple effect is warping our galaxy. Their method, dubbed "galactic seismology," led them to the conclusion that the ripples were likely released hundreds of millions of years ago.

To be precise, they were released at the same time that the Sagittarius dwarf galaxy last crashed through our galaxy — it has done so on more than one occasion, the researchers said, adding that the effect is somewhat like dropping a stone into a pond. Ripples from the collision slowly made their way to the outer reaches of the Milky Way, diminishing in intensity as they went.

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Investigating the Milky Way's cannibalistic past

Though older studies have suggested a collision with the Sagittarius dwarf galaxy — which is a satellite galaxy of the Milky Way — may have caused ripples in our galaxy's center, the new study is the first to show that these ripples reached the outer galaxy.

The Milky Way's interactions with other galaxies play a massive role in the shape and structure of the galaxy that is our home. A 2011 study, for example, suggested the Milky Way's spiral arm was formed due to collisions with the Sagittarius dwarf galaxy.

All in all, the new findings add to a growing body of knowledge about our own galaxy's cannibalistic past. The Milky Way is known to have ingested several smaller galaxies over billions of years, and it is estimated that it may one day be ingested into our neighboring galaxy Andromeda.

Study abstract:

The outer parts of the Milky Way's disc are significantly out of equilibrium. Using only distances and proper motions of stars from Gaia's Early Data Release 3, in the range |b| < 10°, 130° < ℓ < 230°, we show that for stars in the disc between around ten and 14kpc from the Galactic centre, vertical velocity is strongly dependent on the angular momentum, azimuth, and position above or below the Galactic plane. We further show how this behaviour translates into a bimodality in the velocity distribution of stars in the outer Milky Way disc. We use an N-body model of an impulse-like interaction of the Milky Way disc with a perturber similar to the Sagittarius dwarf to demonstrate that this mechanism can generate a similar disturbance. It has already been shown that this interaction can produce a phase spiral similar to that seen in the Solar neighbourhood. We argue that the details of this substructure in the outer galaxy will be highly sensitive to the timing of the perturbation or the gravitational potential of the galaxy, and therefore may be key to disentangling the history and structure of the Milky Way.

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