Astronomers have identified patterns within the motion of stars stretching across the Milky Way, hinting at the presence of a vast wave.
The Milky Way appears to have a vast ripple spreading out across at least a quarter of its disc. If confirmed, the structure might be a relic from a brush with another galaxy, but not all astronomers are convinced that it actually exists.
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| Was the Milky Way set rippling by another galaxy? Buradaki/Alamy |
While illustrations normally show our galaxy as a flat disc with a bulge in the middle, its structure is much more complicated. Astronomers have mapped undulating clouds of stars and gas that create waves across small sections of the Milky Way. They have also found that the disc of the galaxy isn’t actually flat, but warped in a static S-shape, as seen from a side-on view.
Now, Eloisa Poggio at the French National Centre for Scientific Research in Nice and her colleagues have found that the Milky Way appears to contain a faint ripple propagating away from its centre, according to the motion of tens of thousands of stars observed by the Gaia space telescope. “It’s not huge, it’s just a small ripple,” says Poggio. But unlike previously discovered waves, this ripple spreads out over a vast section of the galaxy, she says.
Poggio and her team took the movements of two different groups of young stars, which formed recently enough to be moving with their surroundings, and compared both their vertical and horizontal positions and movement with a model of the galaxy depicting our current understanding of its shape, with the static S-shaped warp.
They found two patterns of motion: stars moving back and forth along the radial axis of the galaxy, and up and down as seen from the side of the galaxy. Both patterns were statistically related, suggesting they might be part of one vast wave. It is difficult to know what exactly may have caused the ripple, but a past close encounter with a smaller galaxy’s gravity could be responsible, says Poggio.
But Ralph Schoenrich at University College London says the two patterns aren’t necessarily linked. There could be two waves, one moving horizontally and one vertically, which may have been caused by the same, or separate, events, but it isn’t clear whether they are part of one overall wave, he says. More careful analysis of how these observations fit with our current models of the Milky Way’s warp will be needed until we can figure out the nature of the wave, he says.
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