Our Milky Way galaxy never sits still: it rotates and wobbles. And now, data from the European Space Agency’s Gaia space telescope reveal that our galaxy also has a giant wave rippling outwards from its centre.

We’ve known for about a hundred years that the galaxy’s stars rotate around its centre, and Gaia has measured their speeds and motions. Since the 1950s, we’ve known that the Milky Way’s disc is warped. Then in 2020 Gaia discovered that this disc wobbles over time, similarly to the motion of a spinning top. 

And now it has become clear that a great wave stirs the motion of stars in our galaxy over distances of tens of thousands of light-years from the Sun. Like a rock thrown into a pond, making waves ripple outwards, this galactic wave of stars spans a large portion of the Milky Way’s outer disc.

Even if no spacecraft can travel beyond our galaxy, Gaia’s uniquely accurate vision – in all three spatial directions (3D) plus three velocities (moving towards and away from us, and across the sky) – is enabling scientists to make these top-down and edge-on maps. 

From these, we can see that the wave stretches over a huge portion of the galactic disc, affecting stars around at least 30–65 thousand light-years away from the centre of the galaxy (for comparison, the Milky Way is around 100 000 light-years across).

“What makes this even more compelling is our ability, thanks to Gaia, to also measure the motions of stars within the galactic disc,” says Eloisa Poggio who is an astronomer at the Istituto Nazionale di Astrofisica (INAF) in Italy, and led the team of scientists that discovered the wave.

“The intriguing part is not only the visual appearance of the wave structure in 3D space, but also its wave-like behaviour when we analyse the motions of the stars within it.” 

“This observed behaviour is consistent with what we would expect from a wave,” Eloisa explains.

Think of a ‘wave’ performed by a crowd in a stadium. Given that galactic timescales are much longer than ours, imagine seeing this stadium wave frozen in time, much like how we observe the Milky Way. Some individuals would be standing upright, some would have just sat down (as the wave passed), and others would be preparing to stand up (as the wave approaches them).

In this analogy, the people standing upright correspond to the regions coloured in red in our face-on and edge-on maps. And, if we consider motions, the individuals with the largest positive vertical motions (represented by the largest white arrows pointing upwards) are those who are just starting to stand up, ahead of the incoming wave.

Eloisa and her colleagues were able to track down this surprising motion by studying the detailed positions and movements of young giant stars and Cepheid stars. These are types of stars that vary in brightness in a predicable way, which can be seen by telescopes like Gaia over large distances.

Because young giant stars and Cepheids move with the wave, the scientists think that gas in the disc might also be taking part in this large-scale ripple. It is possible that young stars retain the memory of the wave information from the gas itself, from which they were born.

Scientists do not know the origin of these galactic shakes. A past collision with a dwarf galaxy could be a possible explanation, but they need to investigate further.

The great wave could also be related to a smaller-scale rippling motion seen 500 light-years from the Sun and extending over 9000 light-years, the so-called Radcliffe Wave.

“However, the Radcliffe Wave is a much smaller filament, and located in a different portion of the galaxy’s disc compared to the wave studied in our work (much closer to the Sun than the great wave). The two waves may or may not be related. That’s why we would like to do more research,” Eloisa adds.

“The upcoming fourth data release from Gaia will include even better positions and motions for Milky Way stars, including variable stars like Cepheids. This will help scientists to make even better maps, and thereby advance our understanding of these characteristic features in our home galaxy,” says Johannes Sahlmann, ESA’s Gaia Project Scientist.

Source: ESA