Our Milky Way is always in motion: it spins, it tilts, and, as latest observations reveal, it ripples. Data collected by the European Space Agency’s Gaia space telescope show that our galaxy just isn’t only rotating and wobbling but additionally sending out an unlimited wave that travels outward from its center.
For a couple of century, astronomers have known that the Milky Way’s stars orbit its core, and Gaia has precisely tracked their speeds and trajectories. For the reason that Nineteen Fifties, scientists have also recognized that the galactic disc just isn’t flat but warped. Then in 2020, Gaia uncovered that this warped disc slowly oscillates over time, much like the motion of a spinning top.
Now, researchers have identified an infinite wave that moves through the Milky Way, influencing stars tens of 1000’s of light-years from the Sun. The phenomenon is sort of a rock dropped right into a pond, where the resulting ripples spread outward — only here, the “ripples” are manufactured from stars, stretching across the galaxy’s outer regions.
The newly revealed wave is illustrated within the figure above. Hundreds of brilliant stars, shown in red and blue, are overlaid on Gaia’s detailed map of the Milky Way.
Within the image on the left, we see our galaxy from a top-down perspective. The best panel shows a side view, cutting vertically through the galactic plane. From this angle, the left portion of the galaxy curves upward while the best side bends downward (that is the warp of the disc). The red and blue regions mark the newly discovered wave: red areas indicate stars positioned above the warped plane, while blue areas show stars lying below it.
Although no spacecraft can enterprise beyond the galaxy, Gaia’s remarkably precise measurements — covering all three spatial dimensions (3D) and three components of motion (toward and away from us, and across the sky) — allow scientists to construct these top-down and edge-on views of the Milky Way.
These maps reveal that the wave extends over an unlimited section of the disc, affecting stars positioned about 30,000 to 65,000 light-years from the galactic center (the Milky Way itself measures about 100,000 light-years across).
“What makes this much more compelling is our ability, due to Gaia, to also measure the motions of stars throughout the galactic disc,” says Eloisa Poggio who’s an astronomer on the Istituto Nazionale di Astrofisica (INAF) in Italy, and led the team of scientists that discovered the wave.
“The intriguing part just isn’t only the visual appearance of the wave structure in 3D space, but additionally its wave-like behavior once we analyze the motions of the celebs inside it.”
Within the edge-on view of the Milky Way linked below (“The Milky Way’s great wave in motion”), white arrows show how the celebs move. The vertical motion of the celebs (represented by these arrows) is barely shifted sideways in comparison with the pattern of their positions (shown by the red and blue colours).
“This observed behavior is consistent with what we might expect from a wave,” Eloisa explains.
She compares the phenomenon to a stadium crowd performing a wave. If we could freeze that moment in time, some people could be standing upright, others would just have sat down (after the wave passed), and a few could be about to face (because the wave approaches). Galactic timescales are far longer, however the principle is analogous.
On this comparison, the people standing upright correspond to the red regions in Gaia’s maps, while those about to rise — moving upward with the best vertical speed — are represented by the longest white arrows pointing up, just ahead of the wave’s crest.
Eloisa and her team detected this remarkable motion by fastidiously studying young giant stars and Cepheid stars, each of which vary in brightness in predictable ways in which make them easy for Gaia to look at across large distances.
Because these stars seem to maneuver with the wave, the researchers suspect that gas within the galactic disc may additionally take part in this large-scale motion. Newly formed stars could retain information from the gas they were born from, preserving a form of “memory” of the wave.
The reason behind the galaxy’s vast oscillations continues to be uncertain. One possibility is that the Milky Way experienced a past encounter or collision with a smaller, dwarf galaxy, but further evaluation is required to substantiate this.
This newly found “great wave” may also have some link to a smaller undulating structure often called the Radcliffe Wave, which lies roughly 500 light-years from the Sun and extends about 9,000 light-years across.
“Nonetheless, the Radcliffe Wave is a much smaller filament, and positioned in a distinct portion of the galaxy’s disc in comparison with the wave studied in our work (much closer to the Sun than the good wave). The 2 waves may or is probably not related. That is why we would really like to do more research,” Eloisa adds.
“The upcoming fourth data release from Gaia will include even higher positions and motions for Milky Way stars, including variable stars like Cepheids. This may help scientists to make even higher maps, and thereby advance our understanding of those characteristic features in our home galaxy,” says Johannes Sahlmann, ESA’s Gaia Project Scientist.