Our home galaxy, the Milky Way, is far from static; it continuously rotates, warps, and wobbles. Now, groundbreaking data from the European Space Agency's (ESA) Gaia space telescope has unveiled an astonishing new feature: a colossal wave, tens of thousands of light-years long, emanating from the galaxy's core.
Astronomers have understood for a century that our galaxy's stars orbit its center, with Gaia providing unprecedented precision on their speeds and trajectories. The Milky Way's disc has been known to be warped since the 1950s, and in 2020, Gaia further revealed that this disc exhibits a precession, akin to a spinning top's unsteady motion.
The latest revelation confirms that a magnificent wave actively influences the movement of stars across vast stretches of the galaxy, extending outwards for tens of thousands of light-years from the Sun. Visualizing this phenomenon, one might imagine a stone dropped into a pond, creating ripples that spread across the surface – similarly, this galactic stellar wave traverses a significant portion of the Milky Way's outer disc.
Remarkable illustrations, derived from Gaia's comprehensive maps, vividly depict this unexpected galactic undulation. These figures plot the positions of countless bright stars in contrasting red and blue, overlaid on the galaxy's structure. When viewed from 'above,' the Milky Way's face-on appearance showcases the wave. A vertical cross-section, offering a 'side-on' perspective, reveals the disc's established warp (curving upwards on one side, downwards on the other). The newly identified wave is delineated by red and blue zones, indicating stars positioned either above (red) or below (blue) the warped galactic plane.
While interstellar travel beyond our galaxy remains a distant dream, Gaia's unparalleled precision in measuring stellar positions in three dimensions, coupled with their velocities (towards/away from us and across the sky), empowers scientists to craft these intricate 3D and edge-on galactic maps. These detailed maps confirm the wave's immense scale, affecting stars situated roughly 30,000 to 65,000 light-years from the galactic center – a substantial fraction of the Milky Way's approximately 100,000-light-year diameter.
"The ability, thanks to Gaia, to also measure the motions of stars within the galactic disc makes this even more compelling," states Eloisa Poggio, an astronomer at Italy’s Istituto Nazionale di Astrofisica (INAF) and the lead scientist behind this discovery. "The truly captivating aspect isn't merely the visual evidence of this wave structure in 3D space, but also its undeniable wave-like characteristics when we scrutinize the movements of the stars involved."
White arrows superimposed on the edge-on view of the Milky Way illustrate these stellar motions. Intriguingly, the vertical motion pattern of the stars (indicated by the arrows) appears slightly offset horizontally compared to the wave pattern defined by the stars' vertical positions (the red/blue coloration). "This observed behavior perfectly aligns with what we would anticipate from a wave," Poggio clarifies.
Consider the familiar 'wave' created by a crowd in a stadium. Due to the vast timescales of galactic events, imagine this stadium wave frozen in time, mirroring our observation of the Milky Way. Some individuals would be standing, others would be just sitting down (as the wave passed), and some would be preparing to stand (as the wave approached). In this analogy, the standing individuals correspond to the red regions in our galactic maps. Crucially, those preparing to stand – exhibiting the largest upward vertical motions – represent stars ahead of the incoming galactic wave.
Dr. Poggio and her team meticulously traced this extraordinary motion by analyzing the precise positions and movements of young giant stars and Cepheid variable stars. These stellar types, known for their predictable brightness variations, are visible to telescopes like Gaia across immense cosmic distances. The observation that these stars move in concert with the wave suggests that the gas within the disc might also be participating in this grand ripple, possibly indicating that young stars retain a 'memory' of the wave from the gas clouds where they originated.
The exact cause of these galactic tremors remains an open question for scientists. A past collision with a smaller dwarf galaxy is one plausible hypothesis, but further investigation is needed. Another possibility is a connection to the Radcliffe Wave, a smaller-scale rippling motion detected closer to the Sun, spanning 9,000 light-years. "However, the Radcliffe Wave is a more confined filament, located in a distinct region of the galactic disc compared to our great wave," Poggio notes. "Whether these two waves are related is uncertain, prompting further research."
Johannes Sahlmann, ESA’s Gaia Project Scientist, anticipates the forthcoming fourth data release from Gaia. "This will deliver even more refined positions and motions for Milky Way stars, including variable stars like Cepheids. Such data will enable scientists to construct even more detailed maps, thereby advancing our comprehension of these remarkable features within our own galaxy." This monumental discovery, ‘The great wave: Evidence of a large-scale vertical corrugation propagating outwards in the Galactic disc’ by E. Poggio et al., has been published in the scientific journal Astronomy and Astrophysics.
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Originally published at: https://www.esa.int/Science_Exploration/Space_Science/Gaia/Gaia_discovers_our_galaxy_s_great_wave
