We finally know what causes the Milky Way’s mysterious “waves.”

These ripples come from the impact of a massive object, such as a pond, where a stone has been thrown.

Looking at the galaxies in the images brought back by the various space agencies, we could easily believe that these structures of astronomical proportions are relatively fixed; but if they consist of objects themselves extremely massive with lifetimes much longer than those of humans, that does not mean they are amorphous.

Far from there; they are extremely dynamic objects, although it is difficult to perceive these phenomena on our scale. Between the gravitational forces emanating from the various celestial bodies, the chemical reactions that take place there and the influence of other objects in the cosmos, there is a whole many external factors that can disturb the tranquility of these enormous objects.

And our good old Milky Way is no exception. It even presents a rather strange phenomenon. In pictures, it traditionally appears as a nice flat and even disc, but the reality is very different. In reality, the central part is constantly shaken by large-amplitude waves – kind of like a pond where a large stone would have been dropped.


A clash between cosmic giants

And this analogy is not chosen by chance. It even corresponds surprisingly well to an international research team’s new theory; its members explain that they have succeeded in determining the origin of these oscillations, which until now remained mysterious. They suggest that the only scenario that would achieve these ripples involves the passage of a massive object — except here, ” Pierre the one in question was about 400 million times heavier than the Sun!

To arrive at this hypothesis, the researchers started from studies centered around a rather popular hypothesis among specialists. In fact, most cosmological models accept the fact that our galaxy would have been started by repeated passages of another dwarf galaxy, dubbed Sagittarius. Crossing the Milky Way from side to side at full speed, the latter would have behaved like the famous rock described above; it is these impacts that would have given rise to the ripples still observed today.

A promising hypothesis, but there was a catch: cosmological models suggest that if this dwarf galaxy had passed through ours in this way, these vibrations would probably extend to its limit. However, this phenomenon had never been directly observed…until now.

A representation of the ballet that Sagittarius and the Milky Way engage in. © ESA

Of ” high-flying galactic seismology

The researchers relied on readings from the Gaia satellite, an ESA machine that records the movement of millions of celestial bodies relative to the center of the Milky Way. He thus seeks to construct a gigantic catalog of the cosmos. Very recently, this data has already enabled astronomers to locate the closest black hole to Earth (see our article).

In this specific case, they compared the motions of about 20 million stars distributed throughout the Milky Way, paying particular attention to those located in the periphery. And it turns out they did discovered the famous ripples all the way to the edge of our galaxy. ” We see that these stars wave up and down at different speeds says Paul McMillan, an emeritus astronomer affiliated with the prestigious University of Lund, Sweden.

They then sought to confirm that their observations were indeed consistent with the passage of Sagittarius. To do this, they engaged in an exercise they describe as “ galactic seismology “. It is not an official or even a common term in the scientific literature, but it has the advantage of being quite explicit.

They worked much like geophysicists who study the wave profile associated with earthquakes, but on an astronomical scale; here astronomers have constructed one extensive computer simulation which represent the current ripples as they have observed them through Gaia. They then extrapolated from this data to see how this phenomenon might have evolved over time.

And this approach has proven to be successful; their model, the current state of the ripple was consistent with the passage of Sagittarius several hundred million years ago, when this now-dying galaxy was far more massive.

For scientists, this confirmation is very interesting, because it lays a very solid foundation that will make it possible to enrich and strengthen a lot of other work that is interested in the interaction between celestial bodies and the global dynamics of the cosmos.

LiveScience, for example, recalls a study also carried out thanks to Gaia and published in 2020; its authors suggest that the collision between the Milky Way and Sagittarius would have given rise to enormous ” baby boomers “starthat is, the birth of an incredible number of stars in a very short time near the point of impact.

Work of this kind is very important for understanding what mechanisms have made it possible to define the current architecture of the universe, and it will therefore be very interesting to follow the fallout from it.

The text of the study is available here.

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