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Our galaxy, the Milky Way, is still suffering from the effects of a disturbance caused by the passage of another galaxy in its vicinity that has created the movement of millions of stars, much like the ripples of the galaxy. A pebble on a pond.
Gaia, the star mapping mission of the European Space Agency (ESA), has shown that this celestial collision probably took place over the past 300 to 900 million years. Astrophysicist Teresa Antoja, of the University of Barcelona in Spain, and her European colleagues came to this conclusion after analyzing the movement of stars in the Milky Way disk – one of the main components of the galaxy.
Many studies have already shown that the appearance of the Milky Way was the result of celestial collisions. For example, in 2011, American astronomer Chris Purcell and his colleagues at the University of California claimed that the spiral arms of the galaxy would have deployed after two impacts with the Sagittarius dwarf galaxy.
The model revealed by the Gaia probe not only establishes the positions of 1.3 billion stars, but also accurately measures their velocity in the celestial plane. For a subset of a few million stars, Gaia even provided an estimate of complete three-dimensional velocities, allowing to analyze stellar motion by combining position and velocity, a concept known as 39, phase space.
Launched in December 2013, this European satellite has been scanning the light sources of our galaxy since 2014. Positioned 1.5 million kilometers from the Earth, it executes no less than 500 million measurements per day. His catalog contains the positions and brilliance of 1.7 billion stars. Thus, in phase space, stellar motions revealed an interesting and totally unexpected pattern of star positions as a function of their velocity.
Teresa Antoja did not believe her eyes when she saw them for the first time on her computer screen. One form in particular caught his attention. It was a shell-snail model that showed the altitude of the stars above or below the plane of the galaxy according to their speed in the same direction. This had never been observed before.
"I was a little surprised and I thought that there might be a problem with the data because the shapes are very clear," she says.
It should be noted that the data collected by Gaia have been subjected to numerous validation tests by the Gaia Data Processing and Analysis Consortium teams. Ms. Antoja and her collaborators also did some tests to try to understand the errors that could be behind such forms. However, no matter what they checked, the only conclusion to draw is that these features are real.
The existence of this structure being confirmed, the authors of the study published in the journal Nature wanted to understand where it came from. Our Sun is located on one of the spiral arms, about 27,000 light-years away (1 AL = 9460 billion km) from the center of the Milky Way. It's like throwing a stone into a pond, which moves the water like waves and waves.
However, unlike the water molecules that settle, the stars retain a kind of memory of their disturbance. This memory can therefore be observed in their movements. Better still, after millions of years, even though the ripple is no longer easily visible in the distribution of stars, it is still noticeable when analyzing the speed of stars.
Astrophysicists have long since established that our neighbor, the Andromeda galaxy, is heading straight for the Milky Way and that a collision between the two celestial monsters will occur in about 4 billion years. Computer simulations developed using Hubble data suggest that it will take 2 billion years for the merger to be complete.
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