The Gaia space probe, the European mission that aims to develop a detailed catalog with the position, distance and speed of more than a billion stars in our galaxy, has just found a surprising "disturbance" in the Milky Way. And it is that the galaxy in which we live is, still, suffering the effects of a past "encounter". In fact, millions of stars do not follow the "sweet and flat" orbit around the galactic center, as would be expected, but move in a similar way to water waves in a pond after throwing a stone. The surprising find is published this week in Nature.
"We made a graph of the z-coordinate (the height of the stars above or below the disk of the galaxy) versus the velocity Vz (velocity with which the stars move in the vertical direction on the disk) and, surprisingly, what appeared was a perfect spiral, similar to the shell of a snail, "the study's lead author, Teresa Antoja, a researcher at the Institute of Cosmos Sciences at the University of Barcelona (ICCUB), told the Sinc agency.
"So perfect it seemed to us the way we saw in the graphics of the computer - adds the researcher - that we thought that we could have been wrong in something, or that there would be some problem in the data. But after multiple checks and knowing that the Gaia data has gone through an exhaustive quality control, we came to the conclusion that this spiral was something real ".
The helical shape detected by the researchers means that the stars are not moving, as would be expected in an equilibrium galaxy, in simple circles around the center of the Milky Way, but rather follow more complex patterns.
These patterns were revealed because Gaia not only accurately measures the positions of more than a billion stars, but also their speeds in the plane of the sky. And for this subset of a few million stars, Gaia provided an estimate of the full three-dimensional velocities, which allowed a study of stellar motion using the combination of position and velocity.
«I was shocked»
When Teresa Antoja first observed the patterns on her computer screen, she could not believe what she was seeing. One form in particular strongly attracted his attention. It was a spiral pattern, similar to a snail, that emerged in the graph that traced the altitude of the stars above or below the plane of the Galaxy. Never before had seen anything like it.
"At the beginning - explains Antoja - the characteristics were very strange for us, I was shocked and I thought there could be a problem with the data". But the Gaia data had been subjected to multiple validation tests before launch. In addition, together with her collaborators, Teresa Antoja carried out multiple tests to look for errors that could explain the observed forms. When not detecting any error, the conclusion was that everything they were seeing was really happening.
The reason why the phenomenon had not been previously detected is simple: never before had the scientists had in their hands an instrument with the power and capacity of Gaia. "It's as if suddenly you put on the right glasses and you could see all the things you could not see before," says the researcher.
Once confirmed that the structures that reflected the data were real, it was time to find out what had caused them and why they were there. "It's like throwing a stone in a pond, and see how the water moves in waves and waves," says Antoja.
However, unlike water molecules, which end up settling down and recovering their original shape, the stars are able to retain a "memory" of what disturbed them. And that memory is precisely in their movements. With the passage of time, and although the waves are no longer easy to see in the distribution of the stars, it is still possible to locate them when studying their velocities.
Cannibal galaxy
The next question was to find out what it was that had hit the Milky Way for the stars to behave that way. From other research, we know that our galaxy is a "cannibal", which has been growing by "devouring" other smaller galaxies throughout its history. But that does not seem to be the case on this occasion.
Amina Helmi, from the Dutch University of Groningen and co-author of the study, had already done other work on a small galaxy, Sagittarius, which barely contains some tens of millions of stars and which is currently being cannibalized by the Milky Way. And it turns out that his most recent approach to our galaxy was not a direct hit, but it happened very close to it, almost touching it. Something more than enough for its gravity to disturb some millions of stars in the Milky Way, just as a stone thrown into a pond would disturb the water.
In addition, it is the case that this last close encounter of Sagittarius with the Milky Way occurred sometime between 200 and 1,000 million years ago, a time that coincides almost perfectly with the one that Teresa Antoja calculated for the origin of the spiral shape of the movement of the stars observed by Gaia.
Now, researchers want to study this galactic encounter in much more detail. "The discovery was easy," Amina Helmi says, "the interpretations are more difficult, and a full understanding of their meaning and implications could take several years."
The Milky Way, of course, has a very rich story to tell. And we are just beginning to read it.
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