Our Milky Way has almost collided with another galaxy | Space

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How do we know the history of our own galaxy of the Milky Way? One way is to observe the current Star movements of the Milky Way (or as current as possible, given the finite speed of light). A revolution in our ability to follow the movements of the stars of the Milky Way began at the end of 2013, with the launch of the Gaia mission of the European Space Agency. His job is to scan the sky several times, observing each of his targets. billion stars averaged 70 times during the five-year mission. In this way, Gaia will see exactly how these stars move. in the long run, scientists want to use this data to build a 3D map of our galaxy. Meanwhile, with each new publication of Gaia data, astronomers discover new information about our galaxy. Gaia's data revealed a near-collision between our Milky Way and a small galaxy hundreds of millions of years ago.

The new work is based on Gaia's second publication of data. This shows that some stars in our Milky Way galaxy are moving "like ripples on a pond," said the astronomers, because of this long-running collision.

The deadline for the close encounter is about 300 to 900 million years. It's relatively recent history, astronomically speaking.

The culprit could be the dwarf galaxy Sagittarius, one of the dozens of small galaxies known to accompany our largest galaxy in space. The Milky Way is cannibalizing this small galaxy, which contains only a few tens of millions of stars, unlike the 100 billion stars of our galaxy.

Teresa Antoja from the University of Barcelona, ​​Spain, led the research. She says:

At first, the features were very strange for us. I was a little shocked and I thought that there might be a problem with the data because the shapes are very clear.

A statement from the Gaia mission explained:

The pattern was revealed because Gaia not only accurately measures the positions of more than a billion stars, but also precisely their speed on the plane of the sky. For a subset of a few million stars, Gaia provided a full three-dimensional velocity estimate, which allowed studying stellar motion using the combination of position and velocity, called "phase space".

In the phase space, stellar motions revealed an interesting and totally unexpected pattern when the positions of the star were plotted against their velocity. [Antoja] could not believe her eyes when she saw it for the first time on her computer screen.

One form in particular caught his attention. This is a snail-shell-like pattern in the graph that represents the altitude of the stars above or below the galaxy plane as a function of their speed in the same direction.

This had never been seen before.

But, according to these astronomers, the Gaia data have been subjected to multiple validation tests. They said the only conclusion to draw was that these features existed in the vast space of our Milky Way. And they believe that the structures they see come from a near-collision. Antoja explained:

It's a bit like throwing a stone into a pond that moves water in the form of waves and waves.

Unlike the water molecules, which are reinstalling, the stars retain a "memory" according to which they were disturbed, these astronomers declared:

This memory is in their movements. After a while, although the ripples may not be easily visible in the distribution of stars, they are still present when you look at their speeds.

Why have not astronomers seen them before? Technology was not advanced enough to make this happen. Antoja said:

Seems like, suddenly, you put on the right glasses and you see everything that was impossible to see before.

Conclusion: a sub-structure of snail-shaped stars in our large galaxy, the Milky Way, indicates that the Milky Way is still experiencing the effects of a near-collision that has moved millions stars, such as ripples on a pond.

Source: A dynamic disk of the young and disrupted Milky Way

Via ESA