Astrophysics: Collisions with the Milky Way

If you look at the history of its formation, our Milky Way is actually more of a milkshake road: The spiral galaxy with a dense central area, powerful arms and a spherical surrounding area, the halo, in which a good 150 globular star clusters rotate their orbits, was formed over the course of its 13.6 billion years Shaken and stirred together from a number of galactic ingredients over a long period of development.

Galaxies interact gravitationally with their neighboring galaxies. Depending on the distance, difference in size and their relative speeds, encounters can lead to a cautious grazing past, a much more dynamic crossing or even a complete merger of the galaxies. Under certain circumstances, this can result in a completely different shape. It is assumed, for example, that today’s structureless elliptical giant galaxies were formed from the merger of disk or spiral galaxies.

New stars are formed

During galactic collisions, there are hardly any collisions between individual stars – the distance between the celestial bodies is too great. However, the interstellar gas and dust accumulations penetrate each other and more stars can form in the hot, dense shock fronts. The new galaxy not only differs in mass and structure, but can also change its luminosity and color due to the large number of newly formed stars.

Cosmological models assume that galaxy collisions were quite common in the early universe. Today, only one or two galaxies in every 100 are in the process of merging – like NGC 4676, also known as “the mice”: a pair of interacting galaxies where a collision left behind two long, conspicuous trails of gas and dust that are connected to something Remind your imagination of mouse tails.

There are also signs of several interactions with slightly larger and smaller galaxies in the Milky Way. These were completely torn apart and all of their matter was swallowed by the young Milky Way – which increased in mass each time but retained its spiral shape.

The chemical properties and orbits of the globular clusters provide initial information about the type, size and mass of the collision partners and when the interaction occurred.

Until now, it was assumed that the Milky Way’s last major collision with another galaxy was between eight and eleven billion years ago. However, using data from the Gaia space probe, which, among other things, measures the positions and directions of movement of more than a billion stars in the Milky Way with unprecedented precision, astronomers have now recognized evidence of a much younger collision.

It was found that clusters of stars in the Milky Way’s halo form a striking “cup-shaped” pattern. This indicates that a smaller galaxy passed through the center of the Milky Way and was torn apart by the tidal forces acting there.

Last collision younger than expected

And just as a spoonful of cocoa in a cup of milk can initially be seen as lumps and streaks and only mixes more evenly over time, disturbances in galaxies caused by interactions are constantly becoming equal to their surroundings. However, Gaia’s data sets show that the observed “star shells” are still clearly structured – that is, that not enough time has passed for them to mix evenly with the environment.

In their studies, astronomers come to the conclusion that the event known as the “Virgo radial merger” could have only occurred three billion years ago – and that another chapter needs to be added to the history of the evolution of our home galaxy.

Milky Way headed for Andromeda

And there is already an outlook. The Milky Way and its largest neighbor, the Andromeda Galaxy, are moving more or less directly towards each other at around 120 kilometers per second. Although it is difficult to make very specific predictions about the merger in a few billion years, astronomers have already thought of a name for the result: “Milkomeda” will be the name of the future, elliptical Andromeda-Milky Way cocktail, which is around two trillion solar masses strong.