How do galaxies collide

The collision between the Milky Way and the Andromeda Galaxy has actually already begun

The two largest galaxies of the Local Group, the Milky Way and the Andromeda Galaxy, race towards each other at a speed of 110 kilometers per second. According to astronomers, a collision is inevitable, but it is not a catastrophe scenario. Star collisions will only occur in extremely rare exceptional cases - if at all. There is simply too much space between the stars for that, even in the "densely packed" regions of a galaxy.

But what will be destroyed are the previous structures. So far, both are spiral galaxies. If they unite - and possibly also take the smaller spiral galaxy M 33, the triangular nebula, with them - an elliptical galaxy will most likely form out of the common mass. This type of galaxy is not as common in the universe as spirals, but many of the particularly large ones belong to it.

Where do you start?

According to previous calculations, the time of the collision is estimated to be around 4.5 billion years in the future. This would at least save us the disgrace of having to call ourselves residents of the galaxy "Milkomeda". Astronomers at Harvard University and the Smithsonian Institution put this name into circulation (and their alternative, Milkdromeda, isn't exactly any better).

Most recently, however, colleagues from the universities of Yale and Notre Dame have come on the scene, who are designing a slightly different time horizon: According to their measurement results, one could definitely say that the collision between the Milky Way and Andromeda has already begun. It all depends on where you place the outer boundary of a galaxy.

Unclear borders

The drawing of boundaries is often not a very clear matter in the cosmos. It already begins in our immediate surroundings: The exosphere, the outermost layer of the earth's atmosphere, does not stop at a certain point, but simply becomes thinner and thinner and thus merges smoothly into the void of interplanetary space.

The outer boundary of galaxies is not so easy to determine either. Around the disk, which is usually regarded as "the actual galaxy", lies the gigantic halo: an almost spherical region in which globular clusters and star currents are the only clearly visible elements. In addition, there are huge accumulations of gas and probably dark matter - and all of this also belongs to the gravitational catchment area of ​​a galaxy.

A team led by Nicolas Lehner from the University of Notre Dame in Indiana has now subjected the Andromeda halo to the most precise measurements yet. The researchers came to the conclusion that Andromeda's halo extends 1.3 million light years in the direction of the Milky Way. That is half the distance between the ("actual") galaxies. Lehner assumes that the same applies to the halo of the Milky Way - therefore one could say that the spheres of influence of the two galaxies already collide. In some regions, Andromeda's halo extends two million light years into intergalactic space.

The researchers also found that this halo consists of two layers that lie inside each other like onion skins. In the outer one, the gas is distributed fairly evenly, while in the inner one it is much more dynamic. Astronomers attribute this to supernova activity, which threw masses of material out of the galactic disk. The halo was thereby inoculated with heavy elements. For example, the signatures of carbon, oxygen and silicon could be determined.

The thin medium of the halo hardly emits any radiation that could be registered. In order to measure the halo, the researchers therefore had to use a trick. As part of the AMIGA ("Absorption Map of Ionized Gas in Andromeda") project, they analyzed the UV light from 43 distant quasars that are located behind the Andromeda Galaxy, as seen from our point of view. Where it crosses the sphere of the halo, it is partially absorbed - from which it can then be inferred which material is in the way there. And how far it goes.

If Andromeda's halo were visible, it would already make out the largest structure in the night sky, calculates NASA's Goddard Space Flight Center - easily three times the size of the Big Dipper. For now, without any technical assistance, we can only see a small cigar-shaped spot of light in its center, the galactic disk of Andromeda. But it is catching up at 110 kilometers per second and will in turn become the dominant phenomenon in the night sky over the next billions of years. (jdo, September 14, 2020)