About 100 million light years away, two galaxies give astronomers a glimpse of the fate of the Milky Way.
So close that they are classified under one name, Arp 91, the spiral galaxies NGC 5953 and NGC 5954 are merging, with the matter of the second extending towards and into the first. Details of this merger can be seen in a new image from the Hubble Space Telescope.
Gradually, the two galaxies will join together, becoming a large elliptical galaxy, according to our models of these colossal cosmic interactions. This is how we expect the Milky Way to end as well, when it does eventually merge with our own closest galactic neighbor, the Andromeda spiral galaxy.
In fact, galactic mergers are not uncommon in the Universe. Space is big and you might think things wouldn’t collide with other things very often, but galaxies don’t drift in a sea of nothing. They are often connected by vast filaments of intergalactic gas, which can act as highways of matter along which galaxies are gathered across the void.
We’ve spotted many such galactic collisions, but they take place on a time scale of around a billion years, so a single collision won’t reveal the entire process. However, each is a snapshot of a moment in the process; by collectively studying the collisions, we can reconstruct the sequence of events, as shown in the 2016 simulation below.
Arp 91 is at a stage where the two galaxies have not yet been significantly disturbed; their spiral structures are still largely intact. However, their interaction triggered a star-forming explosion in both galaxies, as the incoming gas shocks into clouds of molecular star-forming gas, pushing it into denser clusters that s ‘collapse under their own mass to form baby stars.
In addition, both galaxies have active galactic nuclei; that is, the supermassive black holes at their centers actively devour matter. This process generates powerful black hole winds that push into the surrounding gas, which – you guessed it – generates shocks that trigger star formation. The two galaxies are therefore very busy places.
Eventually, the two will merge, their spiral structures dissolving into a type of bright, almost characteristic galaxy called an elliptical galaxy. This, however, is at least a few hundred million years away. Whether humanity will be there to see it is an open question.
The merger between the Milky Way and Andromeda is even further. Scientists predict that it will start to happen in about 4.5 billion years. We don’t have much to worry about, however; by then humanity will almost certainly be dead, extinct, or unrecognizable.
But isn’t it nice to know what will happen to the place after we leave?
You can download the Hubble image of Arp 91 in full resolution or as a wallpaper version from the ESA Hubble website.