Around 100 million light-years away, two galaxies are giving astronomers a sneak preview of the fate of the Milky Way.
So close that they are categorized under a single name, Arp 91, the spiral galaxies NGC 5953 and NGC 5954 are in the process of merging, with material from the latter extending towards and into the former. Details of this merger are visible in a new image from the Hubble Space Telescope.
Gradually, the two galaxies will join together, becoming one big elliptical galaxy, according to our models of these colossal cosmic interactions. That's how we expect the Milky Way to end up, too, when it eventually merges with our own closest galactic neighbor, the spiral galaxy Andromeda.
Arp 91. (ESA/Hubble & NASA, J. Dalcanton; Acknowledgement: J. Schmidt)
Actually, galactic mergers are not uncommon in the Universe. Space is large, and you might think that things wouldn't bump into other things terribly often, but galaxies are not adrift in a sea of nothing. They're often connected by vast filaments of intergalactic gas, which can act as matter highways along which galaxies are drawn together across the void.
We've spotted many such galactic collisions, but they take place on a time scale of around a billion years, so any one collision in isolation won't reveal the entirety of the process. However, each one is a snapshot of one moment in the process; by studying the collisions collectively, we can piece together the sequence of events, as seen in the 2016 simulation below.
Arp 91 is at a stage where the two galaxies haven't yet been significantly disrupted; their spiral structures are still largely intact. However, their interaction has triggered a burst of star formation in both galaxies, as inflowing gas generates shocks in clouds of molecular star-forming gas, pushing it into denser clumps that 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 are actively devouring material. This process generates powerful black hole winds that push out into the surrounding gas, which – you guessed it – generates shocks that trigger star formation. So the two galaxies are very busy places indeed.
Eventually, the two will merge, their spiral structures dissolving into a bright, nearly featureless type of galaxy called an elliptical galaxy. That, however, is at least a few hundred million years away. Whether humanity will be around to see it is an open question.
The merger between the Milky Way and Andromeda is even further. Scientists predict that it will start taking place around 4.5 billion years from now. We have very little to worry about, however; by that time, humanity will almost certainly be dead, gone, or unrecognizable.
But isn't it nice to know what will happen to the place after we're gone?
You can download Hubble's image of Arp 91 in full resolution or wallpaper versions from the ESA Hubble website.