Merging galaxies: the bigger guy wins

As part of our Lab Talk series, Luke Davies explains what happens when galaxies collide.

Photo of the Milky Way which is on course to eventually merge with the Andromeda galaxy. – Miles Bowers Photography / GETTY IMAGES

Don’t panic, but the Andromeda galaxy and our Milky Way are on a collision course. They are hurtling towards each other at around 400,000 kilometres per hour, with the big smash set to take place in four billion years’ time.

We won’t be around to investigate that cosmic collision, but in the meantime there are thousands of merging galaxies we can study. They have been observed as part of the global Galaxy And Mass Assembly survey and they are showing us how galaxies grow and evolve.

Previously, astronomers thought that when two galaxies smash into each other, their gas clouds – the nurseries were stars are born – get churned up and seed the birth of new stars much faster than if they remained separate. But the survey suggests that this is too simplistic.

When two galaxies of similar mass collide, both increase their stellar birth rate. However if one galaxy significantly outweighs the other, it’s more like winner takes all. The more massive galaxy has a fertility spurt and begins rapidly forming new stars, whereas the smaller galaxy will struggle to make any at all. This is probably because the bigger galaxy strips away its smaller companion’s gas, leaving it without star-forming fuel.

The process is not unlike collisions that take place on Earth. If two military tanks hit each other, both come out of it reasonably well. Similarly, when two bicycles collide both will come out of the encounter relatively lightly. But if a tank were to hit a bicycle, we know exactly which one would come out the worse for the encounter. This, in principle, is the same with galaxies – the big guy stops the little guy in its tracks.

So what will happen in four billion years?

The Milky Way and Andromeda are both cosmic tanks – they are both relatively large with similar mass. Even at 2.5 million light-years apart, they probably already affect each other’s star formation. As they get closer that effect will continue and increase, until they eventually merge into a new galaxy some call “Milkdromeda”.

Luke Davies is an astronomer at the International Centre for Radio Astronomy Research in Perth, Australia.