When we think of celestial threats to our planet, we usually think of big asteroids and comets, and maybe the odd gamma ray burst or supernova. What we probably wouldn't think of is an entire galaxy bearing down on us, but according to a new study, that's exactly what's happening right now. The Large Magellanic Cloud (LMC), a nearby dwarf galaxy, is on a collision course with the Milky Way, but there's no need to worry just yet – the starry smashup won't begin for another two billion years or so.
The Large Magellanic Cloud is one of our closest cosmic neighbors, orbiting the Milky Way at a distance of about 163,000 light-years. It's long been thought that this dwarf galaxy would either continue to whizz around the Milky Way for billions of years yet, or zip off into the cosmos thanks to its speedy movements.
But on closer inspection, astronomers have realized that a third scenario may be more likely. Recent studies have found that the LMC is much more massive than it looks, containing almost twice as much dark matter as was previously thought. That means it's quickly losing energy, slowing down and being pulled towards the Milky Way.
To predict what happens next, a team led by researchers at Durham University used a supercomputer called Evolution and Assembly of GaLaxies and their Environments (EAGLE). This allows them to build realistic models of galaxies like the Milky Way and the LMC, and smash them together to see what happens.
These kinds of galactic collisions occur regularly, cosmically speaking, and in this case the two would merge into one galaxy. Basically, the LMC as we know it is toast, but its stars and planets would live on as part of a more inflated version of the Milky Way.
But that's not to say it won't be a bumpy process. The team says the gravitational effects of the collision would hurl many stars out of the Milky Way's main disk and into the stellar halo, a sparsely-populated bubble that surrounds the galaxy. There's a tiny chance the Sun could be one of them, but the researchers say our position in the galaxy would probably protect us from that fate.
The most dramatic effects will be seen in the center of the galaxy, and specifically in the supermassive black hole that resides there. It will feast on all the extra gas and matter that the LMC brings in, swelling as much as 10 times its current size. That could turn the Milky Way into an Active Galactic Nucleus or a quasar, but luckily the gamma rays thrown off shouldn't be powerful enough to harm Earth.
"Barring any disasters, like a major disturbance to the solar system, our descendants, if any, are in for a treat: a spectacular display of cosmic fireworks as the newly awakened supermassive black hole at the centre of our galaxy reacts by emitting jets of extremely bright energetic radiation," says Carlos Frenk, co-author of the study.
Interestingly, it seems this collision is long overdue. The Milky Way is a bit of an oddball – both its stellar halo and central black hole are less massive than most other galaxies its size. But according to the team's calculations, devouring the LMC should bring it up to about average.
"We think that up to now our galaxy has had only a few mergers with very low mass galaxies," says Alis Deason, co-author of the study. "This represents very slim pickings when compared to nearby galaxies of the same size as the Milky Way. For example, our nearest neighbor, the Andromeda galaxy, devoured galaxies weighing nearly 30 times more than those consumed by the Milky Way. Therefore, the collision with the Large Magellanic Cloud is long overdue and it is needed to make our galaxy typical."
This collision isn't the only one in the Milky Way's future, either. Andromeda is also sprinting towards us at about 109 km (68 mi) per second, with first contact due in about four billion years. And given how much bigger that galaxy is than the LMC, the smash will be far more catastrophic.
The research was published in the journal Monthly Notices of the Royal Astronomical Society.
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