Astronomers just spotted a giant galaxy orbiting our own
By studying a collection of images taken by the Very Large Telescope (VST) in Chile, astronomers at Cambridge University recently detected the giant, a wallflower galaxy known as Crater 2 that's quietly lurking at the edge of our own.
Crater 2 is a dwarf galaxy, a type of small galaxy containing only a few billion stars (compared to the Milky Way's hundred billion). It orbits around our galaxy like the moon orbits around the Earth and is located 390 light years from our solar system. It's a sixth of the size of the Milky Way, a hefty size for a dwarf galaxy, making it our fourth largest satellite galaxy. If we could see it with our naked eyes, reports the New Scientist, it would look twice as big as the full moon.
But we can't see it with our naked eyes. Despite its size, Crater 2 has a very low luminosity, which translates to the amount of stars per area of the sky. It's 100,000 times less bright than the Milky Way and 1,000 to 10,000 times less bright than the other three largest satellite galaxies. It's so faint that it has managed to stay entirely off our radar.
Until now.
By looking at accumulations of stars over parts of the sky, these astronomers, including Gabriel Torrealba, have finally pulled Crater 2 into the spotlight.
Aside from its large size and low luminosity, Crater 2 is also unique because of its shape, the researchers found. Unlike other satellite galaxies that are warped by the gravitational forces of the larger galaxies they orbit, Crater 2 is super round. Because it is unchanged by interactions with the Milky Way, the researchers think Crater 2 was formed at its current size.
Galaxies eating galaxies
Galaxies like the Milky Way are galactic cannibals of sorts, formed when smaller galaxies merge. The galaxies that survive this process become satellites, orbiting around the larger galaxy. We've already detected about 40 satellite galaxies in the Milky Way.
Simulations have shown that there's more to this seemingly cannibalistic merging process than meets the eye, however. There's some evidence to suggest, for example, that instead of being swallowed up one by one, smaller galaxies may merge in groups.
Because it's part of another group of dwarf galaxies that may be falling into our own, Crater 2 may add more evidence to this idea.
What's next
Astronomers believe that we should see many more, possibly even dimmer than this one. In order to spot them in future surveys, though, there are two main questions that need to be answered:
1. What's the limit on how big galaxies can be formed?
2. What's the limit on how few stars they could have?
Studying satellite galaxies like Crater 2, Torrealba said, will help us understand how galaxies form, which is a key component of the bigger puzzle of how the universe takes shape.