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One ring (galaxy) to bring them all, and in the darkness bind them

26 May 2020, 13:00 UTC
One ring (galaxy) to bring them all, and in the darkness bind them
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It's the kind of galaxy Sauron would love.

You've probably seen images of spiral galaxies, and elliptical galaxies, and even weird, distorted irregular galaxies. But have you ever seen a ring galaxy?

These probably started out as disk galaxies, much like our Milky Way. But then they suffered a direct hit collision with a smaller galaxy, the latter punching right through the middle of the bigger galaxy. The changing gravitational influence of the smaller galaxy as it approaches and leaves creates an expanding ripple in the bigger galaxy, what astronomers call a radially propagating density wave. It sweeps up gas and compresses it, forming an expanding ring of star formation.

The Cartwheel Galaxy, a relatively close by ring galaxy 500 million light years away. Credit: NASA/ESA/Hubble Borne, processed by Judy Schmidt

They're rare now, happening in only about 0.1% of disk galaxies in the local Universe. Due to a higher collision rate in the past, it's expected there would be more 10 billion years ago than there are today. But in fact they're rare back then, too. Searches for them haven't turned up many.

But a new deep survey of the sky yielded several, including one called R5519, which turns ...

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