Galaxy cluster collision makes a splash... a million light years long!

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Dec 15, 2011
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In what has become an annual tradition here at BA Central, literally the day I post my gallery of best pictures of the year, something comes along that really would've made it in had I seen it even a few hours earlier. In this case, it's a combined Chandra X-Ray Observatory and optical Very Large Telescope image of galaxy clusters colliding that's so weird that at first I thought for sure it was Photoshopped! But it's real, so check this out:

What you're looking at is a collision on a massive scale: not just two galaxies, but two clusters of galaxies slamming into each other, forming this object, called Abell 2052. The total mass of this combined cluster is almost beyond imagining: something like a quadrillion times the mass of the Sun -- 1,000,000,000,000,000 Suns! Note that our galaxy has about a hundred billion stars in it, so Abell 2052 is about 10,000 more massive. Yikes.

Something that big has a lot of gravity, and that's the key to what happened here (PDF). As the clusters approached each other prior to the collision, gas in one cluster was drawn off and headed toward the other. Once the clusters passed, the gas got whipped around by gravity, reversing direction, and essentially, well, sloshed. The analogy the astronomers used was wine in a wineglass as you swirl it; if you suddenly whip the glass a bit faster the wine will slosh up the side in a wave.

That long blue curved streamer? That's the wave: extraordinarily hot gas (30 million degrees C!) that got sloshed around by the cluster's gravity. The scale of it is simply epic; that streamer is over a million light years long! Again, for comparison, the Milky Way is 100,000 light years across, 1/10th as big as that wave. Craziness.

The interior portion is no less amazing. You can see swirls in there too, as well as two holes that look a bit like eyes. Those aren't really holes so much as bubbles of hot gas expanding inside the surrounding cooler gas; they're also buoyantly rising, pushing on the surrounding gas and compressing it. That's what hot gas does, whether it's in a balloon here on Earth or heated to millions of degrees in the center of a cosmic collision 500 million light years away.

But as I looked at that image of the core, I couldn't help but think it looked familiar... and somehow sad. And then I realized, ironically...

Even when two galaxy clusters merge, they're still forever alone.

Image credit: X-ray: NASA/CXC/BU/E.Blanton; Optical: ESO/VLT