Galaxy ripped to its death

Contributed by
Mar 2, 2007

What immense force is tearing this galaxy apart?

The spiral galaxy in the upper right of this newly released Hubble image is getting shredded-- you can see the bits of it getting torn off to the lower left, streaming out behind it. It's disintegrating before our eyes... well, it'll take a few million years, but still. Things are looking grim for it.

There are actually two causes for this galaxy's imminent demise. Both are because of its environment: it has the unfortunate circumstance of living in the massive galaxy cluster Abell 2667. Here's the long view (our doomed friend is in the upper left corner):

Abell 2667 is a rich cluster, fraught with galaxies. It's located about 2.7 billion light years from the Earth. I can't find a good estimate of the entire cluster mass, but one paper quoted the mass as approximately 70 trillion times the mass of the Sun inside a radius of 360,000 light years. For comparison, or own Milky Way Galaxy is about 100,000 light years across, and has a mass of roughly 200 billion solar masses. So this cluster has at least 350 times our Galaxy's mass. In other words, big.

That mass translates into a lot of gravity, and most of it is concentrated in the core. That means tides will be an issue. The gravity an object feels from the cluster core depends on its distance. But galaxies are big! So one side of a galaxy might feel significantly more gravity from the cluster core than the far side. This stretches out the galaxy, and if the difference in force is big enough, it can shred it.


This is in fact part of what's going on with that tattered galaxy. The tides from the cluster are literally pulling it apart. There's more, though.

Inside a cluster is a lot of gas. A lot. As galaxies orbit around the cluster center, they stir that gas up, heating it. In Abell 2667, there is so much gas that a galaxy passing through it feels a pressure, called, appropriately enough, the ram pressure (think of holding a lit candle while riding a bike to get the idea). If the galaxy is moving quickly enough, and the gas is dense enough, the galaxy can have all its gas stripped away, where it will merge with (and heat even more) the cluster gas.

Again, our poor galaxy there in the upper left is getting slammed this way as well. As the tides shred it, its gas is being forcibly blown out. The gas also getting compressed, and you know what that means: star formation. And you can see it! That stream down and to the left is bluish and clumpy, exactly what you expect for gas clouds getting compressed and forming new stars. The color is from newly born very massive stars, which are hot and blue. Those stars live a short, violent life, dying in supernova explosions. Keep an eye on that galaxy; it'll have some fireworks going off in the next few millennia.

Those supernovae heat the intercluster gas up too. How hot is it? It's between 10-100 million Kelvins. Hmmm, for comparison's sake, water boils at 373 Kelvins-- that's 373 with no millions or anything after it; that gas is fracking hot, hot enough to glow in X-rays. One technical paper quotes the total energy emitted in X-rays to be a numbing 1045 ergs/second. So, in X-rays alone, that's a trillion times the Sun's total energy -- in all wavelengths -- every second. And that's just from the cluster gas!

It would suck to live there.

Oh, one other thing. I've talked about gravitational lensing before, where the mass of a galaxy or cluster can warp the images of galaxies behind it. Well, that's happening here too. Did you notice the really weird elongated galaxy near the core in the picture above? Here's a closeup:

That stretched out goofy galaxy is actually probably a relatively normal spiral, distorted and twisty due to having its light passing through the warped and bent gravitational field of Abell 2667. The spiral is about 8 billion light years away, give or take, three times farther away than Abell 2667.

Actually, there's still one other thing. In that last image, the core of the cluster is visible to the right. That's the gravitational center, the bottom of the pit, so to speak. The cluster gas is streaming into that center, where it piles up. Dense gas cools faster than more tenuous gas, so towards the center the gas emits energy in the form of light (from optical up to X-rays), which cools it off. This is called a "cooling flow". Evidently, those blue streamers you can see there are from the gas flowing in; as it compresses it -- what? Bueller? Bueller? It forms stars. Some are massive, hot, and blue, which is again exactly what you're seeing.

So even as a galaxy dies in the outskirts of the cluster, its stripped gas forms blue stars... and when the remaining gas finally hits rock bottom, its destiny is the same. Eventually those stars will merge with the immense galaxy at the heart of the cluster, which will continue to grow as it eats its own.

It's a galaxy-eat-galaxy Universe out there. I'm glad we have a front row seat.

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