Plowing through the electromagnetic spectrum

Contributed by
Nov 8, 2006

One of the many things I love about astronomy is the uncovering of a mystery. As

someone once said (nuts, I can't find the reference) Isaac Asimov once said (paraphrased) science isn't moved forward when someone yells "Eureka!", it gets that nudge when a scientist looks at the data and says, "That's funny..."

Mysteries are everywhere, if you just know where to look. And their solutions can be found, if you know how to look.

So start by taking a look at this image from Hubble:

What you're seeing at here is the cluster MS 0735. It's not a cluster of stars, but of entire galaxies! Each object you see there is a galaxy, much like our own. The cluster is something like a couple of million light years across. There are no big galaxies that close to the Milky Way, yet this clusters has dozens, hundreds, in that same volume. It's a crowded place.

However, it's not that different than lots of other galaxy clusters we see. Most of them are crowded. This one has a big galaxy right at the center, which is also typical. Heavier galaxies tend to "fall" toward the center, merge, and grow into one monster one.

But this cluster hides a secret. And behind that secret is a mystery...

When viewed in X-rays, the cluster's secret is revealed:

This view by the Chandra X-ray observatory shows that the cluster is surrounded by extremely hot gas, millions of degrees hot. Even this isn't too surprising to astronomers: we've seen lots of clusters embedded in hot gas like this, and there are many reasons it happens. Gas from inside the galaxies gets blown out by exploding stars, for example. As galaxies move inside the cluster, orbiting each other, the gas inside galaxies can be stripped away (think of the air inside a convertible getting stripped out as the car speeds down the highway).

Without our X-ray eyes, we'd never know about that gas. So looking at the cluster in a different way reveals something hidden.

But wait a sec... that gas has a shape. It's not just some blob. There are two big circular dark patches where there appears to be less gas. What gives?

Ah, there is the mystery! And to solve it, we need a cosmic detective.

Enter my old friend Brian McNamara. Those of you who watched the PBS NOVA show "Monster of the Milky Way" may remember Brian (I remember him because we went to grad school together). The show opened with him looking at images of this giant hot ball of gas in space. He was the one who took that image of the cluster above, and as soon as he saw it he knew he had something interesting. Those two dark patches are indeed holes, vast cavities carved out of the gas surrounding the cluster.

It's possible to calculate the amount of gas that had to be pushed aside to sculpt those holes. The number is numbing: more than a trillion times the mass of the Sun was moved. Think about the energy involved! What sort of object could wield such vast power on fantastic scales?

The only thing astronomers know of that can do it is a black hole. And not just any black hole, but a supermassive one-- millions of times the Sun's mass. These exist in the centers of galaxies -- there's one that is 4 million solar masses in the center of ours. As matter falls into such a black hole, it heats up. The energy generated can be huge, and in many cases this drives vast beams of matter and energy outward from the black hole. It's ironic that black holes are known for eating up everything near them, yet can actually eject tremendous amounts of matter at incredible velocities... but remember, this matter isn't actually inside the black hole, just very close. And with a big black hole, there's a lot of gravitational energy to tap into.

The galaxy in the center of MS 0735 must have a supermassive black hole at its core. It should even be pretty big, given how many other galaxies must have merged together to form that one beast. But if there are beams of matter ejected from it, there's no hint of them in those images.

But just as we had to switch from visible light to X-rays to see the hot gas, we have to go to another set of eyes to see the beams. Here's a third image of the cluster, as seen in radio waves:

Aha! There you go! In radio, we can see the gas ejected by the black hole, focused into beams (astronomers call them jets) hundreds of thousands of light years long. They're not as hot as the intercluster gas, so they don't emit X-rays, and the gas is too thin to emit visible light. But they glow brightly in radio waves. These beams scream out from the central black hole, moving at nearly the speed of light. Pity any gas that gets in the way... even a trillion solar masses of it.

These jets are what carved those cavities into the gas surrounding the cluster. They plowed through all that material, pushing it aside, powered paradoxically by the immense gravity of the black hole deep inside the cluster. This all becomes clear when all three images are put together:

Click on it for a hi-res version. It's beautiful, stunning.

There's your complete picture. Any one of these images is interesting, even provocative, but none by itself presents the complete picture. Astronomers need many eyes to look into space, and see what there is to see. And what a canvas the Universe reveals... Power on unimaginable scales! Destruction a million light years across! Monsters lurking in the deep cores of galaxies, central engines capable of wreaking such havoc!

Yeah, that's why I love astronomy. Sometimes it's the mystery, and sometimes it's the solution. But always, always, it's the story behind the images.

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