About 65 million light years from Earth — a fair distance, though relatively close by cosmic standards — lies a lovely nearly edge-on spiral galaxy. Called NGC 3079, it looks very much like thousands of other galaxies of its kind, and in fact isn't so different from our own Milky Way. A tad smaller, at 70,000 light years or so across (the Milky Way is 100,000), but otherwise you might not give it a second glance.
Until, that is, you notice that in its center is a towering fountain of gas spewing from its core, looking very much like flames licking out into space.
Sounds cool, right? Yeah, it's actually way cooler than that. It's not a fountain: it's actually a pair of enormous bubbles, vast shells of gas being inflated by some ridiculously energetic engine inside the galaxy's heart, one lobe above and the other below the galaxy's midplane. And those bubbles are blasting out high energy X-rays.
Take a look:
That image is a combination of Hubble Space Telescope data in visible light (the kind we see, shown in orange and blue) and observations from the Chandra X-Ray Observatory (shown in purple). You can see the spiral arms traced out in the Hubble image, punctuated with point-like X-rays sources seen by Chandra along them (probably compact objects like black holes and neutron stars gobbling down material from stars orbiting them).
But then in the center is that amazing structure. The visible light part looks like a popped bubble, but the X-ray data betray that; here is the Chandra observation by itself:
Ah, there's the bubble! And you can see the other bubble below it, much harder to see in the combined image due to the interference of the disk of the galaxy. The upper bubble is a staggering 4,900 light years in size, and the lower one somewhat smaller though still huge at 3,600 light years across.
The X-ray data show that they look like soap bubbles, with a bright rim but a dimmer interior. That's a hallmark of a thin spherical shell, like a … well, like a bubble. Looking around the edge we are looking through more material, so it appears brighter there.
And they are bright. In just X-rays the upper bubble glows with more than a million times the energy of our Sun. That's a huge amount of energy! Whatever is driving these structures is incredibly powerful, both to create that much emission as well as being able to push out so much gas into intergalactic space at speeds of hundreds of kilometers per second.
What could possibly generate that much power?
Actually, in this case, we don't know. But there's another example of such superbubbles much closer to us that provides clues: Our own Milky Way.
Expanding outward from the center of our galaxy are two similar bubbles, detectable in gamma rays (seen by the Fermi space telescope). It turns out our bubbles are much bigger — fully 50,000 light years long, half the diameter of the galaxy! — and moving much faster, at about 1,000 kilometers per second. Astronomers argued for years over their cause: Could they be blown by the supermassive black hole in the heart of the Milky Way, or could they be the result of incredibly vigorous star formation?
Black holes can blow a fierce wind. If materials falls into one too rapidly, it forms a disk around the black hole and heats up to temperatures of millions of degrees. A witch's brew of forces can then drive a flow of subatomic particles screaming away, in what's called a black hole wind.
On the other hand, when gas clouds give birth to thousands or even millions of stars, many of these stars are massive, hot, and incredibly luminous. They too can drive a powerful wind.
So which is it?
In the Milky Way it turns out the culprit is star formation! The way the energy is being emitted is much better fit by zillions of stars pumping this energy into the gas than a black hole wind.
But that's us. What about NGC 3079? That's harder to say. It does have a supermassive black hole, and it is active — you can see it as a purple glow in the middle of the two lobes. But that doesn't mean it's to blame here. It could also be star formation. We're not sure yet.
Interestingly, the sheer power of the X-rays coming from the lobe indicate the presence of fierce magnetic fields there too. As fast-moving gas slams into slower-moving gas, it creates shock waves, and the magnetic field gets tangled up. Subatomic particles trapped inside literally get bounced around, accelerating between them until they gain so much energy the magnetism can't hold them anymore. They then are flung away at just under the speed of light, becoming what we call cosmic rays. In general that sort of thing is more consistent with star formation, but we just don't have enough data to distinguish the two.
I find that rather amazing. By coincidence, two very different phenomena both have about the same ability to drive the formation of these bubbles, and we can't always be sure which one is behind it. Our puzzlement isn't too surprising, though. After all, we haven't been doing these sorts of observations very long, and we still have a lot to learn.
I'm all for that. I like learning, and I love it when we're presented with a mystery we have to untangle. I have little doubt astronomers will eventually figure this one out. And when they do, there will more galaxies seen with these bubbles, and certainly more weird structures previously unseen that we'll want to understand.
Galaxies are immense structures, and filled literally to overflowing with amazing things. We'll be at this for a long, long time.