A Cosmic Jet Ramped Up by the Big Bang

Contributed by
Mar 3, 2016
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I love an astronomy story where a bunch of different amazing events come together to produce something even more astonishing. And I have one for you here! In this case, we have a huge black hole on the edge of the observable Universe spewing material out at fantastic velocities, and we only see it because the material is amplifying the light left over from the Big Bang. And also, it was discovered by accident.

So, yeah. Let me explain.

At the heart of every big galaxy is a supermassive black hole. We think they formed along with the galaxy itself, and some are seriously huge, millions or billions of times the mass of the Sun. Our Milky Way has one more than 4 million times the Sun’s mass.

Sometimes, material like gas falls into one of these monsters. It spirals around and can form a flattened disk called an accretion disk. Close in to the black hole the material is moving at a large fraction of the speed of light, and farther out moves more slowly. This causes tremendous friction, heating the gas to millions of degrees, making them shine brilliantly; sometimes they’re brighter than the entire galaxy of billions of stars.

For reasons still not totally understood (but that have to do with fiercely complicated magnetic fields), the disk can also focus two beams of material and energy that shoot away from the poles, up and down if you will. These beams (also called jets) pack a huge wallop, because the material in them (subatomic particles like electrons and atomic nuclei) are traveling just a hair under the speed of light. They can travel for hundreds of thousands of light years.

We call these kinds of galaxies active, and they are so flippin’ bright we can see them for billions of light-years, clear across the Universe. I talk about them in Crash Course Astronomy:

These jets emit light across the electromagnetic spectrum, from low energy radio waves all the way up to ridiculously energetic gamma rays. They tend to be pretty bright in X-rays, and quite a few have been mapped using orbiting telescopes like the Chandra X-ray Observatory.

In fact, Chandra was being used to map a galaxy cluster (which are sources of abundant X-rays), when astronomers noticed something odd. They saw a bright X-ray source called B3 0727+409 that had an extended tail … and to cut to the chase, it was an active galaxy (called a quasar) with a bright jet.

When they followed up on it, they were surprised to see that this galaxy was very far away: more than 11 billion light-years! Very few X-ray jets are seen from galaxies this far away, so right away their interest was piqued.

And the story got weirder. Usually jets are faint compared to the accretion disk, which is much denser and brighter. But in the case of B3 0727, the jet was very bright. Why would that be?

Now comes the next cool bit. Inside the jets are electrons moving at just under the speed of light. If a photon (a particle of light) happens to stray into the electron’s way, it can get energized, absorbing energy from the electron. That electron has a lot of energy, so the light can get really and truly ramped up. It can get pumped from being in the radio wave range all the way up to X-ray energies. That’s a huge leap.

This is called inverse Compton scattering (click that for a more detailed explanation as well as a good physics joke), and we see it a lot in X-ray jets. That would explain the X-ray brightness of B3 0727, but where are those photons coming from?

And here’s the third cool part. The photons are left over from the formation of the Universe itself! Early on, right after the cosmos big banged, it was dense and opaque. As it expanded, it cooled and became transparent. That happened about 400,000 years after the bang. At that time, the photons were mostly visible light. Now, after 13 billion more years of expansion, they’ve lost a lot of energy, and are mostly in the microwave part of the spectrum.

It’s these photons that the jet from B3 0727 is slamming into. They go from lower energy to super high-energy, and make the jet glow far more brightly in X-rays than it could on its own.

That’s pretty awesome. But then I read just how much energy is being emitted by the jet, and the hair on the back of my neck stood up: It’s blasting out 100 billion times the Sun’s energy every second. 100 billion. Every second. And that’s just in X-rays!

Anything in the way of that jet is cooked. I’m kinda glad it’s so far away.

Still, this is good news. We don’t know much about X-ray jets at this distance, and finding even one means there must be more. Studying it can reveal information about the host galaxy, the black hole, and even conditions in the Universe around it (which we’re seeing as it was 11 billion years in the past).

Every denizen of the Universe is a piece of it, a clue to everything else. It may seem really esoteric to look at gigantic black holes spewing out energy a gazillion kilometers away, but in a real sense we’re just examining where we live, trying to understand it better. This is a great example of that search; it has so many fields of science colliding—literally—and we need to understand each one to grok the whole picture.