Supermassive Black Holes and the Types of Active Galactic Nuclei (Script) (Patreon)

Astrophysicists have discovered a black hole that for millions of years has been blasting vast particle beams in opposite directions across the sky. And has recently swiveled to point its one of these jets directly at us. Is this an intergalactic death ray of an alien civilization that has suddenly noticed us? Absolutely not, and there’s no danger at all. But it’s a pretty cool phenomenon anyway, and something we’ve never seen before.

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The most extreme phenomenon in the universe after the Big Bang itself has to be the quasar, where a gigantic black hole at the heart of a galaxy shreds the matter falling into it, spitting out an enormous amount of energy in the process. The searing whirlpools of quasars - their accretion disks - are so bright that they can be seen at further distances than almost any other object in the universe. Quasars are cool and all, but they’re only one subtype of a larger category of space object called the active galactic nucleus. This is the broad name for all the  different things we might see when a supermassive black hole accrete matter. We have things like Seyfert galaxies, when the relatively piddling black holes in smaller, mostly spiral galaxies start munching. Or we have radio galaxies, when the inflating material gets spat out again in jets that end up blasting radio light. Or we have blazars, when one of those same jets are pointing straight at us.

The different types of AGN are all related by a few parameters — how big the black hole is, how heartily the black hole is currently feeding, whether it’s able to form a jet, and a really big one is the orientation of the system. Which way is the whole accretion disk and jet pointing compared to us. It took us a long time to figure out that all these different weird looking things on the sky were different manifestations of the same basic phenomenon. But once we realised that, it all made a lot more sense. That is, until AGNs of one type started turning into AGNs of other types. And the more recent example of this is the weirdest, where the entire AGN, gigantic black hole included, appears to have rotated on the sky to point straight at us.

To see why this is so weird, let’s take a look at what makes these different objects what they are. This is the basic picture of the active galactic nucleus. We have our supermassive black hole that can be anywhere from a million to several billion times the mass of the Sun. These things live in the cores of all decent sized galaxies. We have the accretion disk—gas from the surrounding galaxy that’s been driven into the core and is spiraling into the black hole. On its way in, friction between the different orbits heats the gas up until in glows, and radiates light from infrared through to extreme ultraviolet and even X-rays right near the black hole. Surrounding the disk we think there’s a giant region of very dusty material—often called the dusty torus—that’s fueling the disk.

We also have winds move material swirling around or being blasted out which produce bright spikes of light that correspond to specific elements found in those winds.

And sometimes those outflows are gathered up—“collimated”—by magnetic fields. They’re channelled into beams of matter that punch their way into the surrounding galaxy. Charged particles spiral in these magnetic fields, emitting synchrotron radiation. The energy of that radiation can be extremely bright in visible, ultraviolet, or even X-rays and gamma rays by some more exotic processes close to the black hole. But the light from the jet has lower energy if produced further from the black hole. On the largest scales, when the jet has punched far into the surrounding galaxy, or even into the empty space beyond the galaxy, it shines as radio light.

Exactly what we see when we look at one of these beasts depends on … a lot. But one of the most important factors is orientation. In the classic quasar, the accretion disk is fully visible. Technically quasars should also have a jet producing radio emission—after all, the word quasar comes from quasi-stellar radio source, with the quasi-stellar part just meaning that it appears small - point-like to our telescopes.

There are also cases where you can’t see the accretion disk at all. If the object is aligned sort of side-on, then the disk is obscured by the ring of material that surrounds in. Then all you see is the radio jets. These are radio galaxies, of which there are also different types—but the coolest are the double lobe giant radio galaxies where you have thin jets of matter extending for up to millions of light years from the galaxy before blossoming out into gigantic radio plumes.

And finally, if these jets are pointing along our line of sight rather than at right angles to it, then we see something even weirder. We can still see the accretion disk in this case, but the disk’s light is faint compared to the light from the jet. That jet contains material travelling at extreme velocities—a good fraction of the speed of light in many cases. That causes the intensity of the jet to be massively increased. This is due to the effect of Einstein’s special theory of relativity, which I can’t explain right now. But this relativistic boosting can magnify the jet’s brightness by a factor of a 1000 or more. The result is the blazar—an AGN with extreme synchrotron emission. Blazars are also prone to rapid flaring activity because the jet if fed by the disk unevenly, and the extreme velocity of the jet speeds up its spluttering bursts.

So, to summarize, radio galaxies and blazars can result from the same underlying physical object, with the difference only being the angle you observe it from. We see radio galaxies side to size, but are looking at blazars down the barrel. So, can a radio galaxy turn its barrel on us?

Because that’s exactly what was seen in the case of the poetically named PBC J2333.9-2343, at least as hypothesized in the very recent paper by Lorena Hernández-García and collaborators. This is the object in question as imaged by the Australian Square kilometer Array pathfinder. t’s a classic double-lobe giant radio galaxy. The actual galaxy is right in the middle there, and the central supermassive black with a mass of 250 million suns would be a dot much smaller than a pixel in this image. In fact, the entire accretion disk would all be millions of times smaller than the highest resolution camera would could point at this object. But these radio lobes are something like a million light years from the galaxy. To produce lobes like this, the jets from that central black hole have to be launched at an angle pretty far from our own line of sight. Radio galaxies are thought to have jets within 15 or so degrees of a perfect right angle from our line of sight.

So that’s what we see in radio light. The team behind this paper observed our weird object all across the electromagnetic spectrum, and at multiple times. Peering as close to the center of the galaxy as possible, they saw the jet across the electromagnetic spectrum. But close to the center, it wasn’t pointing side to side at all. It was pointing right at us - or at least, within 6 degrees of a bullseye in our direction. In short, they saw a blazar. Bright synchrotron, rapid flaring activity, and definitely NOT obscured by the surrounding dust.

This is completely bizarre. Those radio jets definitely aren’t pointing at us, but the visible and X-ray jet close to the black hole definitely is.

What gives? Maybe our whole story of what makes different AGNs different is actually wrong? Possibly, but there’s a more likely explanation. Those radio lobes were produced a long time ago. Like, at least a million years ago if the material traveled at near the speed of light. But the blazar-like jet is close to the core - perhaps as young as a couple thousand years old. So the likely answer is that the jet changed direction some time between the creation of the radio structures and the current day. And it just so happens that it changed so that it’s pointing directly at us.

But what could possibly cause such a shift? Is it even possible? This is a little tricky because we don’t have an exact explanation for how these jets are launched, but we do have some plausible mechanisms. The jet material is very likely pulled up from the accretion disk and channeled by powerful magnetic fields right near the black hole. There isn’t much else that could do the job, and anyway we see jets collimated by magnetic fields in both young and dying stars and in pulsars. Those magnetic fields could be locked into the region just outside the black hole, or could be connected to the inner part of the accretion disk. One popular hypothesis is that to produce a powerful jet, you need the highly ordered and strong magnetic fields that should result from a rapidly rotating black hole. The jet would then be blasted out of the poles of that magnetic field, and would be oriented with the black hole’s spin.

But if that’s the case, then to change the direction of the jet you need to change the orientation of the entire supermassive black hole. More precisely, you need to change the axis that the black hole is spinning around. Which would take a crazy amount of energy.

There are two ways this could happen. One is by changing the orientation of the accretion disk. A black hole gains its spin from the matter it eats. That means it tends to spin in the same direction as its accretion disk. But an active galactic nucleus can go through multiple phases of activity. Fuel from the surrounding galaxy is driven to the center when the galaxy gets disturbed in some way—typically an interaction or a merger with another galaxy. Every time that gas falls in, it might form a disk at a different angle. It may be that the first radio lobes were produced in an ancient event that spun up the black hole in one direction, sending jets off to either side.

The AGN would then have died down, but have been woken up again by a new event. If, say, our galaxies gobbled up a smaller galaxy, then the angle of that merger would define how the new supply of gas fell on the central black hole. That could be completely different angle to the first accretion event. Over time, the spin of the black hole would have shifted towards that new disk.

The second option is that a more significant merger may have happened—a collision between two decent sized galaxies, one of which was our old radio galaxy. These galaxies would then merge and gas would get driven to the center to wake up the active nucleus. But now we would have two supermassive black holes instead of one. These would fall together and merge, and the spin direction of the final black hole would depend on the combination of spins of the two, and also on the angle of the merger. This would most likely be different to the direction of spin of either of the original black holes.

Or maybe we’re seeing a bit of both—a combination of spinning up the black hole and kicking its direction with a black hole merger. However it happened, by pure chance that final orientation has the black hole blasting its jet towards us.

And there’s actually good evidence that active galactic nuclei change their pointing. We see radio jets that have crazy kinks in them, and we have these X-shaped radio galaxies that suggest a change in jet direction over a very short timescale. This new guy looks like one of these things, but it just so happens that the new direction is our direction.

So, what have we learned from all of this? Well, to me things like this highlight how dynamic a place the universe really is. beyond that serene and static canvas of the night sky is a cosmos in constant and often violent motion. And, more pragmatically, we also have a new laboratory for testing our understanding of how astrophysical jets are made, how they’re connected to their black holes, and about the stop-start sputtering lifecycles of active galactic nuclei. And perhaps even about the crazy event that turned this radio galaxy into a blazar, pointing right at us from distant parts of space time.