We first took a peek at supermassive black holes back in our discussion of galaxies. But now that we’ve covered a few types of active galaxies, it’s time to take a deeper dive.

Okay, I guess we won’t really be diving into a black hole…sorry to disappoint!

(Honestly, though, you really wouldn’t want to. It would be very uncomfortable, to say the least…and, of course, it’s a one-way trip.)

Supermassive holes are thought to lie at the hearts of most galaxies, including our own. They seem to be key to galactic structure. Most of them–including our own–are quiet. But a few percent of the galaxies in the universe emit titanic amounts of energy from their nuclei, and supermassive black holes are the ultimate culprit.

But how?

First, let’s quickly review what a black hole is.

The centerpiece of a black hole is a singularity: an object of zero radius, but infinite density.

I know. Whenever I try to imagine that, my brain just kind of goes…whaaaaa?

Trying to actually conceptualize a point that has absolutely zero radius/diameter and yet is filled with so much stuff that it’s infinitely dense is kind of brain-breaking. Instead, let’s explore how an object gets to be a singularity in the first place.

Like white dwarfs and neutron stars, black holes are a type of compact object. All of these objects result when the collapsing core of a star is too massive for “conventional” physics to support them. They must rely on the material strength of degenerate matter.

Degenerate matter is extremely dense because its subatomic particles are packed as close together as they can get.

But if the collapsing core contains more than 3 M_☉ (solar masses) of star stuff, no force in the universe can stop it. There is no material strength that can support an object 3 times the mass of our sun that is not engaging in nuclear fusion. Such an object can’t have any size greater than zero because the stuff it’s made of would have more room to collapse.

It still has 3 M_☉ of stuff, though, and that much stuff in a space of zero radius means infinite density.

Now, when matter falls toward a compact object, it’s a bit like draining a bathtub: the water can’t just pour straight through the drain. It forms a mini whirlpool.

That’s because the bathtub drain is very small compared to the stuff falling into the drain. The water in the bathtub has some motion to begin with; it sloshes around. The law of conservation of angular momentum requires that motion to become spin, and the water must spin faster as it travels down the drain.

Okay, what the heck does a bathtub drain have to do with a supermassive black hole?

Don’t worry, I’m getting to that.

Compact objects are massive but extremely small for their mass. White dwarfs are similar in mass to our sun, but are only the size of Earth itself! Neutron stars, which are even more massive, can have diameters as small as the distance across Los Angeles.

And no matter how massive a black hole is, it is a singularity. It has no size. That’s pretty much as tiny as you get.

Now here’s the crux. Supermassive black holes are the gravitational centers of galaxies, and galaxies rotate. Which means that supermassive black holes are super-small objects that have a ton of matter whirling around them.

When matter from a galaxy falls toward the supermassive black hole at its center, it will act like water down a drain. Conservation of angular momentum forces the infalling matter to spin faster as it approaches the black hole.

Instead of a whirlpool, we get an accretion disk.

Now, all this would be true for any black hole. But we’re not talking about just any stellar-mass black hole. We’re talking about supermassive black holes: black holes that are millions to billions of times the mass of our sun.

That makes for some ridiculously strong gravity.

Supermassive black holes force infalling matter to spin a heck of a lot faster than stellar-mass black holes–and as the material’s particles pick up speed and collide with other infalling particles, the temperature of the accretion disk skyrockets.

(Interestingly, this is the same basic physical process that forms a protostar from a collapsing interstellar cloud. Just, you know, way more powerful and violent!)

The accretion disk itself is a very strange place. It’s hard to get clear images of it, since it’s concealed deep within the heart of its host galaxy. But theoretical calculations can take us some of the way.

In the region closest to the black hole, there is no possible stable orbit, and any orbiting particle must ultimately spiral into the black hole. So, immediately surrounding the black hole is an empty cavity.

Beyond that is the innermost region of the accretion disk itself. This region is where it’s hottest–where the central black hole’s gravity is the strongest and wreaks the most havoc.

When matter is hot, it tends to expand, because the particles have high kinetic energy and bounce off each other, effectively pushing each other away.

Models indicate that the innermost part of the accretion disk should be no different. This region should puff up a bit like a donut.

Beyond that innermost region is a “sweet spot”: conservation of angular momentum forces the accretion disk to flatten, but temperatures aren’t high enough to counteract that. Which means, of course, that this intermediate region of the disk is also cooler than the center.

And then, we have the outermost part of the accretion disk: a fat, cool donut shape much more pronounced than the inner region, made up of dusty gas.

But here’s the million-dollar question.

How the heck do we get from accretion disk to active galactic nucleus?

This research is still in its infancy, but astronomers have a general idea.

Remember that the center of the accretion disk is insanely hot: hot enough that conservation of angular momentum can’t even keep it flat.

This means that the disk is at least partially ionized.

In short, ionization means that the atoms that make up the disk have either lost or gained electrons, giving them either a positive or a negative charge. Which means that we’re talking about magnetism.

Apparently, magnetic fields get trapped in the gas of the disk and drawn inward. The insane orbital speeds at the center of the disk tightly wind up the magnetic fields.

What we end up with, theorists suggest, are powerful magnetic tubes extending along the disk’s axis of rotation. These tubes confine and focus jets of hot gas and radiation.

That’s how we get active galactic nuclei (AGN): the powerhouses at the centers of Seyfert galaxies, double-lobed radio galaxies, and quasars.

Seyfert galaxies, like NGC 1566, are what we see when a supermassive black hole’s accretion disk is tipped to our line of sight, either slightly or completely.

Wait, hold on. Then why the heck do we seem to be staring straight down the center of NGC 1566, above?

Because the galaxy itself and the central accretion disk do not have the same axis of rotation. Even though we see this Seyfert galaxy face-on, the accretion disk is likely tipped a bit to our line of sight.

If the accretion disk is tipped a bit, we can see some of the hot gas at the center, but it’s still largely blocked by the outermost donut-shape. We’ll observe that hot gas to have the broad spectral lines indicative of Seyfert galaxies.

If we see the accretion disk directly edge-on, the outer part of the accretion disk will completely block our view of the center. Some Seyfert galaxies’ spectral lines are a bit narrower than expected; this may be the explanation.

If we see the accretion disk mostly face-on, we see a quasar. We are staring almost directly down the center of the black hole; one of its jets is aimed almost straight at Earth. That explains why quasars are so insanely bright.

Even brighter, though, are the rare blazars: what we see when we are staring directly down the center of the black hole. These AGNs’ spectra are totally featureless, indicating that we are observing synchrotron radiation: radiation produced by electrons spiraling through magnetic fields. That supports the magnetic field model.

The jets emitted by double-lobed radio galaxies also produce synchrotron radiation, so that piece of the puzzle fits.

We have a general idea of how the different AGN are produced. But here’s another question…

Why are so few galaxies active? And what makes them become active in the first place?

We’ll begin to answer those questions in my next post!