Meet galaxy NGC 5128, also known as Centaurus A.

Anyone notice something weird going on here?

If you’ve been following my recent posts on galaxies, you might notice that this does not look like a typical galaxy. It has a clear dust band, so it can’t be an elliptical; elliptical galaxies have no obvious dust or gas. But it doesn’t look much like a spiral, either: it has a bright, spherical cloud of stars you’d more expect from an elliptical!

And it’s definitely not an irregular galaxy. It definitely has a disk shape, and irregulars are not disk galaxies.

So what the heck is going on with this galaxy?

NGC 5128 is what we call a double-lobed radio galaxy.

Like Seyfert galaxies–which we covered in my previous post–double-lobed radio galaxies are a type of active galaxy, a galaxy whose nucleus is producing titanic amounts of energy.

But Seyfert galaxies are defined by the intense radiation produced in their centers. Double-lobed radio galaxies, on the other hand, emit energy from two external “lobes.”

Here’s a better view:

Meet Cygnus A, the brightest radio source in the constellation Cygnus.

So what are we seeing here?

See that bright spot near the center? That’s the actual galaxy here. The “lobes” I’m talking about are those bright, orange pockets of gas to either side.

But, see that faint streak of light between the central galaxy and the lobes? It’s far brighter on the right, but if you look closely between the galaxy and the left lobe, you can still see some faint traces.

This is a jet of high-speed gas, somehow emitted from the galaxy’s center.

The “lobes” themselves are cavities of low-density gas in the intergalactic medium; that is, the stuff between the galaxies. The jets plow open these cavities and impact the far side, creating hot spots. That’s how we get the much brighter spots on the regions of each lobe opposite the galaxy.

But…hold on a sec. Why does the left jet here appear so much fainter than the one on the right?

It’s not the only one.

Take another peek at NGC 5128. Can you even see a second jet at all?

The one on the “upper” side in this image is clearly there: it’s that purple streak that’s at an angle to the galaxy’s dust band. But the only material appearing to connect the galaxy to the “lower” lobe is a faint yellowish plume.

(To be clear, the purple is not actually purple, and the yellow is not actually yellow. This is a false-color image. “Purple” represents x-ray observations, “yellow” represents radio observations, and white is indeed visible light.)

Anyway. What the heck happened to NGC 5128’s lower jet?

Probably nothing. It’s just angled away from us.

Apparently, these jets form when matter flows into the supermassive black hole in a galaxy’s nucleus.

Black holes aren’t the monstrous space vacuum cleaners you see in the movies, gobbling up everything in their path. And for goodness sake, they do not growl. (Star Trek (2009), I’m looking at you.) They are actually quite quiet objects, content to leave you alone–just as long as you don’t cross their event horizon.

Yeah. That bit’s pretty important.

At the heart of nearly every galaxy lies a supermassive black hole, a black hole at least several million times the mass of our sun. They seem to be key to galaxies’ structures. Without a supermassive black hole, our galaxy would look very different, and you and I wouldn’t exist at all.

But they’re sleeping dragons. Throw something massive into one, and you’ll wake it up.

That’s what happened with Swift J1644+57. This is not a galaxy designation–this was the designation for the event, not specifically for an object. In 2011, Astronomers observed a star torn apart by its galaxy’s black hole.

Presumably, this star’s orbit had been disturbed somehow, and it was kicked a little too close.

But…how do we get a pair of jets ejecting material?

Truth be told, this phenomenon isn’t fully understood. But we know that when matter flows into a black hole, it forms an accretion disk–like the whirlpool you see when you drain a bathtub.

Apparently, magnetic fields are drawn into the accretion disk and get tightly coiled. They manage to store enough energy to eject extremely hot gas, and since the magnetic field is all twisted up, it confines the jets to narrow beams.

Since these jets are emitted in opposite directions–perpendicular to the accretion disk–only one can be aimed toward Earth.

Now here’s the funky thing. This gas isn’t just hot. Somehow, the material in these jets travels at up to tens of thousands of kilometers per second–a significant fraction of the speed of light!

Matter traveling that fast tends to emit photons in the direction of travel. So, a jet pointed straight at Earth will appear much brighter than the jet aimed away from us (and the one aimed away may not register on our instruments at all).

That could explain why only one jet from NGC 5128 is visible. But did you notice something else funky going on with that galaxy?

See how twisted around this galaxy’s radio lobes are?

Okay, what is going on here?

Well, this brings us back to how weird the galaxy looks as a whole. Remember how I said it has dust like a spiral, but a spherical cloud of stars like an elliptical?

This galaxy is a combination of both. It appears to be a giant elliptical galaxy and a spiral, passing through one another.

These two colliding galaxies rotate on separate axes. The dust ring rotates about the spiral’s original axis, pretty much just like a frisbee. But the elliptical galaxy rotates about an axis that lies in the plane of the spiral.

Imagine the chaos of the tidal forces going on here. It’s no wonder the lobes don’t appear symmetrical.

And they are far from the only ones.

Meet NGC 1265.

Okay, what the actual heck is going on here?

This galaxy is moving rapidly through the gas of the intergalactic medium. Its jets and lobes are trailing behind it like enormous streamers.

And as for those weird twists in the jets, those are probably produced by movement from their source nucleus, like twirling a ribbon.

Here’s another case in the form of radio galaxy 3C 31:

Presumably, the nucleus of 3C 31 is orbiting some other massive object, perhaps the nucleus of a recently absorbed companion galaxy.

And then there’s radio source 3C 75: a set of two active galaxies in the midst of a close encounter.

In the case of 3C 31, the second nucleus is presumably not active; we only see the twists in the jets produced by their nucleus’s orbital motion. In 3C 75, however, both nuclei are active, and we see two sets of jets that twist with the motions of both nuclei.

Now, you guys wanna hear something super cool?

I don’t know if you remember waaaaay back in February 2019, when we discussed protostars (many of you may not have been reading this blog yet). But back then, we explored some case studies of watching the birth of stars.

And way back then, we discovered that protostars emit jets of material from their poles, which impact the interstellar medium and form Herbig-Haro objects–which look a heck of a lot like radio lobes.

Protostars do not emit jets traveling at near the speed of light, unlike active galaxies. Their jets travel at only a few hundred kilometers per second (rather than tens of thousands). But the geometry is very similar. And it just might be that the fundamental physical processes at work are the same.

Now, back to those active nuclei.

When we discussed Seyfert galaxies in my previous post, I hinted that interactions with other galaxies could explain those active nuclei. And sure enough, many double-lobed radio galaxies also appear to be interacting galaxies.

I mean, just look at NGC 5128, a spiral passing through an elliptical. Radio galaxy 3C 31 is producing twisted jets, presumably due to orbiting a second nucleus. And radio source 3C 75 is definitely due to a close gravitational encounter–and both nuclei are active.

The evidence is mounting that active galactic nuclei are a result of galactic interactions and collisions. And supermassive black hole activity is the ultimate culprit.

But before we dive deeper into that, we’ll cover one more type of active galaxy–a quasar!