Over the course of the last few posts, we’ve explored different types of active galactic nuclei: Seyfert galaxies, double-lobed radio sources, and quasars.

At the heart of each of these galaxies lies a supermassive black hole, feeding off an energy feast of infalling material and producing titanic eruptions of energy.

Most galaxies, though, are not active. The majority of supermassive black holes–like the one sleeping at the heart of our own galaxy–are on “starvation diets,” living off minuscule streams of dust from cannibalized satellite galaxies.

We know what causes these supermassive black holes to erupt. But why are they so rare? What part do they play in galactic evolution?

There’s one very convenient law of physics that makes it easier for us to study galactic evolution: Light doesn’t travel instantaneously.

Moreover, light (in a vacuum) has a set speed.

Because light takes time to travel, the light we observe from distant objects left those objects long ago. When we observe those objects, we’re seeing them as they were when that light left them. We’re looking back in time.

And because light in a vacuum has a set speed, that “look-back time” is measurable.

If we measure the distance to a galaxy, we can find out how far we’re looking back in time. And from our observations of different galaxies, we can piece together a sequence of events–starting from the dawn of time in our universe.

Well, theoretically. That’s why astronomers are always building bigger and better telescopes (and putting telescopes into space).

Anyway.

When we talk about look-back time, we deal in redshift:

The longer light travels, the more time it spends getting stretched to longer wavelengths by the expanding universe. (We’ll get into the expanding universe very soon!)

Light that left a distant object billions of years ago has been stretched, or redshifted, quite a lot. The most distant galaxies–and, indeed, the most distant active galaxies–appear quite red…

…just like this distant galaxy at a redshift of 13, seen as it appeared only 330 million years after the universe began.

Quasars, in particular, are most common at redshifts between 2 and 3. They have been found at redshifts greater than 7, but those are very rare.

So, what does that tell us?

We know why galaxies become active: interaction with nearby galaxies tosses material inward toward their sleeping supermassive black holes and, essentially, wakes the dragon.

If quasars are typically found at redshifts between 2 and 3, those are the cosmological distances when galaxies were interacting the most often.

We also know that galactic evolution is shaped by collisions and mergers…

This is timelapse depicting the eventual collision of our galaxy, the Milky Way, with our neighbor galaxy Andromeda.

See how these two spiral galaxies dance around each other, eventually merging to form one large elliptical galaxy?

That’s how elliptical galaxies form: from collisions and mergers between galaxies. And spiral galaxies themselves likely need gentle interactions and donations of material from small satellite galaxies in order to mature into the beautiful pinwheel-like shapes we see today…

So, if quasars are most common at redshifts between 2 and 3, so are interacting galaxies, and so is galactic evolution.

This look-back time is the time when galaxies were actively growing, colliding, and merging. Quasars were about 1000 times more common than they are in the present day.

Of course, in this period of intense galaxy growth, most galaxies had not yet become large. These were the collisions that produced today’s large, “mature” galaxies. So most mergers would have been between small galaxies and produced smaller eruptions from their central supermassive black holes.

Even during this “age of quasars,” quasars and the even more powerful blazars would have been quite rare.

What about the most distant quasars, then–the ones at redshifts greater than 7?

Now we’re looking at a time when the universe was in its infancy, when galaxies had barely begun to form.

If this is a time when galaxies are rare, then it’ll be even rarer to find two in the same place, and hard for mergers between galaxies to happen. That easily explains why there aren’t many quasars at these look-back times.

But why are there any at all?

Remember that quasars are not galaxies. They are inside galaxies: they are galactic nuclei, and more specifically, they are active nuclei.

Quasars quite literally are erupting supermassive black holes.

If a quasar is an erupting supermassive black hole, then does it necessarily need to be inside a galaxy?

Nearer quasars are caused when galactic interactions “wake up” a sleeping supermassive black hole. But what if the most distant ones are a result of a time before those supermassive black holes had gone to sleep in the first place, before there were galaxies built around them–when they were actively forming?

That’s a hypothesis, anyway. I imagine one of JWST’s jobs is to check it, but it’s not something I can confirm. The jargon-heavy language of brand-new astrophysical research is something I don’t fully understand, and am definitely not equipped to teach on this blog.

Speaking of supermassive black hole formation, though…

Remember when we took just a peek at supermassive black holes in galaxies, and we saw a curious correlation between the mass of a galaxy’s central bulge (home of the nucleus) and the mass of its supermassive black hole?

(That was also the first time I teased quasars–which appear right at the top of this graphic!)

If there’s a correlation between the mass of a galaxy’s central bulge and its supermassive black hole, that would seem to indicate that the two form together.

This relationship goes for both elliptical galaxies and disk galaxies (most commonly spirals)…

In a disk galaxy, the supermassive black hole typically has about 0.5% the mass of the central bulge. And in this case, an elliptical galaxy can be thought of as one big central bulge: its supermassive black hole has about 0.5% the mass of its entire host galaxy.

But here’s where it gets interesting.

There is no relationship between the mass of a disk galaxy’s supermassive black hole and the mass of its disk.

(Then, of course, there is also no relationship in a disk galaxy between the mass of the disk and the mass of the central bulge.)

The explanation may be found in the Perseus Cluster, the brightest X-ray source in the entire sky.

This is a galaxy cluster “only” about 240 million light-years from Earth. Compared to the universe’s estimated age of 13.8 billion years, that look-back time is basically the present day.

See that galaxy toward the left with all the pink cloudiness around it?

That’s NGC 1275, a giant elliptical galaxy. And the pink stuff is the surrounding gas of the intergalactic medium, superheated to emit its own light.

NGC 1275’s supermassive black hole is undergoing an insanely powerful eruption–like I said, it’s literally the brightest X-ray source in the entire sky. And it’s cooking the gas surrounding the galaxy to such a high temperature that the gas is literally escaping from the galaxy’s gravitational influence.

Here’s a close-up:

Soon–astronomically speaking–this supermassive black hole will choke off its own supply of infalling material, and return to a starvation diet.

But here’s the key. It doesn’t just choke off its own food–it’ll choke off the supply of gas and dust available for its host galaxy to form new stars and grow.

Then, as the dragon sleeps, the intergalactic medium can cool off again and eventually begin to fall inward. And the cycle begins anew. That’s how supermassive black holes and their host bulges grow together.

What about disk galaxies, then?

Remember from our exploration of the Milky Way’s formation that a spiral galaxy’s disk component forms late, after the formation of its central bulge.

First, the spherical component of a disk galaxy forms–that is, the bulge and halo. This would’ve taken place during the age of galaxy formation when the universe was only a few billion years old.

Then, once the central supermassive black hole has gone to sleep, new material enriches the galaxy, forming the delicate disk.

That explains why the mass of the disk has no relationship to the mass of the central bulge or the supermassive black hole at its heart: they form one after the other, from separate processes.

But all this leaves us with a question…

Quasars, like 3C 273 displayed here, are only common at look-back times between redshifts of 2 and 3. But there’s no way to get rid of a supermassive black hole. So where are all these erupting supermassive black holes now, in the present day?

All around you.

The Milky Way could have once been home to a quasar. So could our neighbor, Andromeda. And so could all the other trillions–yes, trillions–of ordinary, inactive galaxies.

The dragon has simply gone to sleep. Now it’s waiting for a new galactic merger to throw it a feast of material and wake it up.

When the universe was young–after galaxies had begun to form but before it had expanded much–galaxies were closer together, and collisions were common. And that’s when active galaxies and quasars were most common. Even now, they occasionally occur when there are violent mergers.

Quasars, and active galaxies in general, are evidence of a short-lived stage of galactic evolution: the active formation of galaxies.

Next up, we’ll finish off our exploration of active galaxies with an overview to tie it all together. And then we’ll dive into cosmology!