Active galactic nucleus

It just turns off the star factory.

Astrophysicists call this process active galactic nucleus feedback, or AGN feedback.

It acts as a cosmic thermostat, regulating how large a galaxy can grow and how many stars it can produce.

That's incredible.

Exactly.

But the narrative has almost universally defaulted to negative AGN feedback.

We just assume these active galactic nuclei, or AGNs, operate purely as, like, cosmic quenchers.

Yeah, the classic destroyer of worlds trope.

Well, I mean, it's the dominant paradigm for a reason, you know?

Sure.

If you run a cosmological simulation without heavy-handed negative AGN feedback, your virtual universe just ends up overproducing these impossibly massive blue galaxies.

Right.

Right.

The eighty-eight percent of sources that eROSITA is seeing in the extragalactic void are active galactic nuclei or AGN.

Yeah.

These are supermassive black holes that are actively gorging themselves on a massive, steady supply of galactic material.

Let's dive into the physical mechanics of an AGN because when we casually say accreting black hole, I feel like it drastically undersells the violence of the mechanism.

It really does.

I mean, the black hole itself doesn't emit X-rays, right? Not even light can escape the event horizon.

Right.

That interaction is arguably the most crucial aspect of this entire phenomenon.

It's a process known in astrophysics as AGN feedback, active galactic nucleus feedback.

Okay.

See, we used to view galaxies as relatively simple systems.

There seems to be a natural limit to how big a galaxy can actually get.

Exactly.

We observe a cutoff, and AGN feedback is the primary suspect for why that cutoff exists.

The supermassive black hole basically acts as the galaxy's thermostat.

Now it may have had in the past and lots of other ones do.

So by observing what's happening, can we predict if it will have an AGN in the future, if it had an AGN in the past? And really, what are the physical processes that control that? And those are the questions that really drive me and keep me up at night in the most wonderful way.

You know, I don't blame you for asking those questions or for being kept up at night.

Because when I started learning about astronomy and astrophysics, I was mostly informed by what we can learn and see in visible light, right? What we can see in optical wavelength, the same type of light that our eyes can see.

Can I add one thing to that, Ethan? I just want to just jump in for one second because the Chandra thing is super interesting.

And the fact that we do see these like regular small flares from the, not from the Aegean, from the not Aegean, from the supermassive black hole that is not currently in Aegean.

We see these small little flares, but actually one of the things that my group is working on now that's I find just so fascinating is that the molecular clouds so the gas the clouds of like gas and dust in the central molecular zone kind of surrounding the black hole those are showing signs of x-ray reflections with the idea being that in the past the supermassive black hole did have like a much larger accretion event like probably like an AGN like accretion event and it gave off these really powerful bursts of x-rays and those x-rays would have traveled outward in all directions they would have come to the earth where they saw that we didn't have an x-ray telescope and they just kept on going and then but they also travel and they interact with these molecular clouds and they actually cause those molecular clouds to fluoresce in the x-ray in this special iron k line and um and then we can actually observe with chandra we can see those molecular clouds one year they'll be nothing in the X-ray, then they'll have this X-ray reflection basically of that, that previous X-ray light from one of those flares coming through that cloud and then reflecting back and coming to the earth.

And this time seeing that we do have an X-ray telescopes that we took pictures of that.

And here's the thing, this disk glows and it glows so brightly it can outshine every single star in its entire galaxy combined.

And we call this glowing active region an active galactic nucleus, or an AGN for short.

Now, most scientists would look at that environment and they will think it's so violent, so turbulent, so extreme, and then they say nothing could ever be born there because it's too chaotic, right? But, and it's a big but, a team of scientists decided to look more carefully.

They built a computer model, looked at the variators of these spinning disks, where things get a little calmer, we can say.

Exactly.

In modern galaxies, AGN feedback, which is the massive amount of energy pumped out by the accreting black hole, it regulates star formation.

It pushes the gas out.

Right.

Yeah.

It's a complete outcast just sitting out there in the galactic suburbs.

And the presence of this off-nucleus AGN arguably the most intriguing dynamic piece of the entire puzzle.

Yes, because a two million solar mass object does not simply form in the low density outskirts of a galactic halo.

To really understand how galaxies evolve, we have to look at what happens when these central black holes wake up and start eating.

We have to look at active galactic nuclei, or AGN.

AGN.

Because when a super massive black hole feeds, it's not exactly a gentle process.