How do giant black holes shut off star formation? How do they turn it back on? Which came first, the black hole or the galaxy? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO-GENERATED)

We all carry a dark secret. It's inside of us every day. It's behind everything we say everything we do. It's a part of us, even when we don't want to admit it. Even when we pretend it's not even there. I have a dark secret for me. It's that I like singing along to Ariana Grande songs. I can't get rid of it. I can't change it. I can't put it away This dark shadow, it's a part of me and what goes for us goes for the universe as well. And every galaxy has its own dark secret. We call these secrets super massive black holes. We believe that every single galaxy hosts a supermassive black hole by volume. These black holes aren't very big, typically no larger than a solar system. But by mass. These black holes are the single largest objects inside of each galaxy.

How big are they? Well, the smallest of the supermassive black holes are at least a million times more massive than the sun, and the largest reach to hundreds of billions of times more massive than the sun. But even then, as giant as they are, they hold less than 1% of all the mass of an entire galaxy. And by volume, A typical galaxy is about a billion times larger. Galaxies are huge. Galaxies are beyond the scale of human comprehension. Already, we're not even talking about cosmology. We're not even talking about the universe. Just a single galaxy is far larger and far more complex than we could even imagine. And, yes, black holes are the single largest thing. But if you took the biggest skyscraper out of New York City, you would still have New York City. Life would go on. The skyline would be a little bit different, but nothing major would change if you were to pluck one of these black holes.

These dark secrets from the centers of their Galaxies, you wouldn't even notice the galaxy would keep on rotating would keep on making stars keep on living. But maybe things would be different. Despite the fact that these black holes make up such a tiny percentage of a galaxy's mass, they influence the galaxy far beyond their neighborhoods. You can't take a dark secret out of someone and expect them to be the same person. Galaxies wouldn't be the same without their giant black holes, and those same giant black holes might be responsible for life itself. The first clue that there's an intimate relationship between Galaxies and their giant black holes their dark secrets is something called the M Sigma relation. That's a fun one, so we'll unpack it a little. Astronomers, like all other scientists, love to find relationships or correlations. It doesn't prove anything, but it is a piece of evidence that two things may have a deeper connection.

The these relationships work best, and by best I mean, uh, having the strongest evidence for or connection when the relationship holds over a wide range. If I just look at a couple Galaxies and I notice, uh, something in common between these Galaxies or a property of these Galaxies that connects to another property of those Galaxies, and I just look at two or three Galaxies, that's that's not very interesting. If I look at a couple of 1000 Galaxies and I see some sort of relationship pop out now, you've got my attention again. These kinds of correlations don't prove anything. They don't prove that one thing is causing the other or the other is causing the first one or proving even that there is a common cause between the two. It's just noting a connection a relationship. And this particular relationship is called the M Sigma relation, which was only discovered in 2000, by the way. And we're used to thinking of the frontiers of physics and astronomy, being about dark energy and dark matter particles and and what happened at the beginnings of the Big Bang.

But here's something as simple and mundane as a relationship between black holes in their Galaxies that was only discovered in 2000. This is cutting edge research, and the M Sigma relation tells us that in Galaxies there is a connection between the M and the Sigma, which is not very useful Sigma, of course, being the Greek letter. OK, that name isn't going to win any awards anytime soon, so I'll tell you what the relation is. M is the mass of the supermassive black hole inside the galaxy, and Sigma refers to what's called the velocity dispersion. I know, I know this isn't very fun, but we're almost through it, and it's important, and I decided it's important because I put it at the beginning of the episode. We have to eat our vegetables if we want our dessert folks. OK, so we need to explain velocity dispersion. I like to think of velocity dispersion as the amount of jiggle in something. OK, so Santa Claus has a high. His belly has a high velocity dispersion.

There's a lot of jiggle going on there. If you think about stars moving around or or bees flying around, there's some average velocity to those stars that are zooming around. Or those bees that are zooming around. And you can calculate that average of velocity by taking all their speeds and taking the average of that speed. Pretty straightforward calculation. The velocity dispersion is how much spread there is in those speeds. So if something has a low velocity dispersion, it means that everyone pretty much has the average speed. If there's a high velocity dispersion, it means that everyone has all sorts of crazy speeds. Some are super slow, and some are super fast, but the average is is just some number. This can be applied to stars in a galaxy. Galaxies in a galaxy cluster bees flying around Santa Claus' belly, you name it. If there's a wide or high velocity dispersion.

It means there's a lot of a lot of different velocities going on. And if it's low, then pretty much everyone's just at that average speed. And in this case, in the M Sigma relation, Sigma is referring to the velocity dispersion in the gas in the core of a galaxy. Or, if you want to, you can call it the amount of jiggle in the core of the galaxy. Why this property and literally, anything else? Why are we connecting black hole Mass to this thing of all things? Because it's easy to measure. Astronomers have a lot of tricks for measuring velocity dispersion, and it's it's an easy quantity. You can just observe the core of a galaxy and very quickly get its velocity dispersion. If you want to get something else like mass of a galaxy or size of a galaxy, that that's harder OK, and astronomers love easy stuff. OK, so we're gonna connect the mass to the velocity dispersion because the astronomers are the ones who discovered this and so they get to name it and they decide this is the easiest thing to measure.

And so that's what we're measuring the M Sigma relation tells us that as black holes get bigger, the cores of Galaxies get well, jiggly er, their velocity dispersions go up. If you find a random galaxy, it will have some black hole mass and some velocity dispersion. Some amount of sigma. If you find a different galaxy and it has a bigger black hole mass, you're almost guaranteed to find a higher velocity dispersion. There's this relationship that scales with each one. The bigger the black hole mass, the greater the velocity dispersion. This relation holds through something like five Orders of Magnitude, which is pretty dang impressive and definitely smells suspicious. And it comes down to a simple question. Why should the velocities of stars or gas or whatever very, very far away from the black hole, remember, The black hole itself is the size of, say, a solar system in the core of a galaxy is like 10,000 light years across, if not more.

These things are not connected to each other in space. There's not some line that is connecting the black hole to the outermost edge of the core of a galaxy, let alone the rest of the galaxy. Why should those stars in that gas care at all about the mass of the central black hole, which is just hanging out in the center, minding its own business? Remember the black holes? These dark secrets that every galaxy has aren't very large, both in size and in Mass. They should be insignificant. So why does this relationship exist? Well, we think that the answer is that the black hole, that dark secret that you carry in your heart is doing a lot more than just sitting there. It's eating you alive. Once this relationship was discovered in 2000, astronomers tried to figure out why this relationship existed. Sure, it may just be a coincidence, but that's a pretty big stretch of a coincidence. Over five orders of magnitude in Galaxy size, uh, Black hole mass velocity dispersion measure that that's big.

And in their investigations, they discovered something important. And this concept is so important to the evolution of Galaxies that it gets its own word, its own category, its own identity, the relationship between black holes and Galaxies. Between you and your dark secret is feedback. Say it with me now, this time in italics, everyone together and feedback. Feedback is how this black hole, small but mighty, can influence the rest of the galaxy and how the rest of the galaxy influences the black hole. We believe we being astronomers, scientists, physicists, people in the know we believe that black holes are connected to Galaxies and that the evolution of the black hole is connected to the evolution of the galaxy. They do not evolve separately like the galaxy formed the sun, But the evolution of the sun doesn't impact in any major way at all.

The evolution of the galaxy and the future evolution of the galaxy doesn't impact in any major way at all. The evolution of the sun. They are disconnected, they are decoupled. But not so with these dark secrets. Not so with these giant black holes in our story to explore, this feedback process starts in the core. The core is the densest part of the galaxy. You can have stars there crammed together 1000 to a million times more tightly compressed than they are in the solar neighborhood. A good fraction of a galaxy's mass is in its core. It is probably the oldest part of every galaxy. The first part to form, and then the rest of the galaxy glued onto it. It is The downtown is where all the action is is where all the best shows are. And you know that real. That restaurant that we went to on that one date night last June like that was downtown. It's where all the kids want to be and where rent prices are crazy. But we're not. We're not gonna get into that.

At the very center of the core sits the supermassive black hole, millions to billions of times more massive than the sun, which is large. But again, I'm not. I can't say this enough. It's large, but not insanely large. It is the single largest object in the galaxy, but it is. Less than 1% of the mass of the galaxy is tiny. It's over here in a corner, but gas finds its way into the central black hole. Stars wander too close. Remember, this is a really dense neighborhood. There's a lot going on, so stuff is just going to find its way onto it on the black hole. When it does, it compresses because the gravity of the black hole is really strong. Uh, not like super strong. I me sitting here. Uh, the Milky Way has a giant black hole in its center. We call it Sagittarius, a star. Sagittarius. A star is the single largest object in the galaxy or the single most massive object in the galaxy. I don't feel it's gravitational influence over here, but if I were to get close to it within, like, a light year or something, I would start to feel its gravitational influence.

So any pocket of gas, any star that gets too close starts to feel pulled and it compresses is because the black hole again is not very large in volume, and then it exerts this enormous gravitational influence on its surroundings. And so it pulls all this gas in the gas compresses and heats up because that's that's what gasses do when you compress them. The gas is spinning, and so it flattens into an accretion disk. The accretion disk is whipping around the black hole, frenzied, chaotic, insane, glorious. Even inside of this accretion disk, the temperature skyrocket to like, I don't know, a trillion degrees ball parking here, and you have all these charged particles moving around It's unstable. They're dipping up and down. They're folding in on themselves. This drives the creation of incredibly strong electric and magnetic fields. The magnetic fields amplify, creating what's called a dynamo mechanism that generates an impressively strong magnetic field.

Usually you don't care about magnetic fields like the Earth has a strong magnetic field as astrophysical objects go. But I don't really notice it or feel it. I have to get a little magnet even to be able to sense it. Usually, magnetic fields just stay in the background. But now we have magnetic fields. And yes, I am talking about magnetic fields. They start to drive the action. They get strong enough that they start to funnel gas immaterial around the black hole, following something we call tendex lines. I have no clue where that word comes from, but it sounds super awesome. So I like saying it. Tendex lines, these little cork screw paths around the black hole, and it and it these insane physics and we only ho honestly understand these physics through computer simulations. It's not like we can watch this process play out. It's not like we can go visit a black hole of any size that's creating these kinds of accretion discs. Uh, but we can perform computer simulations of the physics that is happening in the vicinity of a supermassive black hole, and it goes crazy real fast.

These magnetic fields funnel gas looping around the black hole with some of the gas does fall into the event horizon. It's never seen in this universe again. Bye bye, but not all of it. Some of it just looped around and around and around. And then, because of insert complicated physics here, it gets shot out in the form of two jets along the axis axis of the black hole in the north and south spin axis like a giant laser. These jets are enormous. They are traveling at close to the speed of light. They're made of electrons, protons, just subatomic particles stripped off of everything, a giant beam. These things can go out tens of thousands of light years. That's right. They can pierce outside of the host galaxy itself, carrying enormous amounts of energy and now a word from our sponsor. Better help. Burnout can be tough. Sometimes you're just putting in way more into life than you realize and Then out of nowhere, you're tired and unmotivated, and you just wanna sit around listening to mind blowing science.

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We don't fully understand the mechanism. The exact mechanism that launches a jet. It's complicated, but we do have literal pictures of these jets, so we know that they happen. The point of all this, the point of all this is that the black hole is able to project fire power like a cruise missile into the galaxy. It can deliver heat and energy throughout the entire volume of the galaxy. It exerts an outsized influence on the rest of the galaxy way more than it should. And what does all this extra energy do? Well, ironically, it chills the galaxy out, man by heating it up. Hold on, hold on. Let me explain. You see, if I want to make stars, I'm just a random star or a random cloud of gas hanging out somewhere in the suburbs of the galaxy. If I wanna turn myself into stars, I actually need to cool off because the heat of myself is is keeping me inflated.

And I don't want to be inflated. I wanna be a nice, compact, tiny little star. So in order to shrink to that size, I have to cool off. I have to release heat release radiation that will allow me to shrink and reach the densities necessary to trigger star formation. Let's say I'm doing that. I'm a cloud of gas. I'm cooling off, taking millions of years. No rush. Someday soon I'll get around to that star formation and then camo comes this laser death ray from the center of the galaxy heats me up and starts the process all over again. Now I now I gotta dump all this extra. You just reset the clock by like, 10 million years. Thanks a lot, pal. The action of the black hole of the accretion disk, the amount of energy and radiation released by the accretion disk itself just in terms of pure light, because it's hot, approximately a trillion degrees and bright. And then the action of the jet heats up the galaxy, and it prevents stars from forming.

But that's only part of the story. You see, this is a an event that we call a feeding event. When a lot of material falls onto the black hole forms, a large accretion disc forms a large jet. The jet slows down, even stops star formation in the galaxy. It spreads heat throughout the galaxy, but in addition to stopping star formation. The feedback from the black hole also heats up all the gas in the core and all that hot gas now that it's energized, is too hot and bothered to reach the black hole. It's the same physics as as making a star. If the gas wants to reach the black hole, it has to compress down into a tiny volume. But if it's really hot, it can't. It can't shed that volume. It can't condense, it can't compress. And so it just hangs out there, supported by its own heat against its own gravity. So the material stops falling into the black hole and the black hole shuts off the jets turn off the cruise missiles stand down the black hole that dark secret goes to sleep.

Then time passes. Ages go by, the galaxy forgets about its black hole, about its dark secret stars start forming again. Gas in the core cools off, begins condensing, reaches the black hole, and then the black hole awakens, reminding the galaxy of its dark secret, and the cycle starts again. This is feedback. This is regulation. The star formation history of a galaxy is intimately tied to its black hole. The black hole acts as a safety valve for star formation, preventing too many stars from forming all at once. This is what establishes the M Sigma relation. Black hole feeds black hole, ejects a lot of material or a lot of heat into its surroundings. This heats up the gas. This increases the velocity dispersion of the gas because there's more energy to go around. But then over time, the gas cools down. So now you've got some really hot pockets. This episode is not brought to you by hot pockets, so you've got some really hot pockets and some really cold pockets of gas.

The cold pockets of gas eventually cool down enough feed the black hole boom, another blast of energy. But then the cycle continues again and again and again. And every time the black hole feeds, every time there's one of these episodes. It gains mass because some of the material does make its way through the event horizon, and it increases the mass of the black hole. So for large Galaxies, there have been many, many, many cycles of back and forth, back and forth of feeding the black hole, and then shutting off, feeding, shutting, feeding, shutting. And so the black hole mass is increased. And then the entire core is getting destabilized by these blasts. These laser blasts from the core increasing the velocity of dispersion, increasing the jig lines and then for for smaller Galaxies. There's only been a few of these cycles back and forth where you just get a few episodes of feeding. Black hole grows a little bit, destabilizes, energizes, jiggle. Is the core just a little bit?

And then that's it. So we do have a connection between black hole mass velocity dispersion, which we call Sigma, and the mass of the galaxy. But like I said, measuring the mass of the galaxy is harder because you have to define what the edge is, and you have to make very careful observations and blah, blah, blah. And astronomers hate doing all that careful work. But that is the connection. The bigger the galaxy, the bigger the black hole, the more events that they have had. And as a part of this regulation of feedback, of cycles of cooling off and on the black hole acts ironically as a check on star formation. Because if you just take a galaxy and start forming stars. You'll use up all the gas pretty quickly. There's only a finite amount of gas in every galaxy. Yes, some reins in from beyond the galaxy, but that's not very much. And very, very quickly, relatively quickly. We're talking a few billion years, a galaxy, a typical galaxy will run out of gas.

It won't be able to make new stars anymore. It will reach peak star formation and just hang out there until it's done forming stars. And then it will shut off. Galaxies need their black holes. The black holes regulate star formation. They periodically slow it down by heating up the gas in the galaxy. They slow down the rate of star formation to keep it at an even tempered pace to keep it from just blowing up into a star bomb. It just keeps everything burning. It's this tight relationship between black holes and Galaxies, which is one of the reasons why it's hard to decide which came first. The giant black holes or the Galaxies they they co evolve our own supermassive black hole. Sagitta say star evolved with the Milky Way. We don't know which came first our giant black hole or the galaxy itself. It seems like to me they they came together. They evolved together when our galaxy was much smaller in its infancy. We had a smaller black hole and as our galaxy grew, we got a bigger black hole.

They co evolve, the Galaxies need. They're dark secrets. But sometimes that dark secret, the black hole can go too far. It can outright kill a galaxy, not kill, as in, there no longer exists a galaxy, but kill as in totally shut off star formation. Are you ready for another wonderful jargon word? Well, too bad, because here it comes anyway. Patreon go to patreon dot com slash PM Sutter to support the show. I honestly I. I can't express how grateful I am for every single contribution. Whether it's $1 a month or $100 a month, it it doesn't matter. I appreciate it, and it is your contribution. It's your choice to contribute to this show and keep it going. And I I treasure it all. I really do. Thank you so much. That's patreon dot com slash P MS U TT ER When you're done with the episode. Go check it out. No, The actual jargon word was quenching. This is a wonderful word astronomers use to mean what happens when star formation just shuts off.

This is what happens when the black hole goes nuts. When the dark secret takes over, releases too much energy into the galaxy, heats up the gas for too long and even in some cases literally removes gas from the galaxy altogether, physically removes, it just blows it away. And then the galaxy can't form new stars and it ends up dying. All the young stars, the blue stars, the white stars end up dying and you're left with a population of old red stars. We call these red and dead Galaxies or luminous red Galaxies. These are Galaxies whose dark hearts went too far when the black holes went too crazy when they release too much energy at once. Uh, but on the other hand, because physics is life and life is complicated and nice, neat little stories don't always work out. The dark secret of the black hole can sometimes power star formation in the core. Uh, those jets, those blasts as they blast through the core of the Galaxy, they can compress the clouds, which can trigger a new round of star formation.

So sometimes in the core of a galaxy, you get enhanced star formation shortly after these episodes while the rest of the galaxy gets reduced star formation. And that's weird and wonderful and complicated, but that that's life. Speaking of life, what I teased at the beginning that without our galaxy's dark secret, we may not be here today. How does that work? Well, imagine a galaxy without a black hole. Just star formation just stars. Well, in that galaxy, star formation would end quickly. Within a few billion years, it would form all the stars it's ever gonna form and then just shut off. It would stop because it used up its gas too quickly. It wasn't efficient with its use of its finite amount of gas. It used it up all in one burst instead of, you know, being a little bit more measured about this. Which means stars like our sun never appear. Stars like our sun. Solar systems like our own, require multiple generations of stars in order to get where they are to have the composition, the the amount of oxygen and carbon and silicon to make planets and worlds with water and potentially life.

If you make all your stars at once, there isn't this enrichment. You know every generation of stars makes more of the heavy elements that get mixed into the next generation of stars. But if you only have one or two generations of stars, then that's it. You don't get elevated levels of carbon or oxygen or silicon. You just get a little bit and then star formation shuts down. And then that's the end. So for life to happen potentially. I know it's a little bit of a stretch, but it's fun to think about potentially. In order for us to be here today, we needed the Milky Way Galaxy needed its dark secret, its giant black hole. That giant black hole Sagittarius, a star, has regulated and controlled star formation for billions of years, keeping it at a moderate pace, allowing multiple generations of stars to come and go, with each generation adding more to the mix than the generation before to make something like our solar system possible.

This regulation, this feedback that controls and moderates the star formation rate in Galaxies also produces this observational consequence, which we call the M Sigma relation. Sometimes the black holes go too far. Sometimes they don't do enough. Nothing's perfect. But what is? So that's what Galaxies do with their dark secrets. Sometimes they control them. Sometimes they're controlled by them. But no matter what the dark secret is always there, what do you do with yours? Thank you to all the questions that led to today's episode, including Aaron M on email at BD. So Polar on Twitter, Joan L on email. Lieber Tour on Twitter. There we go, Andy on email. Benji on Facebook. BW AD E 88 on email at Alpha Channel. M on Twitter. David Paul on email, Bob P on email and Carissa B on Facebook. Wow! So many questions. Curious about black holes and their relationship with Galaxies, I love it.

Please, please, please send questions to hashtag. Ask the spaceman. Ask the spaceman dot com and don't forget you too, can support the show on Patreon That's patreon dot com slash PM Sutter, thanks to my top patreon contributors this month. Justin G, Chris L Barbeque Duncan M, Corey D, Justin Z, Nate H, Andrew F NAIA Aaron Scott M Rob H Loyalty. Justin Lewis and Paul G, John W, Alexis Aaron J, Jennifer M, Gilbert M, Tom B, Joshua Kurt and Bob H. I really, really do appreciate all the contributions that everyone makes. That's hashtag. Ask us, spaceman, or ask us spaceman dot com or ask us spaceman at gmail dot com. And I will see you next time for more complete knowledge of time and space. Ain't got no tears left to cry So I'm picking it up. I'm picking it up, I'm loving, I'm living, I'm picking it up.

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