How the Universe Works - S06E07 - The Quasar Enigma
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00:00Mysterious lights shine out from the edge of space, brighter than a trillion suns.
00:10They had to be the brightest objects we'd ever seen in the universe,
00:15putting out amounts of energy that we couldn't possibly explain.
00:18Energy so powerful they can incinerate planets and rip stars to pieces.
00:24These are among the most mysterious and most energetic phenomena in the universe.
00:29They can destroy galaxies, but may also be the key to their survival.
00:35These objects are hotbeds of all kinds of crazy physics.
00:41These celestial powerhouses are called quasars, and we may owe them our very existence.
01:00For decades, astronomers have observed brilliant points of light in the night sky.
01:06But there was something strange about them.
01:10They look like a pinprick of light, like a star, and so they were really mysterious initially.
01:17Are they a new type of star, or something else entirely?
01:25One of these strange objects is hiding within the Virgo galaxy cluster.
01:31From Earth, Object 3C273 looks just like a nearby star.
01:38But scientists studying its light made a stunning discovery.
01:44It was tremendously far away.
01:48It not only was not in our galaxy, it wasn't even in any galaxy that they could see.
01:53It was over a billion light-years away.
01:57If it's that far away and as bright as it was,
02:00this must be the most luminous known object in the universe.
02:06And this is one of the reasons these were so mysterious for so long.
02:11These objects are called quasars.
02:16These objects are so bright that despite the incredible distance,
02:21to us they look like nearby stars.
02:24They're called quasi-stellar objects, quasars for short.
02:29But what are they?
02:32With more detailed observations of quasars,
02:35we found that they don't originate from any random place.
02:39They always come from the core of a galaxy.
02:44A quasar is the ultra-bright core of an extremely distant galaxy.
02:49The reason we can see them at all is the result of their incredible power.
02:54A single quasar outshines an entire galaxy.
03:00Hundreds of billions of stars' worth of energetic output
03:04all concentrated into a single source.
03:09One of the most energetic events that human beings have ever been witness to on Earth
03:15is the biggest atomic bomb ever exploded.
03:19We're aware of quasars out there that are putting out more energy
03:23than one trillion trillion of those massive atomic bomb blasts per second.
03:29It is just a huge amount of energy packed into a very, very, very small volume.
03:35That is almost literally unbelievable.
03:40But if they're so small, how can they produce such vast amounts of energy?
03:46How can you generate something that's so bright, so energetic, but not be very large?
03:52What could possibly power something like that?
03:56Hiding within a quasar must be a very powerful engine.
04:01What kind of object can generate that much energy, that kind of power, to create these things?
04:07There's only one thing in the universe that's both massive and dense enough.
04:13A black hole.
04:16That is the only thing that we know of in the universe that could power a quasar.
04:22Stars 25 times more massive than our Sun
04:26eventually lose their lifelong battle against gravity.
04:30They suffer a catastrophic collapse,
04:33and all their incredible mass is compressed into a single point,
04:43giving birth to a black hole.
04:46Black holes are totally unique. There's nothing else like them in the universe.
04:49They're extremely massive and dense.
04:51They have so much mass crammed into such a small space
04:54that they distort space a tremendous amount.
04:57They create regions called event horizons.
05:02The boundary of these monsters, the event horizon, is the point of no return.
05:08Anything that crosses that line is never coming back.
05:12Not even light.
05:15So as light tries to freely fly through space and time,
05:19that space and time is bent back in on itself.
05:22And that means light can never escape.
05:27We see black holes across the universe.
05:31They range in mass from normal, three times more massive than our Sun,
05:36to supersized.
05:39Supermassive, in fact.
05:42So we think that quasars represent the largest black holes that we see in the universe.
05:47We're talking billions of times more massive than our Sun.
05:51It is literally almost at the edge of your ability to perceive, right?
05:56Our ability to even think about black holes being so massive
06:00that they're a billion times the mass of our Sun.
06:05It's this enormous mass that leads to black holes' enormous gravity.
06:09And scientists think that only supermassive black holes
06:13are able to provide the energy to power quasars.
06:16Yet quasars can outshine their entire galaxies.
06:20Black holes suck things down. They're black.
06:23How can they possibly be bright?
06:28Black holes are voracious eaters.
06:31They drag in gas and dust,
06:34which builds up in a ring of material around the black hole
06:37called the accretion disk.
06:40You can think of it as a giant whirlpool of matter
06:43that's trying to fall onto this supermassive black hole.
06:46Well, it can't all fall in at once.
06:49And so there's a lot of friction in this disk.
06:53This friction increases as the motion of the gas and dust speeds up.
06:58That could be a significant fraction of the speed of light.
07:02If you rub your hands together at a significant fraction of the speed of light,
07:05they will vaporize. They will get very hot.
07:08And so the material in this accretion disk can actually get heated
07:11to millions of degrees.
07:15When matter heats up,
07:18it produces radiation,
07:21which we see as light.
07:24The center of the galaxy shines,
07:27visible billions of light-years away.
07:30That's where stuff gets dense and hot,
07:33and there you have a quasar.
07:36So even though black holes are the darkest objects in the universe,
07:39they in turn power the brightest objects in the universe.
07:42For decades, quasars have been baffling astronomers.
07:45And weirdly, it now seems that even the brightest objects
07:48can remain unseen.
07:51But you know what you can't see?
07:54The brightest galaxy we've discovered so far.
07:57November 2015.
08:00Astronomers use the Atacama Large Millimeter Array, ALMA,
08:04to peer into the universe's brightest objects.
08:07Astronomers use the Atacama Large Millimeter Array, ALMA,
08:10to peer inside the brightest galaxy in the universe.
08:13A quasar named W2246
08:16emits as much light as 350 trillion suns.
08:19But to our eyes,
08:22it's invisible.
08:25And that's because it emits in the infrared.
08:28It's not a normal galaxy.
08:31It's a hot dog galaxy.
08:37Hot dogs, hot dust-obscured galaxies,
08:40are surrounded by clouds of interstellar dust.
08:43So the incredible light from this quasar
08:46is hidden from view.
08:49Because a quasar is a supermassive black hole
08:52that has stuff around it falling in,
08:55you can imagine that sometimes there's so much stuff around the black hole,
08:58the quasar is completely obscured.
09:01The quasar's visible light is absorbed by the thick layers of dust.
09:05But what does escape is lower-energy infrared light.
09:08And at this wavelength,
09:11it's staggeringly bright.
09:14The cool thing about hot dogs, besides their name,
09:17is that they weren't expected.
09:20We're finding that half of the most luminous quasars in the universe
09:23are hot dogs.
09:29Quasars are so bright
09:32that they travel from the very edge of the universe,
09:35over 13 billion light-years away.
09:39That means they burst into life
09:42less than a billion years after the Big Bang.
09:48But how could such monsters exist
09:51so soon after the birth of the universe?
10:03We have discovered stunningly luminous quasars across the universe
10:06that are brighter than their whole galaxies.
10:10And that light has been travelling towards us
10:13for billions of years.
10:17As fast as light goes, 186,000 miles per second,
10:20it takes time to cover the vast distances between the galaxies.
10:23So if you look out into space,
10:26you see quasars that are so bright
10:29as they were millions of years ago,
10:32or billions of years ago.
10:35And the light has just arrived at your eyes tonight.
10:39In 2017,
10:42scientists at the Las Campanas Observatory in Chile
10:45focused their telescopes on the most ancient part of the universe
10:48and got a huge surprise.
10:52This quasar is only about 600, 700 million years
10:55after the Big Bang.
10:58And this black hole weighs about 800 million times
11:01as much as the sun.
11:05It's the oldest quasar ever found.
11:09It formed around 700 million years after the Big Bang,
11:13when the universe was mostly a soup of hydrogen and helium gas.
11:21We know quasars are powered by supermassive black holes.
11:24But discovering a black hole this big,
11:27so early in the evolution of the cosmos,
11:30is a huge mystery.
11:34One of the big questions we have in astronomy
11:37is how did these supermassive black holes form?
11:40How did they form so early in the universe?
11:44This is an interesting mystery.
11:47We see quasars about as far as we can see.
11:50That means these objects existed in the very earliest galaxies.
11:53That means million or billion solar mass objects
11:56were able to accumulate.
11:59I don't think we really have a good picture of how that can happen.
12:05The mystery lies in how fast black holes grow.
12:08They grow by eating,
12:11and at an astonishing rate.
12:15When you eat, eventually you get full.
12:18You've had your last bite.
12:21They are never done being hungry.
12:24They are insatiable.
12:29A black hole consumes everything that gets too close,
12:32growing larger and larger.
12:36However, there's a limit to how fast they can grow.
12:40There's not enough time, a billion years after the universe was created,
12:44for them to get to a billion solar masses.
12:47It's just too short a time.
12:50There has to be another process in addition to all this eating
12:53that caused the seed to form.
13:00To swell to a billion solar masses,
13:03we now think these giants grow from smaller seed black holes.
13:16We know that black holes usually form from exploding stars
13:19of around 25 solar masses or greater,
13:22and that's quite small.
13:25To be born supermassive, you need a supermassive star,
13:28a stellar giant,
13:31born from the primordial gases of the early universe.
13:36And so you had these huge clouds of mostly hydrogen,
13:39a little bit of helium, and that's basically all that was there.
13:42And as material cooled,
13:45it would collapse into these big, big stars
13:48of just balls of hydrogen and space.
13:52These supergiant stars lived fast and died young.
13:58When they died, they formed these seed black holes.
14:01So that's one way you could get a massive seed,
14:04is that you just have the earliest stars collapsing
14:07into a black hole when they died.
14:10But there's a problem.
14:13It seems even these supermassive stars
14:16couldn't have produced big enough seed black holes.
14:20There has to be another way
14:23to generate a billion solar mass black hole
14:26in less than a billion years.
14:29How else could the universe create supermassive black holes
14:32from just thick clouds of gas?
14:35One theory of how you get supermassive stars
14:38One theory of how you get these supermassive black holes
14:41is that you just have one big collapse event.
14:44We call it direct collapse.
14:48It's only a theory,
14:51but we think this is how direct collapse would work.
14:55Huge, superdense clouds of hydrogen gas clump together.
15:00Gravity then builds up and drags in more gas,
15:03which becomes more and more dense,
15:06until the gas collapses under its own weight.
15:10Instead of forming a star,
15:13it crushes down straight to a supermassive black hole.
15:18Then a galaxy starts to form around the giant.
15:22Gas streams towards the centre,
15:25getting hotter and hotter
15:28until a quasar explodes into life.
15:31And these are so bright,
15:34we can see them today, 13 billion light years away.
15:39All of these are ideas right now.
15:42Theorists are working really hard to make these models work.
15:46Finding more and more distant quasars will teach us
15:49what those seeds are that form into the quasars.
15:52It might teach us some new physics
15:55to create these big black holes that quickly.
15:58The more we investigate quasars,
16:01the more we discover the universe and the galaxies within it.
16:05Galaxies that appear calm and peaceful places.
16:09Yet closer inspection reveals extreme violence
16:12and huge scars extending from their centres
16:15out tens of thousands of light years.
16:19What could have caused such devastation?
16:23This is galaxy cluster Hydra A.
16:27Here are the scars.
16:30And when viewed in multiple wavelengths,
16:33the cause was revealed.
16:36Two colossal jets of energy
16:39blasting out from the galactic core,
16:42shooting out from the heart of a quasar.
16:45They tear through the galaxy and out into space,
16:48creating voids in the surrounding gas.
16:53The amount of energy in these jets is staggering,
16:56soul-crushing, mind-destroying.
16:59Think about how much energy is wrapped up in these quasar jets.
17:02Something can take many, many times the mass of the sun,
17:05accelerate it to speeds near the speed of light
17:08and throw it out across hundreds of thousands of light years.
17:11These jets seem to launch out from the quasar's core.
17:16Supercharged particles twisted into tight beams of energy
17:19moving at millions of kilometres per hour,
17:22heated to trillions of degrees.
17:26Normal quasars are incredibly powerful.
17:29But quasars with jets are much, much more destructive.
17:35You're taking the power of billions or trillions of stars
17:39and focusing them into narrow jets
17:42which basically march across the universe like death rays.
17:45If you're too close to this thing and you're in its way,
17:48yeah, you're not going to be in its way for long.
17:52And it's not just the galaxy that suffers.
17:55These jets coming out are more powerful than the Death Star.
17:58They would destroy not just a planet going through their path,
18:01but, like, stars, whole solar systems.
18:05This is exactly what's happening in system 3C321.
18:10Viewed in visible light, all we see is a pair of galaxies.
18:15But when viewed in multiple wavelengths,
18:18we see the larger galaxy firing out a huge death ray
18:22that is ripping through its smaller neighbour and out into space.
18:28Imagine being in the direction of a jet.
18:31If a jet's coming at you, it's over.
18:34You're in for a deep world of hurt.
18:39Planets would be destroyed.
18:41Stars would explode.
18:44These jets travel incredible distances through space.
18:48So the largest known jet is about 1.4 megaparsecs in length,
18:53and a megaparsec is about three million light-years.
18:57So we're talking almost five million light-years from end to end.
19:02Eventually, the jets do stop when they slam into the intergalactic medium,
19:07the thin film of gas that surrounds galaxies.
19:10It can go hundreds of thousands of light-years,
19:13striking the intergalactic medium and setting in a blaze.
19:18The impact sends out massive shockwaves,
19:22like in galaxy Pictor A.
19:26They form these huge puffy clouds.
19:28They look like sort of cotton swabs.
19:30You get a narrow jet with these big puffy things at either end.
19:35But it seems that quasars with jets are a very rare species.
19:40As only 10% have them.
19:42And no one really knows why.
19:46The huge jets coming out of quasars are things we can see clear across the universe.
19:50The amazing thing is, we don't even really understand how they're created.
19:54It's such a complicated process that it's hard to untangle the astrophysics going on here.
20:01So we think that the formation of jets arises from the accretion disk,
20:05or the spinning disk of gas that's spiralling close to the event horizon of the black hole.
20:12This is our best theory.
20:15Gas falls towards the supermassive black hole,
20:18which moves faster and faster, getting hotter and hotter.
20:23When you heat gas to extreme temperatures, it becomes a plasma,
20:27full of electromagnetically charged particles.
20:31We think in the regions around supermassive black holes,
20:34you have quickly moving charged particles.
20:36This creates magnetic fields.
20:40As the particles swirl around the black hole,
20:42they build up a powerful magnetic field.
20:46This field builds in intensity and surrounds the black hole.
20:52That strong magnetic field can wrap itself around the black hole,
20:58and any charged particles in the accretion disk will follow the paths of those magnetic fields.
21:06And where there is a body with a magnetic field, there are magnetic poles,
21:10and that's where things can escape.
21:12It gets wound around, spiralled up, and shoved up into a jet.
21:19The pressures in that disk are incredibly high,
21:21and these magnetic fields are incredibly strong,
21:24and they're wound up tightly by the spinning black hole,
21:27and what you end up with are jets that are collimated and incredibly powerful.
21:34The jet blasts out from the poles of the black hole at 99% the speed of light.
21:42These quasars are big generators.
21:45They convert gravitational energy into magnetic energy,
21:49and that magnetic energy gets converted into kinetic energy in the launching of these jets.
21:58They carry so much energy, they can be detected clear across the universe.
22:08Quasars continue to confuse astronomers,
22:11and they're now presenting us with another problem.
22:14Quasars should take millions of years to ignite.
22:19But recently, we've detected one that switched on in a cosmic heartbeat.
22:28Quasars
22:40So far, we have discovered over 200,000 quasars.
22:44And in June 2016, we discovered another.
22:50But this one is different.
22:53It ignited in just 500 days.
22:56In cosmological terms, that's just the blink of an eye.
23:00That's incredibly weird, because remember what we're talking about here.
23:04We're talking about the consumption of galaxy-sized quantities of gas.
23:08How could that be fast?
23:11That's a very rapid process.
23:14Usually, when we think of astronomy, we think of astronomically long timescales,
23:19where we don't get to see things happen in real time.
23:22So when we see these incredible engines in our universe
23:26changing their complete nature by turning on over the course of a few months or a few years,
23:31that's a little bit frightening.
23:34Scientists are theorizing that quasars switching on
23:38may be a natural part of a galaxy's life.
23:43Galaxies are not fixed things. They're constantly changing, evolving.
23:48It's also true that suddenly we see these things switch on,
23:51and it's kind of mesmerizing to imagine a situation
23:54within a year's time a galaxy could go from being normal to being active.
23:58It's thought that most, if not all, galaxies go through a phase of development
24:05that includes forming a quasar, that it's a normal part of a galaxy growing up.
24:12Quasars can act a lot like adolescents, tantrums included.
24:16This was when they were shining brightly, eating mass at an incredible rate.
24:20It's almost like your teenage years.
24:22You know, you ate everything in sight, and things were pretty messy.
24:25You might have undergone a lot of mood swings.
24:28It's kind of like that with a quasar.
24:31When a quasar gets hungry and begins to consume, it can switch on.
24:37So what flips the switch? What turns a quasar on and off?
24:40It fundamentally has to do with gas getting into the centers of galaxies.
24:44When stuff gets in there, a black hole is waiting, and then it gets bright.
24:49So where does a quasar find enough gas to feast on?
24:53To turn on a quasar, you need certain conditions.
24:57You need a supermassive black hole, and you need a lot of material dumped on it.
25:01What does this? A galaxy collision.
25:05Galaxies are not locked. They move through space, and sometimes they collide.
25:12When they do, we think that the supermassive black holes at their cores collide and merge.
25:19Then the gas from the combined galaxies streams towards the new supermassive black hole.
25:25It's almost like when you have these two galaxies merging, that they have all new food.
25:31It's a brand new dinner plate, a brand new buffet of food to eat.
25:34Well, that's a feeding source, right?
25:36You're going to start a feeding frenzy on the black hole that's sitting there.
25:39The gas spirals in, heats up to millions of degrees, and the galactic core lights up.
25:51And a quasar is born.
25:58It appears merging galaxies provide the best conditions for quasar formation.
26:02But when astronomers studied the rapid ignition of these unusual quasars,
26:06they didn't find evidence of merging galaxies.
26:10Something else must have switched them on.
26:15When we think about the size scales of a typical quasar,
26:19we think it must take years to really turn on.
26:22If you're thinking about a flow of gas passing through the central parts of a galaxy,
26:28to encounter the black hole, to activate the dynamo, to launch a jet,
26:32this should take a healthy amount of time.
26:35This is big stuff.
26:38So what could speed this up?
26:41One idea is that something catastrophic happens inside the accretion disk,
26:45the ring of gas surrounding the black hole.
26:50An active quasar means that a black hole is in the black hole.
26:55An active quasar means that a black hole is eating something.
26:59So when you see a quasar turn on, you know that something went very wrong around the black hole.
27:04Maybe some of the disk fell in, maybe even a star got a bit too close.
27:09Accretion disks are not just gas and dust.
27:13Black holes will drag in anything and everything.
27:19There are stars, there's gas, there's gas clouds, there's stars in formation,
27:22there are stars dying, a lot is happening there.
27:25But basically what happens is that star veers too close,
27:28and it is basically stripped, it's pulled apart.
27:33The enormous gravity of the black hole could tear a star to pieces,
27:37giving rise to a sudden surge of energy within the accretion disk.
27:42Or maybe an exploding star could be the catalyst for quasar ignition.
27:48So it could be that a supernova going off in the accretion disk
27:52triggers an avalanche of material suddenly being able to fall in on the black hole.
27:59In both scenarios, a sudden rush of hot gas would heat up the accretion disk,
28:05switching on the quasar.
28:08You need to do something violent to a galaxy to let it really feed enough
28:12to create a quasar activity.
28:17Once activated, a quasar can shine for millions of years,
28:22blasting out energy and destructive jets.
28:28We see them across the universe,
28:30and it seems the basic ingredients are quite common,
28:33even in our own neighbourhood.
28:38Let's put all this together. What do you need for a quasar?
28:40You need a galaxy, you need a central supermassive black hole,
28:44you need gas, and you need stuff falling into that black hole
28:49to create the quasar phenomenon.
28:51Well, we live in a galaxy, the Milky Way,
28:54and it has a central supermassive black hole,
28:56and there's gas orbiting around near there.
29:00That doesn't sound good.
29:02Our future certainly does not look bright.
29:05Yet we used to think of our galaxy as a peaceful place.
29:09But what if it's hiding a violent past?
29:12What's been discovered recently is kind of fascinating.
29:15There's two large evacuated bubbles emanating from the centre of our galaxy.
29:20These bubbles are full of superheated gas,
29:23and they're moving out of our galaxy at 3.2 million kilometres per hour.
29:29These bubbles are huge.
29:31They're on the scale of the galaxy itself.
29:34They're 50,000 light-years long.
29:37They're so big that if you were to map them out on the sky,
29:40they would stretch from horizon to horizon.
29:43These gas bubbles appear similar to those seen
29:46within distant galaxy clusters such as Hydra A.
29:50One big question is what could have put that gas there?
29:53What could have made this gas so hot
29:55that it's able to puff out so far from the galaxy?
29:57One possibility is that this was driven by an active phase
30:01in our galaxy's history.
30:04Far away in the heart of our galaxy,
30:06hidden by gas and clouds,
30:09lies a giant.
30:12A supermassive black hole called Sagittarius A star.
30:18At the moment, it's quiet.
30:20But is it dead or just sleeping?
30:25The only thing that could have created these vast lobes of material
30:29above and below the Milky Way
30:31are giant jets streaming out of the core of our galaxy.
30:34Well, doesn't that sound a lot like a quasar?
30:39But unlike Hydra A,
30:41the expelled gas doesn't date from hundreds of millions of years ago.
30:46It was ejected within the last six million years.
30:50Almost all quasars we see are in the distant universe,
30:53which means they're in the distant past.
30:55But here we have recent activity.
30:59Just six million years ago, our central black hole was feeding.
31:03No-one really expected that because for as long as we can remember,
31:06we've thought about our Milky Way galaxy as being quiescent.
31:09That is to say, not really eating too much,
31:11sort of like a black hole on a diet.
31:14Something must have broken its fast.
31:18Perhaps an unfortunate group of stars strayed too close.
31:23Whatever the method, Sagittarius A star began gorging on its new source
31:28and woke explosively.
31:32It's jets blasted over a trillion, trillion, trillion tonnes of gas out of the galaxy.
31:41This was only a small outburst as our quasar was smaller than most.
31:47But it's possible our sleeping giant could wake up again,
31:51much bigger and much more dangerous than ever before.
31:56There's something coming up.
31:58That means that one day we may walk outside, look at the night sky
32:02and realize the quasar has turned on.
32:06We know quasars can be triggered by galactic collisions.
32:10And we know a galaxy is heading our way.
32:13Public safety announcement.
32:15The Milky Way galaxy is on a collision course with Andromeda.
32:19It's traveling at us at about 70 miles a second.
32:22And in about four billion years,
32:24these two galaxies are going to collide.
32:26Both of us have supermassive black holes.
32:28Ours is four or five million times the mass of the sun.
32:31And Andromeda's black hole is 20 times larger.
32:35Eventually, those black holes will start spiraling around each other
32:38and they're destined to merge.
32:42The new black hole will be much bigger than Sagittarius A star.
32:46And this supergiant will have a fresh load of gas to consume.
32:51This will be an incredible explosion for the Milky Way.
32:54Maybe even the biggest explosion it's ever had in its whole life cycle.
32:59A quasar could be born far bigger than anything we've experienced before.
33:06In the chaos of the merger, our solar system could migrate,
33:10moving closer to the galaxy's core.
33:14Closer to the quasar.
33:17We will have a front row seat to not only colliding galaxies,
33:21but in fact, something even more amazing and terrifying,
33:25a quasar being switched on.
33:29The closer we are, the bigger the spectacle.
33:33It would look like a single bright source in the sky,
33:35almost like a second sun.
33:40But with the beauty will be incredible heat.
33:43Quasar winds and perhaps even jets.
33:48What will this all mean for Earth?
33:52The atmosphere is going to be stripped away.
33:54Oceans are going to boil.
33:56Maybe the rock under your feet will melt.
33:59I mean, we're talking about a tremendous amount of energy here.
34:02There would absolutely be no life on Earth.
34:08The new black hole will be much bigger than Sagittarius A star.
34:12The newly ignited quasar may be so energetic,
34:18it blasts trillions of tons of gas out of the Milky Way.
34:22The basic ingredients for future stars and planets.
34:27That's where stars form and that's where planets form.
34:30And so, if you kick all the normal matter out of the galaxies,
34:34you no longer get stars and planets and humans and civilizations.
34:38That doesn't sound like such a good idea to me.
34:41With their terrible destructive force,
34:44quasars seem like bad news for galaxies.
34:47But we're now discovering there's another side to them.
34:52Quasars are putting out so much energy,
34:55they appear to be so disruptive to their environment.
34:58But it could be that without them, we wouldn't be here.
35:03For all their ferocious power,
35:05quasars might be the ultimate cosmic creators.
35:12EXPLOSIONS
35:21Ejecting unimaginable power,
35:23quasars can cause chaos in their local environment.
35:28But it's possible that this might be essential for a healthy galaxy.
35:33It might even create the conditions for life.
35:38We're discovering that for all their destructive properties,
35:41quasars also have a creative side.
35:43So it very well may be that the universe we see around us
35:46was shaped by quasars.
35:50Stars are the essence of a galaxy.
35:53But too many stars can actually be a problem.
35:58Having a lot of new star formation is kind of a good thing,
36:01but too much of it can be a bad thing.
36:04But too much of it can be a bad thing.
36:06When new stars are born,
36:08many of them are going to be hot, large blue stars.
36:11But eventually, those young hot stars are going to start to die.
36:15And when they do, they're going to explode violently as supernovae.
36:23There will be new black holes, there will be jets,
36:26there will be shocks that actually go through the gas of the galaxy.
36:29All of that is effectively killing the galaxy.
36:35When galaxies produce too many stars,
36:38it creates a chaotic and unstable environment.
36:42Stars and planets are wiped out by intense radiation
36:46from supernovas and black holes.
36:49Left unchecked, the galaxy destroys itself.
36:54But some galaxies have a cosmic guardian,
36:57creating quiet, peaceful neighbourhoods.
37:01Something is controlling star birth.
37:05In order for stars to form, you need cool gas, molecular hydrogen.
37:10And a quasar is anything but cool.
37:13They're cool to think about, but they're really hot if you're near one.
37:16If you're a galaxy and you're ready to just form a bunch of stars,
37:20but then a quasar turns on in your heart,
37:23that's going to impact how those stars form.
37:26So the fate of cold gas in a galaxy is incredibly tightly coupled
37:31to the fate of star formation throughout that galaxy.
37:34And we think that quasars deposit so much energy
37:37into their ambient surroundings
37:39that they can prevent the formation of very, very cold gas
37:44by heating it up.
37:49Quasars pump huge amounts of heat into their environment.
37:53One way they do this is through an extreme cosmic hurricane
37:57called a quasar wind.
38:01You have a wind of light.
38:03There's just so much light that's being powered
38:06by this spinning disk of material around this supermassive black hole
38:09that that light can actually push on the dust and the gas
38:12in the core of the galaxy and push it outwards very rapidly.
38:15And these winds can be really fast.
38:17It's not the kind of wind you and I are used to thinking about.
38:20It's actually a stream of high-energy particles.
38:22In some cases, these particles can be traveling
38:24at hundreds of millions of miles an hour.
38:26The cool star-forming material in the galaxy
38:30becomes hot and turbulent.
38:33Instead of having these nice, tranquil clouds
38:36of molecular hydrogen collapsing under gravity,
38:39now here comes this big, giant, nasty quasar wind disrupting everything.
38:43It's going to basically warm the galaxy too much,
38:46and that galaxy is going to have what we call quenched star formation.
38:50So it's going to have a lower rate of forming new stars
38:53than we would expect it to.
38:57Quasars have another star-suppressing weapon
39:00in their cosmic armoury.
39:04There's another mode that we think exists,
39:06and that is mechanical or kinetic feedback.
39:08And that is literally this, right?
39:10That is momentum injection.
39:12That is a freight train ploughing through a galaxy
39:15in the form of a jet launched by the supermassive black hole,
39:19and that jet, just like a truck ploughing a snowy street,
39:24pushes material out of the way.
39:27The jets bulldoze the gas to the outskirts of the galaxy.
39:31They blow out tremendous amounts of energy,
39:33they spit out gas,
39:35and what happens is it stops the process of star formation.
39:38There's less fuel for new stars to be made.
39:41But quasars are fickle,
39:43and the role of jets is more complex than it seems.
39:47In some cases, they may even be responsible for producing stars.
39:52That's called jet-triggered star formation,
39:55and there's growing evidence that it exists,
39:57at least in some galaxies
39:59and at least over some spatial scales throughout a galaxy.
40:04In 2017, astronomers at ALMA discovered star formation
40:08in the wake of jets emitting from the centre of the Phoenix Cluster.
40:13We also see galaxies where new stars are being born
40:16close to the jets themselves.
40:20Jets drive out the star-forming gas,
40:23so how can they also cause stars to form?
40:26So what happens is you can pile up cold clouds
40:29that otherwise would be having nothing to do with each other,
40:32and you can enhance the amount of star formation along the jet axis
40:36because, just like that snowplough, you're compacting snow together,
40:40and it's from that snow, or cold molecular gas in the case of a galaxy,
40:44that new stars are born.
40:46Quasars are a contradiction,
40:49sometimes destructive, sometimes creative.
40:53It seems they may even bring balance to a galaxy.
40:58However, if they continue ejecting gas,
41:01they would permanently shut off star birth,
41:04killing their galaxy.
41:07Luckily, they stop before causing this fatal damage.
41:12Eventually, the gas in the centre of that galaxy is going to run out,
41:16and when it does, it's like a light switch and you're turning the quasar off.
41:22Quasars are fuelled by cool gas.
41:25Having blown away its fuel, the quasar itself will die.
41:31Gas clouds cool and stars begin to form again.
41:35But the cooling gas also falls back towards the supermassive black hole.
41:41Providing new fuel for the quasar to reignite.
41:46And so the black hole in a lot of ways can shut itself off,
41:50just like a thermostat in your living room,
41:52where if the room gets too cold, your thermostat kicks in,
41:55the heat turns on, and gets to a point where the room is now too hot,
41:58and the thermostat kicks off.
42:00When the quasar turns on, it's like you've stopped star formation,
42:04and then when it turns off, it's like you've turned star formation back on.
42:08Quasars regulate star formation so that not too many stars are formed all at the same time.
42:15Quasars slow down the rate that galaxies use their fuel,
42:19extending the life of the galaxy.
42:22So we think of quasars as these incredibly violent phenomenon.
42:26But in actuality, they're much more subtle and elegant than you might think.
42:29In a sense, they're a force for creation as well,
42:32because they are mitigating how stars are forming in galaxies.
42:36It's very likely that quasar activity is essential for galaxy development.
42:41So in that sense, we have an equilibrium generated by quasar activity.
42:45It could keep them on an even keel.
42:49Quasars are a rite of passage for young galaxies,
42:53graduating from chaotic, star-forming adolescence
42:57into mature, stable galaxies like our own.
43:02The existence of quasars may be essential to the health of a galaxy.
43:06But we think they play an incredibly important role in the evolution of galaxies
43:10throughout cosmic time.
43:13Quasars are the most energetic objects in the universe.
43:16They seem to almost defy the laws of nature.
43:19But they're also tools.
43:21They're tools that allow us to understand our own origins.
43:24We can use quasars to help understand the origins of galaxies,
43:27the origins of stars.
43:31Quasars help create stable galaxies,
43:35the sort of galaxies that could sustain life.
43:40We are connected to the universe in a beautiful and fundamental way.
43:45So quasars, even though they're incredibly violent,
43:49they're a part of galaxies, and they're a part of galactic evolution.
43:55Sometimes, in order to create, you have to destroy.
44:03Quasars have shaped the universe.
44:06Without them, we might not even be here.