# What Happens to a Galaxy When Its Black Hole Jets Finally Die?

An episode of [Bedtime Astronomy](https://invisiblenetwork.ai/podcast/1736397134), 3 Sep 2026, 22 min.

Audio: https://dts.podtrac.com/redirect.mp3/api.spreaker.com/download/episode/74375889/fadingradiosp.mp3

Astronomers have discovered that dying radio galaxies remain active long after their black hole jets shut down—but for far less time than previously thought. These faint remnants typically survive for just 8–42 million years, offering new clues about the intermittent lives of supermassive black holes and the evolution of galaxies across cosmic time. Thank you for listening to Bedtime Astronomy — your guide to the cosmos. New episodes on space exploration, NASA missions & the latest astronomy breakthroughs. This episode includes AI-generated content.

## Transcript

[0:03] Speaker 1: Welcome to Bedtime Astronomy. Explore the wonders of the cosmos
[0:07] Speaker 1: with our soothing Bedtime Astronomy podcast. Each episode offers a
[0:12] Speaker 1: gentle journey through the stars, planets, and beyond, perfect for
[0:16] Speaker 1: unwinding after a long day. Let's travel through the mysteries
[0:20] Speaker 1: of the universe as you drift off into a peaceful
[0:22] Speaker 1: slumber under the night sky.
[0:27] Speaker 2: Imagine, if you will, the most powerful engine in the entire.
[0:32] Speaker 3: Universe, right, something truly massive.
[0:34] Speaker 2: Yeah, we are talking about a super massive black hole
[0:37] Speaker 2: right at the center of a galaxy, and it is
[0:40] Speaker 2: actively consuming matter, and in the process, it's blasting out
[0:44] Speaker 2: these twin opposing jets of energy at nearly the speed
[0:48] Speaker 2: of light, which.
[0:49] Speaker 3: Is just a mind boggling amount of energy.
[0:51] Speaker 2: Exactly. These jets are so unbelievably massive that they actually
[0:56] Speaker 2: dwarf the host galaxy itself. But okay, imagine walking up
[0:59] Speaker 2: to that colossal cosmic engine and just pulling the power.
[1:03] Speaker 3: Courses, unplugging it completely.
[1:05] Speaker 2: Yeah, unpugging it. What happens to all that energy? I mean,
[1:07] Speaker 2: it doesn't just instantly vanish into thin.
[1:09] Speaker 3: Air, right, doesn't. It leaves behind this massive fading signature.
[1:13] Speaker 3: They tend to view space as a series of static photographs,
[1:16] Speaker 3: but it is deeply fluid. It's a really dynamic environment.
[1:20] Speaker 3: So when that central engine switches off, the colossal structures
[1:23] Speaker 3: of plasma and energy that it created over millions of years,
[1:27] Speaker 3: they're just left behind. They get stranded out there in
[1:29] Speaker 3: the dark intergalactic medium.
[1:31] Speaker 2: And the crazy thing is, whenever you look up at
[1:33] Speaker 2: the night sky, you are likely staring right through them. Oh,
[1:36] Speaker 2: absolutely invisible ghosts of dead galaxies. So today we are
[1:42] Speaker 2: setting out to understand the surprisingly short, incredibly violent afterlife
[1:47] Speaker 2: of these radio galaxies. Our mission is to explore what
[1:52] Speaker 2: happens in the dark after those central jets switch off.
[1:55] Speaker 3: And how the universe itself actively erases these structures.
[1:59] Speaker 2: Yes, and how modern astronomy is finally figuring out how
[2:02] Speaker 2: to actually spot these fleeting cosmic ghosts before they just
[2:06] Speaker 2: before they disappear entirely.
[2:08] Speaker 3: It's a massive detective story, really, But to grasp what
[2:11] Speaker 3: happens when the power turns off, we first have to
[2:13] Speaker 3: understand how the engine works when it is running.
[2:16] Speaker 2: Okay, set the stage for us.
[2:17] Speaker 3: So the heart of a radio galaxy is the supermassive
[2:20] Speaker 3: black hole. When this black hole is actively feeding on
[2:23] Speaker 3: surrounding gas and dust. It forms an accretion disc.
[2:26] Speaker 2: Because material doesn't just fall straight into a black.
[2:28] Speaker 3: Hole, right exactly, No, it spirals inward, much like water
[2:31] Speaker 3: circling a drain.
[2:32] Speaker 2: And as it spirals, I imagine the friction generates immense heat.
[2:36] Speaker 3: Immense heat. Yeah, but more importantly, it winds up the
[2:40] Speaker 3: local magnetic fields. The physics here are just intense. Oh so, well,
[2:46] Speaker 3: these magnetic fields get twisted so tightly by the rotation
[2:49] Speaker 3: of the black hole and the accretion disc that they
[2:51] Speaker 3: start to act like giant cosmic particle accelerators.
[2:55] Speaker 2: Oh wow.
[2:56] Speaker 3: Yeah. So a fraction of the infalling matter gets caught
[2:58] Speaker 3: in these magnetic fields and gets fired outward from the
[3:01] Speaker 3: poles of the black hole perpendicular to the disk, and
[3:04] Speaker 3: those are the jets.
[3:06] Speaker 2: Okay, so the black hole is feeding, and it's a
[3:08] Speaker 2: messy eater, very messy. It's basically spitting a portion of
[3:12] Speaker 2: its meal back out into space at relativistic speeds.
[3:15] Speaker 3: Relativistic meaning an incredibly high fraction of the speed of light.
[3:19] Speaker 3: And these jets are primarily made of electrons and positrons.
[3:22] Speaker 3: They basically act like cosmic fire hoses is blasting outward,
[3:26] Speaker 3: blasting straight out of the galaxy and into the thin
[3:28] Speaker 3: intergalactic gas that surrounds it. And when they hit that gas,
[3:31] Speaker 3: they create shock waves inflating these gigantic structures that we
[3:35] Speaker 3: call radio lobes.
[3:36] Speaker 2: I picture it like leaving a garden hose running on
[3:40] Speaker 2: a relatively flat dirt driveway for a few hours.
[3:42] Speaker 3: That's a great analogy.
[3:43] Speaker 2: Actually, you got the water blasting out of the nozzle,
[3:46] Speaker 2: carving a channel, and then pulling into these massive puddles
[3:49] Speaker 2: on either side of the yard. And that is the
[3:51] Speaker 2: active face. The engine is on, the hoses pumping, and
[3:54] Speaker 2: the puddles the radio lobes are growing.
[3:56] Speaker 3: Exactly. The lobes are essentially these massive balloons of tracked
[4:02] Speaker 3: plasma and highly energetic particles, and because of the mechanics
[4:07] Speaker 3: going on inside them, they glow brilliantly in radio frequencies.
[4:11] Speaker 3: But you know, nothing feeds forever. The gas fueling the
[4:14] Speaker 3: accretion disk eventually runs out, or maybe the dynamics of
[4:18] Speaker 3: the galaxy shift, and then the central engine switches off
[4:21] Speaker 3: the jet stop.
[4:22] Speaker 2: The spigot is turned off.
[4:24] Speaker 3: Yep.
[4:24] Speaker 2: So exactly what happens to the puddles then, I mean
[4:26] Speaker 2: they don't just pop out of existence. The water is
[4:28] Speaker 2: still sitting there on the driveway.
[4:30] Speaker 3: That marks the beginning of the remnant phase. The supply
[4:33] Speaker 3: line of fresh, highly energetic particles is completely cut off.
[4:38] Speaker 3: The giant radio lobes remain suspended in space, but without
[4:41] Speaker 3: the jets replenishing them, the particles inside begin to lose
[4:45] Speaker 3: their energy.
[4:46] Speaker 2: Which raises the big question, how do you actually go
[4:50] Speaker 2: about finding one of these fading puddles, Because if they
[4:53] Speaker 2: don't emit visible light and their radio signals are dying down,
[4:57] Speaker 2: it seems like looking for a needle in a haystack.
[4:59] Speaker 3: Oh, it gets worse, really, Yeah, it's a haystack where
[5:02] Speaker 3: the needle is constantly changing color to match the hay Historically,
[5:06] Speaker 3: relying on limited observational data has been wildly misleading for
[5:10] Speaker 3: that very reason. Why is that well, astronomers would scan
[5:15] Speaker 3: a region of space with a single radio telescope, pick
[5:18] Speaker 3: up a faint smudge of a signal and just misclassify it.
[5:21] Speaker 2: Because they didn't have enough context exactly.
[5:24] Speaker 3: A fading remnant can look identical to a strange, dim
[5:27] Speaker 3: active galaxy or even a distant cluster of galaxies. If
[5:30] Speaker 3: you don't have enough detail.
[5:32] Speaker 2: So you are basically just seeing a blob of radio
[5:34] Speaker 2: waves and guessing at its life story.
[5:36] Speaker 3: Precisely, to confidently identify a remnant radio galaxy, you absolutely
[5:41] Speaker 3: cannot rely on one instrument. You need a multi frequency dragnet.
[5:44] Speaker 2: Okay, so bringing them big guns right.
[5:47] Speaker 3: Recently, astronomers aimed an unprecedented combination of technological firepower at
[5:52] Speaker 3: a specific patch of the sky known as the XMM
[5:55] Speaker 3: Newton Large Scale Structure Field.
[5:57] Speaker 2: And the hardware list for this is just staggering. I mean,
[6:00] Speaker 2: the combined data from the MEERCAP Mighty Survey, the UGMRT
[6:04] Speaker 2: Super Mighty Survey, the Low Far Array, the GMRT, and
[6:08] Speaker 2: the JANSKI very Large Array.
[6:10] Speaker 3: It's massive. And by coordinating all of those arrays, they
[6:13] Speaker 3: achieved continuous radio frequency coverage stretching from one hundred and
[6:16] Speaker 3: forty four megahertz all the way up to one point
[6:19] Speaker 3: five digger heerds.
[6:20] Speaker 2: Wow.
[6:20] Speaker 3: Yeah, And they isolated fourteen specific candidate objects, fourteen smudges
[6:26] Speaker 3: in the sky that previous models suggested might be remnant
[6:29] Speaker 3: radio galaxies.
[6:30] Speaker 2: And what do they find?
[6:31] Speaker 3: While they applied detailed spectral modeling across all these different
[6:34] Speaker 3: frequencies to these candidates, and out of the fourteen, twelve
[6:37] Speaker 3: were confirmed as genuine remnant radio galaxies. Two actually had
[6:41] Speaker 3: to be reclassified as active sources.
[6:43] Speaker 2: Okay, hang on, I want to push back on this
[6:45] Speaker 2: methodology for a second here. Sure, you were talking about
[6:47] Speaker 2: stitching together data from completely different telescope arrays built in
[6:52] Speaker 2: different decades, operating on different continents, and listening on completely
[6:56] Speaker 2: different frequencies.
[6:57] Speaker 3: This is a logistical nightmare.
[6:58] Speaker 2: Yeah, doesn't layering all that introduce massive room for error?
[7:03] Speaker 2: Like couldn't a calibration glitch between low Far in Europe
[7:07] Speaker 2: and Meerkat in South Africa make a regular galaxy look
[7:11] Speaker 2: like a dead one? Why not just build one massive,
[7:15] Speaker 2: incredibly sensitive telescope at one frequency.
[7:18] Speaker 3: It's a fair question, But building one giant telescope wouldn't
[7:21] Speaker 3: actually solve the problem because a single frequency only gives
[7:25] Speaker 3: you a single slice of time.
[7:26] Speaker 2: What do you mean?
[7:27] Speaker 3: The necessity of combining these disparate arrays really comes down
[7:30] Speaker 3: to the mechanism of how these particles emit light in
[7:32] Speaker 3: the first place. It is a process called synchrotron radiation.
[7:36] Speaker 2: Okay, let's walk through that mechanism. How does the plasma
[7:39] Speaker 2: actually create a radio wave.
[7:41] Speaker 3: Right, So inside these massive radio lobes you have magnetic fields. Okay,
[7:45] Speaker 3: When a high energy electron travels through a magnetic field,
[7:48] Speaker 3: it doesn't just move in a straight line. The magnetic
[7:50] Speaker 3: force causes it to spiral like a quirk screw exactly,
[7:54] Speaker 3: and as it spirals it changes direction, and in physics,
[7:58] Speaker 3: any charged particle changing direction emits radiation. In this specific case,
[8:02] Speaker 3: it emits radio waves.
[8:04] Speaker 2: So it's essentially shedding energy by broadcasting radio signals as
[8:07] Speaker 2: it spirals.
[8:08] Speaker 3: Yes, and exactly how much energy it sheds depends entirely
[8:12] Speaker 3: on its speed. The fastest most energetic electrons spiral the
[8:16] Speaker 3: tightest and emit the highest frequency radio waves. Makes sense,
[8:20] Speaker 3: But because they are emitting so much high frequency energy,
[8:22] Speaker 3: they exhaust their energy reserves incredibly fast.
[8:26] Speaker 2: They burn bright and die young.
[8:28] Speaker 3: Exactly the lower energy electrons, the ones emitting lower frequency
[8:32] Speaker 3: radio waves, don't burn through their energy nearly as quickly.
[8:35] Speaker 3: They can hold onto it for much much longer. Okay,
[8:38] Speaker 3: So when the black holes jet turns off, the supply
[8:40] Speaker 3: of fresh high energy electrons stops, the high energy electrons
[8:44] Speaker 3: already in the low burn themselves out very quickly, so
[8:47] Speaker 3: the high frequency radio emission just drops off a cliff.
[8:50] Speaker 2: Ah. Okay, I see the flaw on my single telescope.
[8:53] Speaker 2: I da now.
[8:54] Speaker 3: Yeah.
[8:54] Speaker 2: If you only look with say the Janski very large
[8:58] Speaker 2: array at one point five gigger her such as a
[9:00] Speaker 2: high frequency, the remnant might be completely invisible to.
[9:04] Speaker 3: You, right because it's already burned out in that spectrum.
[9:07] Speaker 2: Wow.
[9:07] Speaker 3: And conversely, if you only look with low far at
[9:10] Speaker 3: one hundred and forty four megahertz a low frequency, you
[9:13] Speaker 3: will just see a glowing blob of low energy electrons.
[9:17] Speaker 3: But you have absolutely no way of knowing if it's
[9:19] Speaker 3: an old dying blob or a young active blob that
[9:22] Speaker 3: just happens to be faint.
[9:24] Speaker 2: So you need the whole spectrum because you were specifically looking.
[9:26] Speaker 3: For the drop off exactly. The steepness of the curve
[9:29] Speaker 3: between the low frequencies and the high frequencies is essentially
[9:33] Speaker 3: a cosmic clock. It is called the spectral age.
[9:36] Speaker 2: That is fascinating. So by measuring where the high frequencies disappear,
[9:40] Speaker 2: you can calculate exactly how long it has been since
[9:42] Speaker 2: the engine shut off and those electrons were freshly injected. Yes,
[9:45] Speaker 2: the spectrum is literally a timeline.
[9:47] Speaker 3: It is a highly precise timeline. And when astronomers applied
[9:51] Speaker 3: this multi frequency clock to those twelve confirmed remnants we
[9:54] Speaker 3: talked about, the results completely upended our expectations about galactic
[9:59] Speaker 3: life spans.
[10:00] Speaker 2: Really, what did the clocks say?
[10:01] Speaker 3: They are incredibly young. The total spectral ages of these
[10:05] Speaker 3: massive galaxy spanning structures ranged from roughly eight to forty
[10:10] Speaker 3: two million years. Wow, and the median age was about
[10:14] Speaker 3: twelve million years.
[10:15] Speaker 2: Okay, listeners, think about that scale for a moment. To
[10:17] Speaker 2: human minds, twelve million years is in eternity, but in astrophysics,
[10:22] Speaker 2: where stars burn for ten billion years and galaxies take
[10:25] Speaker 2: billions of years to slowly merge and evolve, twelve million
[10:29] Speaker 2: years is nothing.
[10:30] Speaker 3: It's basically a rounding error, right.
[10:32] Speaker 2: It is a cosmic blink of an eye. Finding a
[10:34] Speaker 2: structure this massive that is only twelve million years old
[10:36] Speaker 2: is like finding a vast, sprawling Roman ruin and realizing
[10:40] Speaker 2: through carbon dating that it was actually built in abandoned
[10:42] Speaker 2: just last Tuesday.
[10:43] Speaker 3: That's a perfect way to look at it. The brevity
[10:45] Speaker 3: of their existence is staggering, and furthermore, the researchers calculated
[10:49] Speaker 3: the fraction of the total lifetime these galaxies spent in
[10:52] Speaker 3: this fading remnant.
[10:53] Speaker 2: Phase, and what was the spread there?
[10:55] Speaker 3: That fraction ranges wildly from about four percent to eighty
[10:58] Speaker 3: three percent of their total life life span.
[11:00] Speaker 2: Okay, meaning this survey caught some of these ghosts right
[11:04] Speaker 2: after their jets switched off, literally just as the power
[11:08] Speaker 2: cord was pulled right and caught others much later in
[11:11] Speaker 2: their decay phase, right before they dropped below our ability
[11:15] Speaker 2: to detect them completely exactly.
[11:17] Speaker 3: The data tells us that earlier astronomical surveys completely missed
[11:21] Speaker 3: a massive population of these remnants.
[11:23] Speaker 2: Because they just vanished so fast. Right.
[11:26] Speaker 3: If a structure that large only stays visible for a
[11:29] Speaker 3: few million years before fading into the background static of
[11:32] Speaker 3: the universe, you have to be looking at the exact
[11:35] Speaker 3: right coordinates at the exact right moment to catch it.
[11:38] Speaker 3: They are disappearing much much faster than the older models predicted.
[11:41] Speaker 2: But wait, if they are fading so incredibly fast, where's
[11:45] Speaker 2: all that energy going? I mean, these radio lobes are
[11:47] Speaker 2: millions of light years across in some cases huge, yeah, right,
[11:51] Speaker 2: and energy signature that massive doesn't just quietly put her
[11:54] Speaker 2: out in a few million years without a reason. Something
[11:57] Speaker 2: has to be actively draining the puddles, and.
[11:59] Speaker 3: You're right, something is the environment is draining them. To
[12:02] Speaker 3: understand the mechanism erasing these ghosts, we have to look
[12:06] Speaker 3: at redshift. Redshift in cosmology, redshift is our measure of
[12:10] Speaker 3: distance because light takes a finite amount of time to
[12:14] Speaker 3: travel through space. Looking at an object with a high
[12:17] Speaker 3: redshift means we are looking at something very far away.
[12:20] Speaker 2: And because it's far away, we are seeing it as
[12:22] Speaker 2: it existed far back in time. Distance equals time exactly. Now.
[12:26] Speaker 3: A major finding in this data is a significant negative
[12:29] Speaker 3: relationship between red shift and spectral age. The further away
[12:34] Speaker 3: and further back in time these remnants are located, the
[12:36] Speaker 3: faster they seem to fade. Wait, really, yeah, their spectral
[12:40] Speaker 3: ages are demonstrably shorter at higher redshifts, But.
[12:43] Speaker 2: Why would being further away make a giant cloud of
[12:46] Speaker 2: plasma evaporate faster? That doesn't intuitively make.
[12:49] Speaker 3: Sense because the universe itself was physically different in the
[12:52] Speaker 3: distant past. The main mechanism erasing these ghosts is the
[12:56] Speaker 3: cosmic microwave background, or the cmb.
[12:59] Speaker 2: AH, the leftover thermal radiation from the big bank.
[13:02] Speaker 3: Exactly. It is a bath of photons that permeates every
[13:06] Speaker 3: single inch of the universe. Now remember our high speed
[13:10] Speaker 3: relativistic electrons trapped inside the radio lobes.
[13:13] Speaker 2: The one spiraling around.
[13:14] Speaker 3: Yes, they are zipping around, occasionally colliding with things. When
[13:19] Speaker 3: one of these super fast electrons collides with a low
[13:22] Speaker 3: energy photon from the cosmic microwave background, a process called
[13:26] Speaker 3: inverse Compton scattering occurs.
[13:28] Speaker 2: Okay, let's break down the mechanics of that collision. What
[13:31] Speaker 2: exactly happens.
[13:32] Speaker 3: So, in a normal collision, you might expect two things
[13:35] Speaker 3: to just bounce off each other, But in inverse Compton scattering,
[13:38] Speaker 3: the fast moving electron transfers a massive amount of its
[13:41] Speaker 3: kinetic energy directly to the sluggish CMB photon. Oh wow, Yeah,
[13:47] Speaker 3: the photon gets a massive energy boost, sometimes jumping all
[13:49] Speaker 3: the way up into the X ray spectrum, and the
[13:52] Speaker 3: electron is completely robbed of its momentum. It slows down dramatically,
[13:56] Speaker 3: losing the energy it needs to emit radio waves.
[13:59] Speaker 2: It's like a speed train slamming into a pingpong ball. Okay,
[14:02] Speaker 2: the bing klong ball gets launched into orbit and the
[14:04] Speaker 2: train loses a chuck of its forward momentum.
[14:07] Speaker 3: That is exactly it. The cosmic microwave background is acting
[14:10] Speaker 3: like cosmic friction. It is actively scrubbing these energetic electrons
[14:15] Speaker 3: out of existence by stealing their energy one collision at
[14:18] Speaker 3: a time.
[14:19] Speaker 2: That is wild. But how does that connect to redshift?
[14:22] Speaker 3: The connection is the density of that friction. As the
[14:25] Speaker 3: universe expands over time, the cosmic microwave background cools down
[14:29] Speaker 3: and dilutes the photons spread out. Okay, but if we
[14:32] Speaker 3: look at a high redshift, we're looking back in time
[14:34] Speaker 3: to an erow when the universe was much smaller and
[14:37] Speaker 3: more compact. Back then, the cosmic microwave background was much
[14:41] Speaker 3: denser and hotter.
[14:42] Speaker 2: The fog of photons was just thicker, much thicker.
[14:45] Speaker 3: A radiolobe forming back in that era would be slammed
[14:48] Speaker 3: by a vastly higher number of CMB photons, So the
[14:51] Speaker 3: inverse compton scattering would happen at a much higher rate,
[14:54] Speaker 3: draining the electrons energy much faster.
[14:56] Speaker 2: That makes perfect sense, right.
[14:58] Speaker 3: That is why distance appears to speed up the fading.
[15:00] Speaker 3: The remnants at higher redshifts are being actively erased by
[15:03] Speaker 3: a more intense, denser cosmic microwave background. They lose their
[15:07] Speaker 3: energy so fast that their detectable lifespan shrinks dramatically.
[15:11] Speaker 2: So the universe itself is the eraser. That completely changes
[15:15] Speaker 2: how I picture the vacuum of space. It's not this empty,
[15:19] Speaker 2: peaceful void. It's an active environment tearing down these structures.
[15:23] Speaker 3: It's very violent in its own quiet way.
[15:25] Speaker 2: So if this dense cosmic fog is erasing them, does
[15:29] Speaker 2: it erase them evenly? Like if you look at one
[15:31] Speaker 2: of these fading ghosts, does the whole puddle just slowly
[15:35] Speaker 2: dim or is the decay messier than that?
[15:38] Speaker 3: Oh, it is highly complex, and we can actually see
[15:41] Speaker 3: the messiness thanks to spatially resolved spectral age maps.
[15:45] Speaker 2: What are those?
[15:46] Speaker 3: Well, the astronomers didn't just calculate a single age for
[15:49] Speaker 3: the entire remnant. By cross referencing the different frequencies from
[15:53] Speaker 3: all those telescope arrays, they were able to map the
[15:56] Speaker 3: age gradients across the physical structure of the radio lobes.
[15:59] Speaker 2: They can see different ages in different parts of the
[16:01] Speaker 2: same cloud.
[16:02] Speaker 3: Yes, they can point to one side of the plasma
[16:05] Speaker 3: cloud and see that it is millions of years older
[16:07] Speaker 3: than the center.
[16:08] Speaker 2: So you can actually see the timeline physically printed on the.
[16:11] Speaker 3: Structure you can, and they found a distinct behavioral difference
[16:15] Speaker 3: based on the shape of the remnant. In extended massive sources,
[16:19] Speaker 3: they see systematic age gradients meaning what exactly, meaning the
[16:23] Speaker 3: outer edges of the lobe or older, and the regions
[16:26] Speaker 3: closer to the host galaxy where the jets used to
[16:28] Speaker 3: be are younger.
[16:30] Speaker 2: Okay, going back to the garden nose on the driveway,
[16:31] Speaker 2: that makes sense. Yeah, even after you turn off the spigot,
[16:34] Speaker 2: the water that was already blasted out doesn't stop moving instantly.
[16:38] Speaker 2: It still has momentum. It continues to flow outward across the.
[16:41] Speaker 3: Concrete exactly, and in astrophysics that is called plasma transport.
[16:45] Speaker 3: The material in these extended sources is still moving and
[16:49] Speaker 3: expanding outward into space, carrying its energy with it, even
[16:52] Speaker 3: after the central black hole has shut down. So the
[16:55] Speaker 3: older electrons are pushed to the fringes right, but compact
[16:59] Speaker 3: remnant the smaller, denser ones behave entirely differently. They display
[17:04] Speaker 3: irregular chaotic aging patterns. There is no neat gradient.
[17:08] Speaker 2: Why the difference, I mean, if it's the exact same physics,
[17:10] Speaker 2: why is a small puddle aging differently than a big one.
[17:14] Speaker 3: Because a compact source is heavily influenced by its immediate
[17:17] Speaker 3: local environment. It might be trapped in a dense pocket
[17:20] Speaker 3: of intergalactic gas, or tangled up in complex localized magnetic
[17:25] Speaker 3: fields that act like a cage ah. Instead of flowing
[17:29] Speaker 3: outward smoothly, the plasma is basically trapped. It churns and mixes.
[17:34] Speaker 3: The older electrons get folded back in with the younger ones,
[17:37] Speaker 3: causing the entire structure to age irregularly.
[17:40] Speaker 2: It's really like cosmic forensic science. An extended source is
[17:44] Speaker 2: like a clear set of footprints walking away from a
[17:46] Speaker 2: crime scene. You can trace the direction of travel and
[17:49] Speaker 2: the timeline really clearly right. But a compact source is
[17:52] Speaker 2: an unpredictable scatter, heavily influenced by the rocky terrain it
[17:57] Speaker 2: happened to be standing on when the engine died.
[17:59] Speaker 3: That's a brilliant way to put it. And this forensic
[18:01] Speaker 3: science forces us to ask a crucial question about the
[18:04] Speaker 3: supermassive black holes themselves, which will for a long time.
[18:07] Speaker 3: The assumption was that these massive jets were relatively permanent fixtures,
[18:12] Speaker 3: burning continuously for hundreds of millions of years, but Finding
[18:15] Speaker 3: all these young, rapidly fading remnants and realizing how quickly
[18:19] Speaker 3: the universe erases them proves that supermassive black holes have
[18:23] Speaker 3: duty cycles.
[18:24] Speaker 2: Duty cycles like a home furnace turning on to heat
[18:29] Speaker 2: the house, shutting off when it reaches the right temperature,
[18:32] Speaker 2: and then turning back on an hour later.
[18:33] Speaker 3: Exactly they breathe. A black hole will acquire a massive
[18:37] Speaker 3: cloud of gas, turn on, blast out these jets and
[18:41] Speaker 3: inflate the radio lobes. Then the local fuel runs out,
[18:45] Speaker 3: the engine.
[18:45] Speaker 2: Switches off, and the fading begins.
[18:47] Speaker 3: The lobes begin to fade, scrubbed by the cosmic microwave background.
[18:51] Speaker 3: But millions of years later, another gas cloud might fall in,
[18:54] Speaker 3: or the galaxy might merge with the neighbor. The black
[18:56] Speaker 3: hole powers back up, firing new jets into the fading
[18:59] Speaker 3: remnants of the old ones. The cosmos is just flickering,
[19:02] Speaker 3: with these massive engines powering up and shutting.
[19:04] Speaker 2: Down, a flickering, breathing universe. We're looking at an ecosystem
[19:08] Speaker 2: of energy that is constantly recycling itself, and the observations
[19:11] Speaker 2: we are discussing today are just a preview of what
[19:14] Speaker 2: is coming, aren't they?
[19:15] Speaker 3: Oh, we are just scratching the surface. The current findings
[19:18] Speaker 3: relied on an impressive combination of existing telescopes. But the
[19:22] Speaker 3: future of this field is the Square Kilometer.
[19:25] Speaker 2: Array or SKA, the Big One.
[19:27] Speaker 3: The really big one. It is a massive radio telescope
[19:30] Speaker 3: project currently being built, and its deep radio continuum surveys
[19:34] Speaker 3: will be vastly more sensitive than anything operating today.
[19:36] Speaker 2: If the current arrays can find twelve confirmed ghosts in
[19:40] Speaker 2: one small patch of sky, what will the SKA find.
[19:44] Speaker 3: The SKA is expected to uncover a tremendously larger population
[19:48] Speaker 3: of these faint, high redshift remnant radio galaxies. We are
[19:52] Speaker 3: going to be able to map the graveyard of the
[19:54] Speaker 3: universe in unprecedented detail, finding structures that faded so long ago.
[19:58] Speaker 3: Their signals are currently way below our threshold of detection.
[20:01] Speaker 2: So we are finally building the right kind of ghost
[20:04] Speaker 2: hunting equipment to see the things that the universe has
[20:06] Speaker 2: tried to hide.
[20:07] Speaker 3: From us exactly. We are moving past just studying the
[20:10] Speaker 3: bright active things in the sky. We are finally developing
[20:14] Speaker 3: the capability to study the things that have stopped.
[20:16] Speaker 2: That is incredible. Let's try to bring all these mechanics
[20:19] Speaker 2: together for a second. We started by pulling the power
[20:21] Speaker 2: cord on a supermassive black hole.
[20:23] Speaker 3: YEP.
[20:24] Speaker 2: We explored how the accretion disk and twisted magnetic fields
[20:27] Speaker 2: act as a particle accelerator, blasting plasma into intergalactic space
[20:32] Speaker 2: to form massive radio lobes, and we learned that when
[20:35] Speaker 2: the engine starves and the jets switch off, those lobes
[20:38] Speaker 2: are stranded out there, left in the dark, and they
[20:41] Speaker 2: fade incredibly fast, sometimes in just twelve million years. The
[20:45] Speaker 2: mechanism behind that rapid decay is inverse Compton scattering, where
[20:49] Speaker 2: the very background radiation of the universe, the leftover heat
[20:52] Speaker 2: of the Big Bang, acts like cosmic friction. It violently
[20:57] Speaker 2: scrubs the high energy electrons out of.
[20:58] Speaker 3: Existence the train hitting the ping pong bong exactly.
[21:02] Speaker 2: But by layering multiple frequencies of radio astronomy, we can
[21:05] Speaker 2: read the spectrum as a clock, allowing us to spot
[21:08] Speaker 2: these fleeting cosmic ghosts before they vanish entirely.
[21:11] Speaker 3: It's a huge leap forward, it really.
[21:13] Speaker 2: Is, which leads us with a totally mind bending reality
[21:17] Speaker 2: to ponder if massive galaxy spanning structures powered by the
[21:22] Speaker 2: most violent energetic engines in physics can be scrubbed away
[21:26] Speaker 2: and completely erased in a cosmic blink of an eye
[21:29] Speaker 2: by the universe's own background radiation.
[21:31] Speaker 3: Oh I see where you're going with this.
[21:32] Speaker 2: How many countless other colossal, magnificent structures have already lived, died,
[21:37] Speaker 2: and completely faded from the cosmic record, leaving absolutely no
[21:40] Speaker 2: trace for us to ever find.

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