Welcome to Bedtime Astronomy. Explore the wonders of the cosmos with our soothing Bedtime Astronomie podcast. Each episode offers a gentle journey through the stars, planets, and beyond, perfect for unwinding after a long day. Let's travel through the mysteries of the universe as you drift off into a peaceful slumber under the night sky.
I want you to start by visualizing something with me. Picture a massive, heavy velvet tapestry and it's pitch black, absolutely lightless. But woven into this black fabric are these brilliant shimmering threads of gold and silver. It's a pretty classic image, right, Yeah.
That's the standard metaphor for the universe. People call it the cosmic web.
Right because when we look up at the night sky, or you know, we look at those incredible composite images from NASA, that is exactly what we see. We see the light. We see billions of galaxy whirling in the void, glowing with the heat of trillions of stars. We fundamentally define the universe by what shines.
Well, because that is what our eyes are built to do. We are biological light detectors. Our entire history of astronomy, from the ancient Greeks drawing constellations to Hubble taking deep field images has been biased toward luminosity.
If it glows, it exists exactly.
And if it doesn't glow, we assume it's just empty space the void.
But and this is why we are here today doing this exploration. What if that assumption is fundamentally flawed.
It's a huge question.
What if that tapestry is actually full of things woven with black thread against a black background, Objects that are massive, complex, and gravitationally significant, but effectively invisible.
It is a really disquieting thought. When you dig into the research, it suggests that our map of the universe is not actually a map of mass. Oh interesting, Yeah, it's merely a map of light bulbs, and we might be missing the vast majority of the furniture in the room, so to speak.
That is exactly what we are unpacking today. We're looking at a specific discovery from our sources that challenges the very definition of what a galaxy is. Yes, we are talking about a ghost hiding in the Perseus cluster, a galaxy named CDG two.
CDG two is just a fascinating case study. It's an object that breaks the usual rules of galactic formation.
Breaks the rules, feels like an understatement to me. When I was reading the research on this, one statistic jumped out and just sat there staring at me.
The mass ratio, Yes.
We usually think of galaxies as star factories, right, but this thing, this galaxy appears to be ninety nine percent dark matter.
Ninety nine percent. It is an immense figure.
So if I'm doing the math right, that leaves roughly one percent for the stuff we can actually see, you know, the stars, the gas, the.
Dust, the buryonic matter, baryonic right, the normal stuff that makes up you, me, the Earth, and the Sun in this galaxy, all of that is essentially a rounding era that is just wild.
To think about. But before we get too far into the wow factor of it all, I want to push back on this a little bit. We talk about dark matter a lot. We know our own Milky Way has dark matter. We know it's the gravitational glue that keeps galaxies from spinning apart. So is ninety nine percent actually that weird? What is the standard ratio?
That is a very important distinction to make you are right? Almost every galaxy has a dark matter halo. But typically the cosmic ratio is about five to one fave to one, five parts dark matter to one part visible matter.
Okay, so five to one versus ninety nine to one.
Exactly In a normal galaxy like the Milky Way, the dark matter provides the structure that the visible matter, the gas and stars, has collapsed into the center to form a bright, churning.
Disc right the spiral we always see.
Yeah, the baryonic physics are very active. Yeah. In CDG two, that process seems to have short circuited.
Entirely short circuited.
How well, It's like a house built almost entirely of invisible bricks, with just a few effects of dust floating inside to prove it's actually.
There, Which brings us to the definition problem. If you have a clump of dark matter with almost no stars, is it even a galaxy or is it just a halo that failed to do anything?
That is the existential crisis as strongers are grappling with right now.
Really, oh yeah.
Traditionally a galaxy is defined by its starlight, but objects like CDG two suggest that a galaxy should really be defined by its gravitational potential. It's dark matter halo so.
The stars are just an afterthought exactly.
The stars are just the ornamentation, they aren't the structure itself.
So our mission today for you listening is to understand this ghost. We aren't just going to gawk at the ninety nine percent number. We really need to understand the detective work here. The methodology is the best part, because finding something that emits almost no light in a universe that is incredibly big and incredibly dark seems like an impossible task.
It borders on the impossible. This isn't just an observation, it's really a forensic investigation.
It really is a cosmic crime scene. We have a ghost galaxy hiding in a very rough neighborhood called the Perseus Cluster, very rough. We have a team of detectives using three of the most powerful telescopes ever built. And they used a technique that I found absolutely brilliant in the source.
Material, always so clever.
They didn't look for the galaxy itself. They looked for the well, let's call them the ornaments.
Globular clusters. They were hunting for globular clusters.
So let's set the scene. We are going to the Perseus Cluster.
Very chaotic noisy place in the universe.
Let's start with the visual or I guess the lack thereof. The research comes from a collaboration evolving NASA, the European Space Agency, in the University of Toronto, and they released an image from the Hubble Space telescope. Now, usually when you see a Hubble press release, you expect fireworks, WAPs, you expect nebulas, pillars of creation giants, sweeping spiral arms.
You expect high contrast, bright whites, deep blacks, vibrant colors all over the place, right.
But I looked at the image for CDG two, specifically the crop they provided in the material, there is a dashed red circle drawn on the blackness of space, just to tell me where to look. And what did you see inside that circle? I literally had to wipe my monitor. I thought it was a smudge on my screen.
It is incredibly underwhelming to the naked eye, isn't it.
It looks like noise. It's just a faint, barely there haze.
That is what astronomers classify as a low surface brightness galaxy.
Low surface brightness the term.
Is quite literal, the surface brightness meaning the amount of light emitting from any given square arc second of the sky is so low that it's barely distinguishable from the background radiation of the universe.
So let's try to ground this for you listening. If I were in a spaceship floating just outside this galaxy looking out the window, what would I see? What I see? A galaxy?
Honestly, you wouldn't see a structure. You wouldn't see a disk or spiral arms, or a glowing core, nothing like that. No, you would see a very sparse scattering of faint stars. It wouldn't look like a cohesive object. It would just look like you were in a slightly more populated region of empty space.
It's a galaxy running on one percent battery.
That's a good analogy to put a number on it. The paper notes that CDG two has a total luminosity equivalent to roughly six million sun like stars.
Okay, stop there, six million sons. To me and probably to you listening, six million suns sounds like a blinding amount of light. That sounds huge.
It sounds massive in human terms. Yes, but we have to adjust our scale to galactic terms.
Okay, adjusted for me.
For context, the Milky Way contains somewhere between one hundred and four hundred billion stars.
Wow, one hundred to four hundred billion.
To do the math there, six million versus four hundred billion CDG two is putting out less than zero point zero one percent of the light of the Milky Way.
That is nothing.
It is a whisper compared to a shout.
And it's not just that it's faint, it's where it is. We are looking at an object inside the Percus galaxy cluster. How far away is this?
We are looking at a distance of roughly three hundred million light years.
Three hundred million light years. I want to pause on that number because I think we get really numb to millions and billions in space talk.
It's hard for the human brain to process.
Right. The light we are seeing from this faint smudge left the galaxy three hundred million years ago. That's the Palaeozoic.
Era on Earth, long before the dinosaurs. We are talking about the time when the first reptiles were just emerging on Earth. The continents were in completely different positions.
So we are trying to spot a faint collection of six million stars, which is basically nothing, from a distance that is incomprehensibly far away. It's like trying to see a candle in the moon, but the candle is behind a giant searchlight, the searchlight being.
The rest of the Perseus cluster. You have to understand, Perseus is one of the most massive objects in the entire known universe. Really, Oh yeah, it contains thousands of galaxies. It is full of hot X ray emitting gas. It is bright, it is crowded, and it is gravitationally noisy. Finding CDG two in there is statistically nightmarish.
And that leads me to the how if you can't just stand the sky and see it, Because as we establish, it looks like nothing, how do you find it? You can't just zoo in on every single pixel of the sky.
Now you would never finish. Yeah, the universe is too big. You need a marker, you need a proxy.
Entered David Lai. He's the lead researcher from the University of Toronto on this project. Yes, and his team didn't start by looking for the galaxy itself. They started by hunting for those ornaments we mentioned earlier.
They went looking for globular clusters.
Okay, let's unpack globular clusters because to me, it sounds like a type of candy or maybe a geology term. What exactly is a globular cluster In this astronomical context, it is.
One of the most ancient and fundamental structures in the cosmos. A globular cluster is a very compact, spherical group of stars. Spherical, right, Imagine a ball maybe ten to thirty light years across, but packed with tens of thousands, sometimes even millions of stars.
So it's a super dense city of stars.
Extremely dense. In the solar neighborhood where we are, stars are light years apart in the core of a globular cluster. They are packed cheek by jowl, and the key characteristic for this study is that globular clusters are typically found orbiting normal.
Galaxies, orbiting them like moons around a planet.
Kind of The Milky Way has about one hundred and fifty of them. They swarm around the galacticiclo like bees around a hive.
Okay, so the clusters are the bees.
The galaxy is the high right now, imagine you are looking at a hive at night from a mile away.
It's pitch black.
You can't see the hive itself. It blends completely into the darkness of the trees. But suppose the bees are bioluminescent. Suppose they glow.
Ah, if I see a swarm of glowing bees hovering in a specific tight formation, I can safely assume the hive is sitting right in the middle of them.
That is precisely the logic. This is the big breakthrough. CDG two is the first galaxy detected solely through its globular cluster population.
I want to make sure I really get the difficulty of this though. At three hundred million light years away, does a globular cluster look like a ball of stars in a telescope?
No, even with hubble at that immense distance, a globular cluster is what we call a point source, meaning it looks like a single pixel, looks exactly like a single faint star in our own milky way that just happens to be in the foreground. Yeah. Or it looks like a very distant background galaxy.
So you have a field of view full of thousands of tiny dots. Some are foreground stars, some are background galaxies, and a very small few are these globular clusters exactly? How do you know which a witch?
That is the real trick of this whole endeavor. You can't just look at the dots individually. You have to look at the statistics of the dots. Statistics explain that this is where David Lye's work is so incredibly impressive. They used a sophisticated algorithm to scan the overall distribution of these points of light. Okay, random noise, meaning your foreground stars and your background galaxies, tends to be distributed somewhat uniformly across the sky, or at least randomly scattered.
This is a general spread.
Right, But globular clusters that belong to a single galaxy aren't random at all. They're gravitationally bound to each other and to the host galaxy. They cluster together.
So they were specifically looking for a tight grouping of dots.
Exactly, they were looking for a statistical anomaly. The algorithm essentially asks, is there a region in this patch of sky where I see three or four of these specific point sources huddled together in a way that is highly unlikely to happen just by random chance.
I see. It's like walking into a crowded sports stadium. You can't tell who knows who just by looking at the giant sea of faces.
No, you can't.
But if you see four people standing in a tight little circle facing each other, ignoring everyone else, you assume they are a group. They came together.
That's a perfect analogy. The algorithm flagged these friends. It said, Hey, at these specific coordinates, there are four objects that seem to be hanging out together.
And because gravity works the way it does, yes.
If you have four massive star clusters hanging out in the tight group, there must be something massive in the middle holding them there.
And that unseen massive something is the dark matter halo of CDG two.
Correct Using this method, they identified ten previously confirmed low surface brightness.
Galaxies, which prove the math works exactly.
It validated the tool, and then using that validated tool, they found two new candidates. CGG two was one of them.
I love that so much. They didn't see the galaxy, they calculated that it absolutely must be there. It's a triumph of math as much as it is observation.
Which is really where modern astronomy is heading. As a field. We are rapidly moving from point and shoot astronomy to massive data mining.
But as with any good detective investigation, just having a suspect isn't enough. You can't publish a paper saying the math says there's a galaxy here.
Trust us, you definitely cannot. You need to verify it and see the body to keep the crime scene metaphor going.
And that requires the heavy artillery. It does verification triad. I really like the sound of that from the research.
It does sound official, doesn't it. It refers to the three major observatories they used in tandem to confirm this candidate, because no single telescope on Earth or in space could do the whole job.
Let's break down why that is. Why did they need three different telescopes. Let's start with the most famous one of the bunch, NASA's Hubble Space telescope.
Hubble is the sniper of the group.
The sniper.
Yes, it has incredible pinpoint resolution. They needed Hubble to confirm that those four suspicious dots were actually globular clusters and not just something else masquerading A Is that?
So Hubble gives you the fine detail.
Hubble provided the sharpness to resolve the immediate local environment of those four points.
Okay, so Hubble confirms the ornaments are real. Who is next in the triad?
Next is ESA's EUCLID Space Observati Tory, the European Space Agency. This is a newer mission and it's critical for a totally different reason. Hubble has a very, very narrow field of view. It basically looks at a tiny straw hole of the sky at any given moment. EUCLID, on the other hand, is wide angle. It surveys huge swaths of the cosmos all at once.
But why do you need a wide angle to see a tiny, faint galaxy.
You need context. You need to know that this tight grouping of four clusters is truly isolated in space. Oh, I see, you need to see the surrounding neighborhood to ensure these clusters aren't just the far outskirts of some other, much bigger galaxy nearby. EUCLID confirms the isolation.
Got it. Hubble zooms in to check the details. EUCLID zooms out to check the neighborhood. And the third telescope.
The Subreu telescope. This is a ground based monster located on Monachia in Hawaii.
Now, why do we need a ground telescope if we already have two amazing space telescopes. Usually we think space is better because there's no atmosphere to distort the image.
Space gives you clarity. Yes, the ground gives you size. You can build much bigger things on the ground.
Size matters here very much.
The Subaru telescope has a primary mirror that is over eight meters wide. Hubble's mirror is only two point four meters.
That's a huge difference.
It is. Think of it like catching rain. A bigger bucket catches more rain. In this case, the rain is light photons.
So Subro is essentially a giant light bucket exactly.
They needed Subru's immense light gathering power to try and detect the incredibly faint glow between those clusters the actual galaxy itself.
And this is the real moment of truth in the story. They combine the data from the sniper, the wide angle, and the light bucket. What did they actually find.
They found the ghost when they process the deep imaging, specifically looking at the empty space between those four colobular clusters.
Yeah, they detected a signal, a signal like a radio sign row.
An optical signal, a faint, diffuse structural glow. It wasn't just empty black space, was a very very weak stellar population surrounding those clusters.
That is the body of the galaxy.
That is the body. It confirmed once and for all the clusters weren't just randomly floating in the void. They were embedded in a dark matter halo. They were anchoring a hidden stellar population.
So the Christmas tree analogy holds up perfectly. They saw the glowing ornaments, they did the math to prove they were a group, and when they turned up the exposure time with the big telescope, they finally saw the faint outline of the trees branches precisely.
And David Lay makes a very conservative, but very interesting point here in the findings.
What's that?
He notes they given the mass calculations of the halo, these four clusters likely represent the entire globular cluster population of CDG two.
Wait only four? You just said our Milky Way has one hundred and fifty.
Yes, CDG two is hanging onto these four clusters for dear life. They're the only major structural features it has left.
That brings us to the why. This is the part of the exploration that really hooked me. We have a gaps galaxy that is ninety nine percent dark matter, has almost no stars and only four Measley clusters.
It's a barren place.
Why is it so empty? Did it form this way right after the Big Bang? Or does something terrible happen to it?
Now we are moving from detection to forensics. We're looking at the cause of death, or perhaps more accurately, the cause of sterility.
Sterility, that's a strong word for a galaxy.
Well, a galaxy lives in astronomical terms, as long as it forms new stars. Ok, deform stars. You need fuel, and the fuel of the universe is hydrogen gas, cold, dense hydrogen gas.
And the research says CDG two has none.
Of this gas. It is completely depleted. It is what we call gas poor. This means it stopped forming stars a very long time ago. It's a dormant system.
So where did the gas go? Did it use it all up?
No, The culprit is the environment, the Perseus cluster itself. We mentioned earlier that Perseus is a rough neighborhood. Let's dig into what that actually means physically.
Yeah, you mentioned it's chaotic and noisy.
A galaxy cluster isn't just a collection of galaxies floating in a vacuum. The space between the galaxies is filled with something called the intracluster medium or.
Icm intracluster medium.
This is a plasma, a superheated gas that permeates the entire volume of the cluster.
So space there is not a vacuum.
It's not a vacuum in the way we usually think of deep space. It's very diffuse, sure, but it is physically there, and it's hot, millions of degrees hot. Now, I want you to imagine CDG two falling into this cluster from the outside. Okay, it is moving at thousands of kilometers per second.
Thousands of kilometers per second, so it's moving incredibly fast through this plasma soup exactly.
Yeah, And even though the plasma is thin, when you hit it at two thousand kilometers per second, it creates a massive amount of drag. It creates a physical.
Wind, a cosmic headwind.
We call this ram pressure.
Yeah.
The standard analogy we used to teach this is driving a convertible down the highway with a top down.
Okay, I'm in the car, top is down.
You have a stack of loose paper sitting on the passenger seat next to you. Those papers represent the hydrogen gas in the galaxy. Okay, it's fluffy, it's light, it's not gravitationally bound very tightly to the center.
I can see exactly where this is going.
You hit the gas pedal, you go out to one hundred miles per hour. The wind, the ramp pressure whips inside the car and rips the loose papers right out.
The papers go flying out the back of the.
Car right but the car itself, the head, steel chassis, in the engine.
The car keeps moving forward.
The car represents the dark matter halo and the existing stars. They are massive and gravitationally down tightly enough where they simply don't interact with the wind, so they just plow right through the plasma. But the gas, the future potential for new stars, is violently stripped away.
That is such a violent image. The galaxy is essentially being flayed alive as it falls into the cluster.
It is a violent process. It's called ram pressure stripping. It effectively kills the galaxy. It completely removes its ability to grow or evolve.
But wait a minute, here's the plot hole for me. What's that If the cosmic wind is strong enough to rip out all the gas, why are those four globular clusters still there? Why didn't the ornaments get blown off the tree with the leaves.
This is a fantastic question, and it all comes down to density. Physics is all about density contrasts. Do you remember how I described a globular cluster a few minutes ago.
A super dense ball of.
Stars keywords there, super dense. A globular cluster is not a stack of loose papers. It is a bowling ball.
Ah, the bowling ball on the passenger seat.
Right. If you have a twelve pound bowling ball on the passenger seat of your convertible and you drive one hundred miles per hour, does the wind blow it out?
No, it just sits there. The wind flows right over.
It, exactly. The globular clusters are gravitationally tightly bound. They are tough little knots of.
Matter, so they survive.
They are highly resistant to gravitational tidal disruption, and they are way too dense to be pushed around by the ram pressure wind.
So the galaxy gets stripped of the gas, but the bowling ball the globular clusters stay behind.
Precisely, they are ultimate survivors. They act as reliable tracers for us because they are the only things tough enough to survive the extreme environment of the Perseus cluster.
That really paints a picture of CDG two not just as a weird scientific object, but almost as a survivor character in a story.
It is.
It's been battered, it's been stripped, and it's been starved to fuel for millions of years, but its dark matter skeleton and its four toughest star clusters are still hanging on.
It's a ghost with a history. It tells the story of its own abuse just by its current state.
This brings us to the bigger picture of our deep look today. We found this one ghost. We used this amazing new method finding the clusters to find the galaxy. We verified it with the triad of telescopes. What does this mean for the future of astronomy. Is CDG two a unicorn, a one off anomaly, or are we about to find a whole herd of these things?
The general consensus in the field is that we are definitely about to find a herd. This discovery is a proof of concept.
It proves the tools work.
It shows that the statistic method works in the real universe, and we are entering a golden age for this specific kind of low surface brightness detective work.
The source material mentions some specific upcoming missions that are going to blow this wide open.
Yes, there are several. We are currently in the era of EUCLID, which is great, as we discussed, but coming up very soon, we have NASA's Nancy Grace Roman Space Telescope.
The Roman Telescope. I've heard this described by astronomers as Hubble with a panoramic lens.
That is a very fair and accurate description. Roman has the same visual resolution as Hubble, so it can see those tiny faint point source dots, but its field of view is one hundred times larger.
One hundred times larger at the exact same sharpness.
Yes, so instead of looking at a tiny patch of the Perseus cluster and hoping to get lucky, it can map the whole cluster and high resolution in one go. You'll be able to spot these globular class populations across huge swaths of the sky.
That's incredible. And there's another one mentioned in the research too, the Verisi Reuben Observatory.
The Ruben Observatory is a massive ground based project currently being built in Chile. It is going to conduct what's called the Legacy Survey of Space.
And time legacy survey of space and time. What does that actually mean.
Basically, it's going to make a high definition movie of the entire Southern sky, scanning the whole thing repeatedly every few nights.
Every few nights, So we are going to go from having a few static snapshots to having a continuous moving stream of data.
We are going to be drowning in data, petabytes of it. And that leads to the final necessary piece of the puzzle. We can't do this with human eyes anymore.
The source mentions astronomers turning to artificial intelligence.
It's inevitable. As we mentioned, finding CDG two required complex statistical analysis a relatively small area. Now imagine doing that for billions of objects detected by Reuben and Roman over the entire sky.
A human astronomer cannot manually scan the sky for these tiny mathematical patterns. It would take lifetimes.
You need a machine to find the needle in the haystack, more.
Like you need a machine to find the needle in a stack of needle Exactly.
We are actively training machine learning algorithms to recognize the specific statistical fingerprint of these globular clustered groupings.
So the AI will flag it.
The AI will scan the data and say, hey, look at these specific coordinates, there is a ninety five percent probability of a dark batter dominated galaxy hiding there. And then the human astronomers will turn the big telescopes like subru or James Web there to verify the body.
So we are literally building machines to hunt for ghosts.
We are automating the discovery of the invisible universe.
It feels like we are on the edge of a major paradigm shift. For so long, we've thought of the universe as just the stuff we can see, but this suggests the universe is much much more crowded than we thought.
I think that is the inevitable conclusion here. We were realizing that our cosmic map has huge blank spots, not because nothing is there, but simply because we were wearing the wrong glasses.
So let's bring this all together for you listening. We started with a wild headline, a galaxy that is ninety nine percent dark matter CDG two. We learned that it's nearly invisible, just a faint scattering of stars three hundred million light years away.
A textbook low surface brightness galaxy.
We learned that the way David Laie and his team found it was sheer cleverness. They didn't look for the galaxy, They looked for its ornaments, four globular clusters huddled together in the background noise.
Using advanced statistical clustering algorithms, we.
Verified it with the avengers of astronomy Hubble, Euclid, and subru, proving that there was indeed a faint diffuse glow holding those clusters together, confirming.
The presence of the massive dark matter halo.
And we learned that this galaxy is essentially a victim of its environment. It was stripped of its star making gas by the ram pressure of the Perseus cluster, leaving only the dark matter skeleton and the tough dense clusters behind.
A cosmic skelet left on the wind.
So what is the big takeaway here? Why does this matter to the person listening right now on their commute or doing dishes?
It matters because it fundamentally changes how we perceive reality on a cosmic scale. For all of human history, seeing is believing. In astronomy, light is matter. This discovery tells us that the map is dangerously incomplete. There are massive continents on this cosmic map that we haven't drawn yet simply because they are dark. We are moving from a light based cartography of the universe to a gravity based one.
We are finally seeing the bones of the universe, not just the glowing skin.
Exactly, and that gives us a much more accurate picture of how the universe actually works and how it evolved.
I want to leave you the listener with a thought that's been sticking with me since we started prepping this research. We found CDG two, this massive dark matter dominated object, by spotting just four faint clusters, just four tiny ambiguous dots in the darkness.
It really was a needle in a haystack.
So if we found this one ghost using just four faint clues, how many billions of invisible galaxies are sitting right in front of our faces right now?
It's a staggering thought.
How many times have we looked at a pitch black patch of empty sky thinking it was the void when really we were staring directly at a massive galaxy just waiting for the right algorithm to reveal it.
The universe is likely a lot more crowded and a whole lot darker than it looks to our eyes.
Thanks for exploring the dark with us today.
It's always a pleasure to be here.
Keep looking up everyone, even at the empty parts. Catch it next time.
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