Chapter 1: What is the main topic discussed in this episode?
You're listening to Shortwave from NPR.
Chapter 2: What is dark matter and how much of the universe does it comprise?
The universe that you and I see with our eyes, things that make up matter like galaxies, stars, planets, grass, that's only 15% of the universe's total mass. The rest is called dark matter. This is mass in the universe that doesn't interact with light. Astronomers know that dark matter is there, but they don't know what it's made out of.
So we don't know what dark matter is, but we know what it's not.
Chapter 3: What is Cloud 9 and why is it considered a failed galaxy?
So when you think of a galaxy like our Milky Way, it has three main components. It has stars, it has gas, but most of it is dark matter.
That's Jorge Moreno, a computational astrophysicist, cosmologist, and professor at Pomona College in California. Like many scientists out there, he would love to find out what exactly dark matter is. And a new clue just dropped. It's called Cloud 9.
Chapter 4: How does the discovery of Cloud 9 contribute to our understanding of dark matter?
Cloud 9, I'd like to think of it as a bit of an underachiever. It actually had all the resources it needed to make a galaxy. It had the fuel, it had all the conditions, but it just chose not to.
Cloud 9 is a failed galaxy. It's a dark matter halo with a cloud of gas devoid of stars. It's on the outskirts of a beautiful spiral galaxy, M94. But what's a dark matter halo?
A clump of dark matter.
And while it may be an underachiever, Cloud 9 is a big deal.
Chapter 5: What role do dark matter halos play in galaxy formation?
The current model of our universe predicts this kind of dark matter halo exists, one that didn't help make a galaxy or stars. But this is the first time astronomers have observed one.
It not only teaches us about the nature of galaxy formation, but also the nature of dark matter itself.
Today on the show, why this failed galaxy could be the key to finding out one of astronomy's biggest mysteries. Answering, what is dark matter? I'm Regina Barber, and you're listening to Shortwave, the science podcast from NPR. I'm talking with Andrew Fox and Gagan Deep Anand, two astronomers from the Space Telescope Science Institute in Baltimore, Maryland.
Chapter 6: What is the Lambda CDM model and how does it relate to dark matter?
And you two were on the team that found this dark matter halo. First of all, Andy, how did you feel when you realized what you found?
We were excited because we'd been studying this cloud near the galaxy M94. This cloud's been known for a few years. But we pointed the Hubble Space Telescope at this cloud, expecting to find some stars. And if we'd seen stars that would have confirmed that this cloud is really a small galaxy, something like many other galaxies that are out there.
But what we found was that there are no stars, even though we pointed at this object for a very long time with the Hubble telescope. And that told us that it's a different type of object, an object that is gas rich, but that is almost completely starless. And so we were excited because in a way that was a surprise. We didn't find the stars we were expecting to see.
Chapter 7: How do astronomers study and confirm the existence of dark matter halos?
We found just a blank piece of sky, a completely empty cloud. And that's a really interesting clue about what the nature of this object is.
And Deep, can you help our listeners understand what is a dark matter halo? Why is this such a big deal?
Yeah, so galaxies form inside dark matter, a halo. So the dark matter is the prevailing structure, actually, even though we can't see it. And then all of the matter that makes up, you know, the things we know, like stars and planets, normal matter or baryonic matter, that is, you know, that's the stuff that we know and love and that you see in the pictures. But...
The prevailing model of our universe, what's called Lambda CDM, or the model that describes dark energy and dark matter, it predicts that you should have dark matter halos that are actually not massive enough to form stars in the centers.
And so this has been a prediction of the theory, and with the discovery of this relic object, Cloud 9, it's a confirmation that you actually do indeed have dark matter halos that are not massive enough to form stars, just like the simulations predict.
Okay, so Andy, can you help explain this Lambda CDM model and how it really tells us how galaxies are being put together?
Sure.
Yeah, it's a great question because, as Deep mentioned, most of the matter in the universe is not thought to be in atoms and molecules, the regular matter we can see around us on Earth and in our solar system. Most of the matter is thought to be dark, which means it doesn't emit any light.
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Chapter 8: What implications does the discovery of Cloud 9 have for future astronomical research?
We can only infer it by seeing its effects on the matter around it. for example, the gravity that the dark matter can provide. Now, this lambda CDM model, it's accepted as the prevailing, the commonly accepted model of where the matter is in the universe and the matter condenses into galaxies. And those live in what we call halos because the matter is concentrated into different patches.
We call those halos. Most massive halos have galaxies in them. So when we look out into the night sky, we see those galaxies. But the cool thing about this model is that it predicts there are smaller halos, smaller halos that are beneath the scale that can form a galaxy, essentially failed galaxies, things that didn't quite have enough mass
to form a galaxy, emit light, and become like any other galaxy we can see. Those smaller halos are called relics. That's a technical term, but you can think of it as a relic, a leftover from this time when galaxies formed. But there were some leftover clouds that were not massive enough to form galaxies. And this is part of this theory, the CDM theory.
How big are we talking about? Like, if you can kind of compare it to our galaxy and things maybe around it, when we're talking about these relics, how big are they?
So this relic we've been studying called Cloud 9 is about a kiloparsec across. That's the unit astronomers use. That's about 3,000 light years. The Milky Way would be, I don't know, maybe 50 kiloparsecs or 150,000 light years across. So this object is much smaller than the Milky Way. It's small and it's dense with no stars in it.
But so we think of these relics as the leftover clouds that didn't quite make it to become galaxies. And they've just been hanging out there in the universe, but they're very hard to observe. And that's why we had to look really to very deep levels, look with very sensitive imaging with the Hubble telescope to actually confirm this thing and show that it had no stars in it.
And that's really the side of this story we're most excited about, is that these objects have been predicted by theory. They come out of this Lambda-CDM theory. But with Cloud9, we finally have a chance to observe one and see what its real properties are like.
And Deep, I think this is a good time to, like, take a step back for people, you know, who aren't astronomers. How do galaxies form? Like, what is the consensus, you know, in the astronomy community?
Yeah, so before, you know, if you go back way in time before galaxies formed, you have the universe and you have certain parts of the universe that are just slightly more dense than other parts. And those are the regions that become galaxies. And so we think that galaxies form in halos or roughly spherical regions of dark matter.
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