A Mysterious Gas Is Discovered In The Milky Way

11:55 minutes

a linear stretch of clouds illuminated by glowing stars. a panoramic view of the milky way galaxy
Milky Way Galaxy. Credit: ESO/S. Brunier

Recently, a group of scientists studying the Milky Way through the world’s largest ground-based radio telescope identified something they had never seen—a cold, dense gas that had been ejected at high speed from the galaxy’s center. 

The mystery of this gas—what caused it, how it could move so fast, and where it will end up—prompted research by Enrico Di Teodoro, a scientist in the department of astrophysics at Johns Hopkins University. He joined Science Friday producer Katie Feather to talk about the new discovery, as well as answer some fundamental questions about what is happening at the center of our galaxy.

Further Reading

  • Read the full study in the journal Nature.

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Segment Guests

Enrico Di Teodoro

Enrico Di Teodoro is a researcher in the Department of Astrophysics at Johns Hopkins University in Baltimore, Maryland.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Later in the hour, we’ll hear how indoor ventilation is impacting the spread of COVID-19 and why hardly anyone is talking about it. And we’ll talk to a researcher studying connections between the microbiome and cancer immunotherapy.

But first, the Milky Way. It’s a swirling galaxy of billions of stars, including our own. But what is going on far away from our little celestial neighborhood at the center of the galaxy? With help from the world’s largest ground-based radio telescope, scientists recently looked there and uncovered something strange. Science Friday producer Katie Feather has more.

KATIE FEATHER: When I first heard the news researchers had discovered mysterious clouds of dense gas at the center of our galaxy, I realized that I don’t know where the center of our galaxy actually is. Do you?

So we asked Dr. Enrico Di Teodoro, one of the scientists behind this discovery, to help us answer some fundamental questions about what is happening at the center of our own Milky Way. Dr. Enrico Di Teodoro is a researcher in astrophysics at Johns Hopkins University. Dr. Di Teodoro, welcome to Science Friday.

ENRICO DI TEODORO: Thank you very much for having me.

KATIE FEATHER: So the first question I have is actually a deceptively simple one. Where is the center of our galaxy located?

ENRICO DI TEODORO: Yeah, that’s actually a very good question. Because, as you probably know, our sun and ourselves, we live in a spiral galaxy that we call the Milky Way. And you can imagine our galaxy as a thin rotating disk of some several hundred billions of stars and lots of gas, mostly hydrogen gas.

And our sun is located quite far away from the center of this disk because we are at a distance of about 27,000 light years from it, which means that life takes 27,000 years to travel from the galactic center to us.

And this also implies that when we look at the center of our galaxy, we are looking at events that happened 27,000 years ago. And if you want to see the center of our galaxy at night, you should look in the direction of the Sagittarius constellation. And if it is dark enough, you will notice a clear patch of diffused light. And that’s exactly the center of our Milky Way.

KATIE FEATHER: Oh, wow. So you can actually see it from outside at night if it’s dark enough.

ENRICO DI TEODORO: Exactly, yes.

KATIE FEATHER: What does a clear patch of diffused light look like? Like, what exactly am I looking for?

ENRICO DI TEODORO: I mean, when you look at the night sky, you see all the single stars. But there is also what we call the Milky Way. I mean, and it’s like a diffused emission, so diffused light that goes across the sky. And the most luminous part of this diffused emission is the center of our galaxy.

KATIE FEATHER: OK, so that’s what it looks like if we were standing outside at night looking up at the sky. But if we could travel to this spot, the center of our galaxy, what would you actually see there?

ENRICO DI TEODORO: Yeah, I mean, the galactic center is like the downtown of our galaxy. So there are a lot of very interesting things happening there. So, first of all, the galactic center is a very crowded place in terms of stars. So there are millions of stars all packed together in a very small volume.

And the night sky for any particular inhabitant in the galactic center would be absolutely spectacular. Because there would be a million stars, very bright with brightness greater than the brightest stars in our sky, which is serious.

And this also means that you could easily read a newspaper at midnight, just relying on the starlight alone. So it’s a very different environment from where we are. And beside the stars, the galactic center is also interesting because there is a supermassive black hole, which is four million times more masculine than our sun.

And however, it’s also very dangerous because all these stars are very young and very massive. And they can easily explode as supernovae. And they can release a lot of energy. And the black hole as well can release a huge amount of energy when the matter falls onto it.

KATIE FEATHER: So you recently used a ground-based telescope to look at the center of our galaxy. And you noticed this weird type of gas emitting from it. What was strange about it? What caught your eye?

ENRICO DI TEODORO: So, basically, all this energy is like a huge bomb that creates a shockwave that blasts away everything, and in particular, gas, which is light and easy to move around. And this gas can be even pushed out the galaxy and escape if the shockwave is strong enough.

So this is what we call a galactic point, OK? And for an analogy with winds here on Earth. But this particular wind moves with velocities of a few million miles per hour, so it’s very fast. And it extends for several ten thousands of light years.

So, yes, as we said, for this particular study, we used a telescope in the mountains of Chile in South America. This is called apex. And this particular telescope can see microwave light. But this light is coming from the coldest place in the universe. In particular, we observed carbon monoxide gas, which is very abundant in the universe. And this gas looks exactly like the clouds that we see here on Earth.

KATIE FEATHER: So mixed in with this hot galactic wind that we already knew about, you’ve now discovered these clouds of colder gas. What does that mean? What are you expecting is happening there?

ENRICO DI TEODORO: Yeah, exactly. As you said, this galactic wind is very hot plasma gas, and I’m talking about temperatures of millions degrees. And this is what we call the Fermi Bubbles, which are exactly that. There are two enormous bubbles filled with fast-moving, hot gas.

And now we have discovered that within these hot bubbles, there are a bunch of cloudlets of cooled and condensed gas with temperature of minus 449. And these clouds are floating around and moving with a hot flow. So this is what we call molecular gas because it’s the only gas cooled enough to contain complex molecules.

And the existence of these cold gas islands within the hot gas motion was very surprising and puzzling because it is like to see ice cubes floating in all kind of lava. You wouldn’t expect them to last a very long time without melting.

And also, another very puzzling aspect is how those bands of gas clouds got there. Because hot gas is light and very easy to push, but cold gas is not. It is very heavy. And it is like if you blow on a bowling ball, and you try to make it move.

So we think that the energy released by our galaxy is not enough to easily push this dense gas out. Therefore, we do not know yet how this happened. This is a second mystery that we are still trying to actually investigate with new observation.

KATIE FEATHER: Do we know how long this gas has been there?

ENRICO DI TEODORO: Well, we don’t know exactly for how long this wind in the center of our galaxy has been active. But we think it’s at least 10 million years, probably much more. And, of course, the condition of the center of the galaxy can change with time.

And the black hole can become more or less active. The star formation can be more or less advanced. Therefore, we expect that the strength of this galactic wind also can change with that.

KATIE FEATHER: So why do we think this gas is there? I mean, what’s it doing there? What’s it used for?

ENRICO DI TEODORO: OK, this gas is outflowing from our galaxy. So it’s just being pushed out. But what is very important about this discovery is that this gas may have a strong impact on the future of our galaxy.

That’s because the cold condensed gas that we have observed flowing from the Milky Way center is the same kind of material that galaxies used to form stars. And if a significant amount of this gas is lost because it’s driven out with this wind, like our data suggests, you can imagine that at some point, our galaxy will run out of the fuel that it needs to assemble new stars.

And it is like when gas is leaking from the fuel tank, and there is no replenishment of gas, at some point, we finish it and we stop it. And if the Milky Way stops its star formation, well, this would have very important consequences for the future of our galaxy. Because the life of a galaxy that actively forms stars is very different from the life of a galaxy that doesn’t form stars anymore.

But, anyway, this is not going to be a problem for us human beings because this transformation of the Milky Way would not happen earlier than several thousand years.

KATIE FEATHER: Wow. So the center of our galaxy is like a fuel tank leaking this star-forming gas that it needs to form more stars. When it’s leaking this gas, where is that gas actually going? Is it getting blown away out into the galaxy? Or is it affecting other things in our galaxy?

ENRICO DI TEODORO: Yeah, the fate of the gas in this wind depends on how strong the wind actually is. So if this wind is strong enough, all this gas will be just pushed out of our galaxy and just escape. And it will go in the space in between galaxies, just hanging out there.

But if the wind is not strong enough, then, at some point, the outflowing gas will lose its momentum. And because of the gravity of our galaxy, it will fall back onto the galactic disk. And it will be recycled again to form your stars. So it’s kind of a cycle. And the weird thing with in the Milky Way, the current wind may be strong enough to blow away this gas. But we are not 100%.

KATIE FEATHER: Are there other galaxies we know about that have stopped making stars that we can kind of study to see where we’re headed in about 10 million years or so?

ENRICO DI TEODORO: Yes, there are many galaxies in the universe, actually, that do not form stars anymore. And astronomers broadly classify galaxies in two categories. So, on the one side, we have galaxies like the Milky Way, which have active star formation, a lot of gas, young blue stars. And these galaxies look like flat, rotating disks.

But on the other side, there are galaxies that have stopped their star formation a long time ago. And these galaxies are more spherical. They do not rotate. They do not have gas. And they are just made up of old, red-looking stars. So these galaxies are kind of dead. And we think that they represent a late stage in the evolution of a galaxy.

So, and the Milky Way may actually be in a transitional phase between these two stages because it is still a disk forming stars, but its star formation is declining with time.

KATIE FEATHER: Well, I’m sort of glad that we don’t have to worry about this for another 10 million years. But I hope that you are able to solve these mysteries of the dense, cold gas at the center of our galaxy soon. Thank you so much for joining us to talk about this.

ENRICO DI TEODORO: Thank you very much to you for having me.

KATIE FEATHER: Dr. Enrico Di Teodoro is a researcher in astrophysics at Johns Hopkins University.

IRA FLATOW: Science Friday producer, Katie Feather.

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