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  • NATURE PODCAST

How galaxies could exist without dark matter

Listen to the latest science news, with Nick Petrić Howe and Noah Baker.

In this episode:

00:47 The mystery of the missing dark matter

Dark matter makes up most of the matter in the Universe, and is thought to be needed for galaxies to form. But four years ago, astronomers made a perplexing, and controversial discovery: two galaxies seemingly devoid of dark matter. This week the team suggests that a cosmic collision may explain how these, and a string of other dark-matter-free galaxies, could have formed.

Research article: van Dokkum et al.

News and Views: Giant collision created galaxies devoid of dark matter

08:39 Research Highlights

How fossil fuel burning has caused levels of helium to rise, and a high-efficiency, hybrid solar-energy system.

Research Highlight: Helium levels in the atmosphere are ballooning

Research Highlight: Flower power: ‘Sunflower’ system churns out useful energy

10:49 Researchers experiences of the war in Ukraine

We hear the stories of scientists whose lives have been affected by Russia’s invasion of Ukraine, including researchers who have become refugees, soldiers and activists in the face of a horrifying conflict.

Nature Feature: How three Ukrainian scientists are surviving Russia’s brutal war

20:46 Imaging the black hole at the centre of the Milky Way

Last week, a team of researchers released an image of Sagittarius A*, the supermassive blackhole at the centre of our galaxy. We hear how they took the image and what it is revealing about these enormous objects.

Nature News: Black hole at the centre of our Galaxy imaged for the first time

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doi: https://doi.org/10.1038/d41586-022-01403-w

Transcript

Listen to the latest science news, with Nick Petrić Howe and Noah Baker.

Host: Nick Petrić Howe

Welcome back to the Nature Podcast. This week: how galaxies could exist without dark matter.

Host: Noah Baker

And stories of Ukrainian researchers caught in the conflict. I’m Noah Baker.

Host: Nick Petrić Howe

And I’m Nick Petrić Howe.

[Jingle]

Host: Nick Petrić Howe

First up on the show, astronomers have observed a string of galaxies that seem to exist without dark matter. Now, dark matter is a mysterious thing that nonetheless appears to make up most of the matter in the Universe, but physicists are pretty sure that it exists because without it, star and galaxy motions just don’t seem to play by the rules of gravity. So, to make the motions make sense – and even to form galaxies in the first place – we need something invisible to be there, exerting a gravitational pull. That something else seems to be dark matter. But then a few years ago, researchers saw something that seemed to challenge this idea – a celestial body that did play by the rules.

Interviewee: Pieter Van Dokkum

So, here we had a galaxy that behaved as it should, which made it an anomaly because every other galaxy does not play by the rules. So, in that galaxy there was no need for dark matter.

Host: Nick Petrić Howe

That’s Pieter Van Dokkum, who four years ago observed two unusual galaxies that seemed to lack dark matter. When this result was published, many researchers were critical of it, questioning how such galaxies could form without dark matter’s pull. Well, this week in Nature, Pieter has a new paper that looks at those galaxies in detail and makes another bold claim, this time about how they formed, and it involved something called a bullet collision. Reporter Lizzie Gibney gave him a call to find out more, and she started by asking Pieter to talk her through this bullet collision and how it could have led to galaxies without dark matter.

Interviewee: Pieter Van Dokkum

So, what happens here, we think, is that they were 2 ordinary galaxies, 8 billion years ago, just minding their own business, both of them mostly composed of dark matter, like every other galaxy, and a lot of gas. Gas is the stuff that galaxies are built from. And those galaxies did something unusual. Namely, they collided almost head on with very high velocity. And what happens then in a collision like that, is that the dark matter and the stars essentially pass through one another. The galaxies are so empty that this chance of a collision between two stars, or in between two dark matter particles, is extremely small. And so, the galaxies essentially pass through one another, they see each other, they feel each other, but then they say goodbye very quickly and they go on their merry way. The gas, though, the gas in these galaxies can’t do that. It collides with the other gas, and it compresses, it cools, and it starts to form stars. So, what happens is the gas stays behind and is separated from the original galaxies. And that gas then turns into stars, and the dark matter is gone, right, and the original stars are gone. So, you get these pristine galaxies formed in one go, effectively, out of this gas that was left behind in the collision. And what we think is that there wasn't just one galaxy that's formed but that this led to this trail of galaxies that were formed out of individual gas fragments that all came from these two galaxies that are still going on their merry way, having lost their gas, but still retaining their dark matter.

Interviewer: Lizzie Gibney

So, we have this string of galaxies that were the gas that was left behind in the collision, and what about the, I guess, the ends of the trail where the dark matter presumably still resides? Are they something you're able to study or see?

Interviewee: Pieter Van Dokkum

Yeah, this is where this story becomes somewhat mythical. Those two objects that originally collided should still be out there in these scenarios, but they should be incredibly dark. Because they've lost all their gas early on, they probably didn't have a lot of stars to begin with because it was such a long time ago. They hadn't formed many stars yet. And when we looked at the ends of the trail, there are actually two galaxies, so one on each end, that are very unusual. And interestingly, it had been noted before, completely independently, that there are these unusual galaxies in this group. They're very big, very fluffy, and one of them in particular is extremely faint. It's one of the faintest galaxies ever discovered. And what we think now is that those galaxies are the dark remnants, that they actually are extremely dark matter dominated, and that they're mostly composed of dark matter. So, ironically, we think what we're seeing is two things. One, a string of galaxies that have no dark matter, only what you see is what you get galaxies. And then two galaxies that are almost invisible, where all the dark matter still resides. That will be extremely exciting if we can show that this is where all the matter is, and that these galaxies also behave differently, and that they have too much mass, too much dark matter, from what you expect based on how faint they are.

Interviewer: Lizzie Gibney

And can this really unusual system of galaxies, can that tell us anything about dark matter itself, which is obviously still so much of a mystery?

Interviewee: Pieter Van Dokkum

Yes, in several ways. The first is a seemingly trivial point but important. Namely, if we can show that dark matter can separate from normal matter on the scale of galaxies, so that the galaxy can have dark matter, extra dark matter, no dark matter, it means that dark matter is real. It means that it is stuff, substance, something you can touch. And that is important because there are alternatives. Some people have suggested that dark matter isn't actually a particle or a thing but a manifestation of our lack of knowledge of physics. That is the laws of physics may behave differently out there in the Universe on these very large scales, such that these motions that we see we interpret as extra mass, but in fact it’s a modification of Newton's laws that leads us into believing there is extra mass. Those theories have come and gone over the years, but showing that dark matter can be separated from galaxies, that would not be possible in those alternative theories.

Interviewer: Lizzie Gibney

And four years ago, you made a discovery that proved somewhat controversial. So, how much evidence do you have that this amazing narrative of a single collision creating a whole string of dark-matter-free galaxies is what actually happened? Is it just plausible, or can we say it's the most likely explanation for what we see?

Interviewee: Pieter Van Dokkum

Well, I would say, at the moment, it is the most likely explanation in the sense that it is a single explanation that explains all these very different observations that we've gathered over the past four years, including the observation now that there's this trail of all these galaxies. At the same time, we do not have a smoking gun piece of evidence that proves that this is what's going on, and that's what we need to do. We need to verify the predictions that this model makes. In the past, over the past four years, that has happened repeatedly that people said, well, are they really sure. And then we went out and other people went out, got more data and what happens is, it's not really that we were proven right, although that happened, it’s that we learn more. And so, we learn more about the system and then found new things, including now this bullet explanation. And so, our hope is that that will continue, that others will question our results, we will question our results, we’ll gather more data, and then learn more about this collision and these remarkable galaxies.

Host: Nick Petrić Howe

That was Pieter Van Dokkum from Yale University in the US. For more on these curious galaxies, make sure you check out the show notes, where there’ll be a link to the paper, and Lizzie’s news article on the topic.

Host: Noah Baker

Coming up, we'll be hearing stories of researchers who've been caught up in the war in Ukraine. Right now, though, Dan Fox is here with the Research Highlights.

[Jingle]

Dan Fox

The amount of helium in the Earth's atmosphere is rising, caused in part by fossil fuels. Scientists have expected that as we continue to extract and burn fossil fuels, helium would become more common in the atmosphere as an isotope of helium, called helium-4, is known to seep into fossil fuel reservoirs. But they've not been able to measure the element with enough precision to detect such a trend until now. Researchers analysed 46 air samples from Australia and the US and found that the amount of helium-4 rose by around 0.2% between 1974 and 2020. While helium doesn't contribute to global warming, it is another sign of the increased use of fossil fuels. Read that research in full in Nature Geoscience.

[Jingle]

Dan Fox

A prototype solar energy device has been shown to be more than twice as efficient as the best conventional solar cells, converting more than 65% of the Sun's energy receives into electricity or heat. Solar cells convert some of the electricity that they absorb into electricity, with the rest becoming waste heat. But hybrid solar devices seek to capture this heat. This prototype dubbed the Sunflower system uses a dish that concentrates solar energy on a module of highly efficient solar cells – the centre of the ‘sunflower’. The cells’ waste heat is then captured by a water-cooling system. This system heats the water to more than 245 °C, preventing the solar cell from overheating and providing useful hot water for other processes. The team say the sunflower system is more robust and cheaper to produce than similar hybrid solar devices, making the system’s cost competitive with that of natural gas. The device could be particularly useful in industrial sectors that are hard to decarbonise. If that's captured your interest in this research, read it in full in Cell Reports Physical Science.

[Jingle]

Host: Noah Baker

The war in Ukraine is coming to the end of its third month. And in recent episodes of the Nature Podcast, we've been looking at how the conflict might affect the big picture – things like energy and climate, research funding and future collaborations. But of course, there are countless stories of individuals who've had their lives turned upside down by war, and our colleague Nisha Gaind has been reporting on some very personal stories of Ukrainian researchers who've been affected, and how the scientific community is coming together to help. She spoke to Benjamin Thompson about these stories, and started by explaining the current state of the conflict.

Interviewee: Nisha Gaind

It's three months into this brutal and horrifying conflict, and we are in the midst of a large humanitarian crisis in Europe. And things on the military front are developing now with the active conflict moving more to the east of Ukraine. And the length of this conflict has given us the opportunity to do some deeper reporting into the human stories. Research happens in basically every corner of the Earth now, so wherever there are people, there are researchers, and those are the voices that we're very keen on hearing and the stories that we really want to tell.

Interviewer: Benjamin Thompson

And let's talk about some of those researchers then that you spoke to as part of your feature article. Let's start maybe with Olena, a plasma physicist and, well, the epic journey that she had to escape the conflict.

Interviewee: Nisha Gaind

Yeah, that's right. She is one of the millions of Ukrainians who fled the conflict, and one of about 20,000, we estimate, researchers that might have fled. She responded in the way that many citizens of Kyiv did. She left the city. She went to her home village where she grew up, and they hoped that they would be able to avoid direct fighting, but sadly that came to them quite quickly. And she related to me the moment she will never forget is when a Russian rocket landed in her neighbour’s garden. At that point, she knew that she and her family had to get out of Ukraine, and eventually made their way to the Polish border and then to the Netherlands. And it's worth mentioning that the reason that they went to the Netherlands, her and her mother and her sister and her dog, is because they just got on a bus and they took whatever bus would take their dog, regardless of destination, which I just think is incredible.

Interviewer: Benjamin Thompson

And so, you detail this escape in your feature article, and she now finds herself, as you say, in the Netherlands, and she's picking up research again, which is astonishing.

Interviewee: Nisha Gaind

I have been filled with admiration for my sources, who are living through a war and who are, many of them, seeking to return to work as fast as they can. And they've been aided by the research community, who very quickly have jumped into action to support their colleagues from Ukraine. And Olena, when she was still in Kyiv, was contacted by a Ukrainian acquaintance who had offered her help and said, ‘Do you want to leave Ukraine?’ At the time she said no, but once she had made it to the Netherlands, and after some time of recuperating from the trauma of war and of their flight, decided that she wanted to start working again. She told me how important it was to start healing, to start getting back on their feet, to start contributing again. And so, she got in touch with her acquaintance who is part of this network called Science for Ukraine, which is collating these job offers for Ukrainian scientists. And through that, she has managed to link up with a research institute and she is discussing how she might continue her research. And her sister is also a physicist, so they’re that doing that together.

Interviewer: Benjamin Thompson

And this kind of organisation then, Science for Ukraine, is really worldwide, right? People really are stepping up to offer support or places wherever they can.

Interviewee: Nisha Gaind

Yeah, that's right. So, one of the other people I profiled, Taras who is a genomics researcher in the US but is originally Ukrainian, he has been part of coordinating that effort and also on his own has gone to extraordinary lengths to help Ukrainian scientists and students who are having to flee. And with great success at his university, Oakland University in Michigan, he already had several Ukrainians in his lab and, on top of that, he's created another position for a Ukrainian graduate student who he’s trying to assist to get to the US, and has also created a place in his lab for a young data engineer Valerii, who I also profile, for him to do a master's once he gets back from the frontline, where he is currently being a medic.

Interviewer: Benjamin Thompson

Yeah, the story of Valerii really is something else, and goosebumps for me, Nisha, when I read about it. So, as you say, he is literally on the frontline in Ukraine right now.

Interviewee: Nisha Gaind

Yeah, that's right. He was very keen to speak to us and to tell us his story, and it truly is an extraordinary story. He has a degree in medicine, and he was working for the genomics company Illumina as a data engineer. Like many of his compatriots who signed up to fight, and because of his medical training, he was put into a unit of the Ukrainian army as a medic, and went off to train for three weeks. And then we spoke, finally, on the evening that he was deployed to the frontline just hours before. And we had a conversation that I will likely remember for the rest of my life, speaking to somebody so calm, so collected, so in touch with what he was about to do with his life.

Interviewer: Benjamin Thompson

Did you not get a sense that he was frightened about what he was going to do?

Interviewee: Nisha Gaind

I mean, that's the first question that occurred to me, and I asked him, in fact, and his very sort of frank answer was, ‘No, not really.’ He caveated that by saying, ‘We'll see after the first shelling,’ which I believe he has now experienced.

Interviewer: Benjamin Thompson

I mean, I have to ask, have you managed to get in contact with him since you interviewed him, since he was deployed to the frontline?

Interviewee: Nisha Gaind

Yes, so we continue to WhatsApp when he has signal. He has told me that they've been experiencing quite heavy Russian bombardments on several nights, but no casualties yet in his unit, which obviously is very relieving to hear. And we had a little back and forth when I sent him the article, which he was really, really pleased to see, and said that it really boosted morale, which was a wonderful thing to hear and such a rare piece of feedback that you would get from a source.

Interviewer: Benjamin Thompson

And one of the little things that stood out to me about his story in particular, even among all the things that are going on right now, in his small amount of free time, he was finding space to do a bioinformatics course online and writing code on his smartphone, and he isn't the only one doing things like this.

Interviewee: Nisha Gaind

Yeah, I can only imagine what my response to a live conflict on my doorstep would be, and I would imagine that work or science would be going out of the window immediately. But here we have these three individuals, and I know this story is repeated many times over because of the people that I've spoken to, that Ukrainian researchers are logging in from basements to do genomics classes and science lectures. Taras teaches a genomics class in Ukraine once a week. He says many students log in from air raid shelters. Olena says the same. She also continues to teach her classes in Ukraine. So, it's this incredible show of strength that life continues and science continues.

Interviewer: Benjamin Thompson

And what of science then? I know you spoke to them all about the future and their hopes. It seems like maybe a strange thing to think about in the middle of a conflict, but what were their thoughts on the future of Ukrainian science?

Interviewee: Nisha Gaind

Well, that is a difficult question and a difficult topic to talk about because of the destruction that has been wrought by this war. We know that many science centres have been destroyed by Russia through bombing and that the devastation is almost too much to comprehend. And that is something that will take time. This conflict is still ongoing. We don't know how long it's going to go on for. But it unfortunately looks like Ukraine will have to do a complete reconstruction of its science system. But that will be aided by its very young cohort of scientists. It's notable that a lot of Ukrainian science departments are led by these very young, vibrant leaders. And it was really starting to kind of shake off its Soviet association after it gained independence and after its revolution. But the question of science in Ukraine after this war is still a very uncertain one. And I know that many like Olena hope to return home. They don't know whether they have a home to return to and that is, obviously, the first focus for many refugee scientists.

Interviewer: Benjamin Thompson

Nisha, if we can broaden things out a little bit. Of course, we are science reporters. We report on the latest paper or the latest discovery or what have you. This couldn't be more different. I mean, how was it for you reporting the story? What are some of the practicalities? And how did it affect you personally?

Interviewee: Nisha Gaind

Yeah, that's a great question. As science reporters, we are often reporting on good news, developments that would benefit humanity. And in this case, we're reporting on the very worst news. So, that does affect me as a reporter. But I must caveat that by saying anything that I experienced as a reporter is nothing that compares to what the people of Ukraine are going through right now. Overall, yeah, it's been a different story to report because usually when you speak to sources, you set up a very civilised time to chat to them. But in this case, I'm trying to speak to people who are fleeing their country or are on a frontline, so that meant speaking at times when they're available, which aren't necessarily sociable. So, for example, when I spoke to Valerii, who was about to deploy, I was in the back of an Uber and waiting a couple of days to hear back from him, if he has signal because there's no guarantee that he will. So, it's been a very different experience in that sense because typically we do report on wars, but we don't do a vast amount of war reporting. So, it has been a different and humbling experience to tell the stories of these scientists.

Host: Noah Baker

Nature’s European Bureau Chief Nisha Gaind there. Look out for a link to her feature article in the show notes.

Interviewer: Nick Petrić Howe

Finally on the podcast, we have some more astronomy news. Last week, you may have seen a picture of a black hole doing the rounds. In fact, it was the black hole at the centre of our own Galaxy – Sagittarius A*. So, here to help me catch up on what this is all about, and why physicists are so excited, is Davide Castelvecchi, Nature’s own black hole correspondent. Davide, how are you doing?

Interviewee: Davide Castelvecchi

I’m very good. How are you?

Interviewer: Nick Petrić Howe

I'm not so bad, thanks. So, my first question is, as our sort of black hole correspondent/physics buff, how excited are you about this picture?

Interviewee: Davide Castelvecchi

Just to give you an idea, this picture has been more than 20 years in the making. People started proposing this could be done in the late 1990s. And it is the largest black hole in the Milky Way by far. On the other hand, it's only the beginning. This is a relatively new technique. It's only the second black hole that’s been imaged this way, and it is still at the very rudimentary phase. At the same time, there's a lot of information that scientists have already been able to extract from it, so it is very exciting.

Interviewer: Nick Petrić Howe

This, as you said, is the second ever direct image of a black hole. And from a sort of lay perspective, it kind of looks similar to the one we saw in 2019, the one from M87. So, I guess, how does it differ from the first?

Interviewee: Davide Castelvecchi

So, to some extent, they are reassuringly similar because, according to Albert Einstein's general relativity, we know that they should be very simple looking objects and their shape is just determined by their size. What's different here is that this one is a lot closer. It's more than a thousand times closer than the previous one, but also more than a thousand times smaller, and so the way it appears in the sky is about the same. What's different is, because it's so much smaller, the matter that we see orbiting the black hole doesn't have to go as much distance to go around it. And so, if you see a blob of matter of like superheated plasma orbiting the black hole, it will orbit in a matter of minutes, maybe ten minutes, whereas in the previous case, it would have taken days. One source I talked to told me, ‘Taking a picture of our Galaxy’s black hole is a little bit like trying to take a picture of a two-year-old child. It never stays still. He or she never stays still. Whereas in the case of the 2019 unveiling, it was a black hole that you could take data for hours and basically see no variation.’

Interviewer: Nick Petrić Howe

And this two-year-old sort of nature of this black hole, as I understand it, that's also part of the reason why even though these observations were taken back in 2017, it's taken this long to actually get the picture.

Interviewee: Davide Castelvecchi

Indeed, and the picture we see is actually not a picture that they took over any period of time. They were taking data over several nights, and they took more than a thousand separate pictures, and then what we see is the result of a sort of averaging of all those pictures.

Interviewer: Nick Petrić Howe

And one thing that is specific about this new picture that people have commented on are these three, bright spots on it. Do researchers have an idea of what these are and what they might show?

Interviewee: Davide Castelvecchi

Taking these images is an extraordinarily difficult technical achievement. What they're trying to do is image very, very tiny details in the sky, using telescopes that are spread around the planet. So, that way, it's as if you had a telescope dish that is as large as the planet. But of course, you don't have the whole dish. It's like you only have tiny little shards of one dish. And then you have to reconstruct an image. And that, as you can imagine has its own limitations, and probably these brighter spots are artefacts of the way that the image is reconstructed.

Interviewer: Nick Petrić Howe

And can this reveal anything more about black holes as we know them?

Interviewee: Davide Castelvecchi

So, this is still the early days, and what we've seen so far is absolutely consistent with other observations and with theoretical predictions. The size of this image is in the ballpark of what it should be, given its mass, the mass that we already knew it had. Once the technique improves, it will not be just a ballpark measurement. There will start being a precision check on the theory. And then the other thing is, it's really interesting to see how the environment around the black hole evolved. And in particular, the matter that's orbiting it. Is it still orbiting in the same way, or will there be another cloud of matter that will fall in and maybe will orbit in a different way, and how does that relate to everything else that we see in the central region of the Galaxy?

Interviewer: Nick Petrić Howe

So, this could tell us more about our Galaxy as well and how things orbit the black hole there?

Interviewee: Davide Castelvecchi

Absolutely, because most galaxies are known to have such a supermassive black hole at their centre, and we know from observing many other galaxies that these black holes, they play a crucial role in the evolution of the galaxy itself. They can undergo periods where they engorge themselves with matter at a very high rate, and this matter that is falling in also produces enormous amounts of radiation and shockwaves, and all of this can move matter around in a galaxy. It can shut down the formation of stars in a galaxy, or it can start the formation of new stars in a galaxy. So, it will be interesting to go also back in the history of our own Galaxy and see how the black hole might have affected it.

Interviewer: Nick Petrić Howe

And you've been speaking to some of the researchers involved in this. How are they sort of feeling? Are they excited, thrilled? What was their sort of feeling when they revealed this?

Interviewee: Davide Castelvecchi

I think a lot of them felt vindicated because they had worked on this for decades, and there had been sceptics along the way, people who said you'll never be able to do it. For many of these researchers, it's a culmination of their careers. For many others, especially the younger ones, it's the beginning because we're still in the early stages of this new kind of astronomy.

Interviewer: Nick Petrić Howe

Well, that's a good point. What are the sort of next steps beyond this? Are we going to see many more images like this in the future?

Interviewee: Davide Castelvecchi

Well, many more in the sense of they are still observing these two black holes every year. Not more images in the sense that there aren't many other objects in the sky that you can observe. Basically, these are the two black holes that we know of that appear large enough in the sky to be imaged with this technique.

Interviewer: Nick Petrić Howe

That was Nature’s Davide Castelvecchi. For more on this story, check out the news article written by Davide, and we’ll link to that in the show notes.

Host: Noah Baker

And that’s all for this week. As always, you can keep in touch with us on Twitter – we’re @NaturePodcast. Or you can email us – podcast@nature.com. I’m Noah Baker.

Interviewer: Nick Petrić Howe

And I’m Nick Petrić Howe. Thanks for listening.

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