So, you get these pristine galaxies formed in one go, effectively, out of this gas that was left behind in the collision. And that gas then turns into stars, and the dark matter is gone, right, and the original stars are gone. So, what happens is the gas stays behind and is separated from the original galaxies. It collides with the other gas, and it compresses, it cools, and it starts to form stars. The gas, though, the gas in these galaxies can’t do that. 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 galaxies are so empty that this chance of a collision between two stars, or in between two dark matter particles, is extremely small. And what happens then in a collision like that, is that the dark matter and the stars essentially pass through one another. Namely, they collided almost head on with very high velocity. And those galaxies did something unusual. ![]() Gas is the stuff that galaxies are built from. 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. 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. 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. When this result was published, many researchers were critical of it, questioning how such galaxies could form without dark matter’s pull. That’s Pieter Van Dokkum, who four years ago observed two unusual galaxies that seemed to lack dark matter. ![]() So, in that galaxy there was no need for dark matter. 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. But then a few years ago, researchers saw something that seemed to challenge this idea – a celestial body that did play by the rules. That something else seems to be dark matter. 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. 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. I’m Noah Baker.įirst up on the show, astronomers have observed a string of galaxies that seem to exist without dark matter. This week: how galaxies could exist without dark matter.Īnd stories of Ukrainian researchers caught in the conflict. Head here for the Nature Podcast RSS feed. Never miss an episode: Subscribe to the Nature Podcast on Apple Podcasts, Google Podcasts, Spotify or your favourite podcast app. Subscribe to Nature Briefing, an unmissable daily round-up of science news, opinion and analysis free in your inbox every weekday. Nature News: Black hole at the centre of our Galaxy imaged for the first time We hear how they took the image and what it is revealing about these enormous objects. Last week, a team of researchers released an image of Sagittarius A*, the supermassive blackhole at the centre of our galaxy. 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 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. Research Highlight: Flower power: ‘Sunflower’ system churns out useful energy 10:49 Researchers experiences of the war in Ukraine ![]() Research Highlight: Helium levels in the atmosphere are ballooning How fossil fuel burning has caused levels of helium to rise, and a high-efficiency, hybrid solar-energy system. News and Views: Giant collision created galaxies devoid of dark matter 08:39 Research Highlights 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. But four years ago, astronomers made a perplexing, and controversial discovery: two galaxies seemingly devoid of dark matter. In this episode: 00:47 The mystery of the missing dark matterĭark matter makes up most of the matter in the Universe, and is thought to be needed for galaxies to form.
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