Dark matter in galaxy groups

2:19So, today I'm joined by John Mulchaey, the president of Carnegie Sciences. John, thank you so much for taking your time. Happy to be here. Um so, I wanted to start by focusing on the science because that's what we do on the podcast. And you have a stellar career in cosmology. Right? Um one of the things that I was reading about is how you focused on galaxy groups, not just like large galaxy clusters, but sort of smaller galaxy groups like the kind that the Milky Way is a part of, and trying to characterize the dark matter that's in these galaxy groups because it's it's tends to be a bit harder than if you have a giant cluster and you can

3:01just like chart the velocities and then do virial theorem, what's the mass? >> That's right. >> Yeah. Um so, could you briefly talk about, you know, some of the challenges there and what your approach was when you were dealing with this problem? >> Sure. Absolutely. So, I think the clusters get all the attention, right? Because they're these big grand things. >> Yeah, with the gravitational >> with the gravitational lenses and all that. The lenses and the big x-ray halos and all that. But they're just really really amazing systems, right? But but they're pretty rare, right? I think this is the thing people don't really appreciate that. Most galaxies actually are in small groups like the Milky Way in our very small collection. Milky Way's in the local group, which is a very very small group. But that's a much more typical environment than the clusters that get

3:42all the attention. And the nice thing about the clusters, as you mentioned, is that there's lots of galaxies so you can you can measure the velocities of, you know, 100 galaxies and get a good measurement of the mass of the system. for instance. It's very hard with a group because you have two or three velocities and remember we only measure the velocity away from us. We can't really measure it in this in the plane of the sky cuz you're doing that even that's right. We can only measure the red shift and so that's only a velocity in one dimension and then if you you know say you have three galaxies those velocities are it's not a very reliable measurement of of of the mass of a system. And >> just to briefly pause it's true that even with the large gal gal gal galactic clusters you can only measure this that's right but maybe

4:24because there's so many you kind of do an ideal gas type thing >> That's right. >> and you're like oh you know equal equal partition >> That that's right. >> Okay. >> It should be a somewhat random process and so if you do enough of them eventually you kind of you you you aren't losing out you know. Imagine you have a group and everything's moving in this direction we wouldn't see a measurement at all right or you know so for instance you know with three objects you really cannot measure that and so the >> historic challenge has been um because of that people really had a hard time estimating the mass of these systems um and then in 1993 we made a remarkable discovery when I was in grad school um that um these systems also contain hot gas just like the clusters do that glows

5:04in the x-ray >> Mhm. >> and that was a really interesting um measurement because you can use the temperature of that gas to get an estimate of the amount of mass in the system. And so it was the first really kind of reliable measurement of a mass of a group and by the way the masses actually were aligned with kind of what you might expect based on the pretty poor statistics we had from the galaxies. But it did demonstrate really for the first time that there was a lot of like everything in the universe there's a lot of dark matter. Um and so