Hot gas in a galaxy group emits X-rays, giving astronomers a way to estimate the gravitational mass holding the system together. John Mulchaey explains to Krishna Choudhary how the gas temperature relates to the depth of that gravitational pull under an assumption of hydrostatic equilibrium. He also recalls how a low-ranked ROSAT observing proposal produced a discovery that helped establish the importance of dark matter in galaxy groups.
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Hydrostatic equilibrium describes a balance between a gas pressure gradient and gravity.
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X-ray measurements provide information about gas temperature and distribution that can be used to infer a group's mass.
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Mulchaey's account illustrates why observing programs sometimes need room for proposals whose outcomes are uncertain.
739 words · auto-generated from the episode video
5:32these groups just like clusters are really dominated by the dark matter. >> Ah and just to just to follow that thread of logic a little bit more closely with the x-rays you get something like the how much gas there is because the gas is glowing in x-ray. >> Right. >> And then from that you get velocity. >> You Well, what you really measure is the temperature. >> Ah, okay. >> So, yes. So, you measure the temperature of it. And the temperature is a reflection. So, why is the gas hot, right? Why is it emitting in x-rays and not in the radio or in optical? It's because the gas is being heated by the basically the gravitational force of the system. And so, it I know you guys talk about hydro-static equilibrium a lot. >> Yes. This is the same exact thing
6:13>> here, right? Yes. >> It's the gas falls into the system, it heats to a certain temperature to maintain the gas cloud is basically hydro-static equilibrium. And so, the temperature of the gas is enough to balance the gravity. >> Yeah. >> And so, it's the same thing that happens in the sun with, you know, the the nuclear fusion pushing outward in the um the reason the sun isn't doing this, right, in a huge ways is because of hydro-static equilibrium. >> Right. >> Same exact argument. >> Same exact argument. So, you get like, you know, the size of the gravitational potential >> Heats the gas up. >> Yeah, that's right. >> Yeah. >> So, the clusters being more massive have much higher x-ray temperatures. >> Oh, okay. >> than the groups. >> Yeah. >> And so, by measuring the temperature of the group, which you can do with a like Chandra space telescope or something, um that temperature then can be directly
6:54can be either a direct measure of the mass of the system. >> Right. Okay, we we just recently covered um a paper in the Astrophysical Journal Letters about the new relics. >> Yes. >> Like the you know, they found Cloud 9. They did a very similar argument where they looked at the temperature of this gas, and then oh, there's no galaxy there, there's no stars, but it's still there. Um okay. That's that's all clicking. And so, that was in the '90s. >> Mhm. >> And what was it you know, this is a massive discovery because now you've found a way to measure the mass of smaller groups. >> Mhm. >> And you know, what was that moment like? Like was it was it sort of a single moment where you like unblind all the
7:35parameters of your model and you know, you know how they do it now where it's like everything's blind and then and then somebody presses a button and all of a sudden H not pops out. >> It was a very similar moment for us. It was I should say that um uh we wrote a So, interesting story is we wrote a proposal to use the ROSAT X-ray telescope which was the big telescope in the '90s, the X-ray telescope prior to Chandra. >> And this was in space? >> It was a space telescope. Yeah, all the X-rays have to be done in space because the atmosphere absorbs the X-rays. Fortunately for us as humans, but not so fortunately, you know, if you're an X-ray astronomer. >> Yeah. >> Um and so we wrote a proposal saying let's we want to look at some groups and see if we see anything there. Um and the proposal was very poorly ranked. Um they said, "Oh, you're not
8:16going to see anything. This is just We don't Nobody really expected to see this gas at the level we see it." >> Yeah. >> And so, but we got lucky because um ROSAT had this weird system where they would rank proposals to A, B, and C and a very small fraction of the C proposals got through. We ended up getting a C target and so we ended up getting data that we probably shouldn't have gotten otherwise. >> Wow. >> And that was the discovery image. Um and so it always reminds me and I always every time I sit on a scientific panel or reviewing proposals, I always remind people, "It's very important that we try