How you find dark galaxies: globular clusters as tracers

21:30challenge is how do you defeat sky glow? Sky glow being dark galaxies, these these types of dark galaxies that I mean there's no there's no stars. So what are you going to look at? Right? They're super faint. The signal is often so faint that there's a statistical noise limit when it comes to like your CCD, your charge couple device, the sensor in the back of your camera that is like looking at this thing. There's a noise, right? It's going to catch stray photons and that noise is going to be higher than the signal that we're getting. So, we can't just like look at the thing. Right. >> Right. So, how do we actually find this thing? Well, >> this is for earthbased >> even. No, this is even Hubble, dude. >> Oh, interesting. >> Yeah. >> Interesting. >> Because the Hubble has a CCD on it. >> I see what you're saying. >> And even though it's in outer space,

22:10like that thing has a noise. And then there's there's something actually called zodiac light. >> Okay. >> There is dust in the solar system, right? And that dust is going to scatter sunlight into your detector. Wherever you look, there's going to be dust from the solar system. And that is going to have a faint glow. And if the thing that you're looking at is a smaller glow than literal dust in our solar system. So what? You're we're going to put a Hubble like outside the solar system. We're not doing that. And then wait like 4 days to get signal back. Like well, first of all, getting out there is like, you know, it's ridiculous. So, so, so we need to figure out other proxy ways to to look for these ultra

22:52low density galaxies, these candidate dark galaxies. Okay. >> Okay. >> And we you the proxy that comes up is called the globular cluster. These are some of my favorite objects. They're dense, ancient, they're bright, and they're point-like sources. They're way, way smaller than galaxies. They're like these old old relics of galaxy formation. >> Okay. Not to be confused with the acronym relic, just to be clear. >> Yeah. Yeah. Yeah. Not to be confused with the acronym relic. There's so many of these that are around our Milky Way. They're really small. Okay. They're going to be like hundreds of solar systems to thousands of solar system. Really small. >> It's really small. Like like and the density of stars in that in that like

23:34region >> is is insanely small. like you'll have like like you know the closest star to us is like four light years away over there. I think it's like on the order of light days. >> So it's like it's like comparing the population density of Korea Town in Los Angeles to like Pikipsy New York or something else. >> Yes. Yes. We are we are that New York. >> Yeah. Yeah. We are we are Pikypsy New York but these galaxies look like Korea Town. >> Yeah. Yeah. Yeah. I You shouldn't say New York because whenever I think New York I think New York City. It's like So try let's try like uh >> Pikipsy is like the middle of New York State, but let's say let's say like I don't know where that is.

24:15>> I only know New York City. >> There's only one New York. You could say like >> you know Nebraska, >> right? Right. You know Omaha. >> Yeah. Omaha. Nebraska. >> Om. Okay. Fair. Fair. Exact. Let's do that. But that's an that that's a that's a helpful reference point to so these things are really small and so imagine and these globular clusters they're actually really important for um just history astron astronomical history um hero Shappley at Mount Wilson at the Carnegie Observatories which we're going to have a special coming out soon. >> He mapped that the sun was not at the center of our galaxy by using these globular clusters. He observed a bunch

24:55of globular clusters, their positions, >> how far they were relative to the galactic disc, and he could figure out we're actually off center. >> Uhhuh. >> Right. We're not we're not in the center of this galactic >> Yes. >> um these globular cluster distributions. So they're really tiny little specks around the around giant massive galaxies, right? And so they become really nice for observing a dark galaxy because the galaxy couldn't be there. But if we observe a bunch of globular clusters, there's got to be something that is gravitationally binding the globular clusters in that area. >> This this vaguely reminds me, and I know I'm always making these stretch connections. When we talked about the

25:36radius of the proton story last week, >> the idea that you cannot directly observe it because it's so small. And so we have to use these like second and third order >> reference points in order to like compute the value. M and similarly because we can't because dark matter does not interact with light we're using the proximity and the movement and acceleration and angular momentum and etc etc of these globular clusters to derive understanding of this dark matter. >> That's exactly right. Yeah. And so this is what they finally did. They they looked at archival data from the Piper survey which is of the Perseus cluster. There's a giant galaxy cluster in the

26:17Perseus constellation. Okay. And um