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I thought I'd talk about NGC 2403. Oh, at last. What is it? It's a galaxy. It's actually a pretty bright galaxy. It's a Here it's a picture that I took. You took that? Yeah. This is one of mine. That was from my back garden. Nice. And it's actually quite a big bright galaxy, right? It's one of the brightest galaxies that Messier missed. So, it could have been a Messier galaxy, but for some reason, you never saw it. Why do you think he didn't see it? The Messier catalog is not done in a very systematic way, right? It's, you know, things that Messier happened to spot while he was looking for comets. It's
things that he happened to have read that other people had seen. So, it's not like he did a systematic sweep of the sky trying to find every object. So, it's just by chance that this is probably one of the brightest ones that he missed. All right. It's it's the fish that got away. Yeah. And it's a very nice little spiral galaxy, right? It's a shame to have missed it cuz it's it's nice. It looks a little bit like Messier 33. It's that kind of spiral galaxy, but very pretty. Probably the thing that this thing is most famous for that no one much knows is it's where
dark matter was first sort of fairly definitively detected in a galaxy and it's sort of a lost bit of the story right in that you know whenever people talk about dark matter in galaxies everyone knows about Vera Rubin and the work that she did but it turns out that there were a lot of other people working in similar fields and she was building on a lot of things that have already been done um and in fact the person who discovered dark matter in this galaxy is a guy called Seth Shustak who in the early 1970s wrote a thesis in which he looked at a couple of galaxies. is you know so this is early 1970s so it's quite early for doing high resolution radio astronomy but he was looking at
that 21 cm line from hydrogen using a radio array in California um and making images quite high resolution images in the radio part of the spectrum which essentially told him where the gas was in his galaxy and also because you're measuring an emission line you can measure the doppler shift whether it's coming towards you or away from you from the shift in its frequencies so he was able to measure the rotation speed of the galaxy and the rotation speed because he had this high resolution data as a function of how far out you are. And so he ended up producing one of the early versions of one of these plots which is how fast the galaxy's rotating versus how far out you are in the galaxy. And it has that
what became, you know, a very well-known structure that it rises in the middle and then it stays flat. And this is this evidence of dark matter. The fact that you would expect as you get further away the pull of gravity should be getting smaller and smaller which means that to stay in orbit you should be going slower and slower. Just like you know in the planets in the solar system as you go out in the solar system because all the mass is concentrated in the middle as you get further and further from that mass the planets go around slower and slower. In this case things don't get slower and slower. They stay at the same speed. That tells you that there's more and more mass as you go to larger and larger radi.
So, dark matter, I know we don't know what it is. It's like a mystery thing, isn't it? Does it live in galaxies? Does it live between galaxies or what do we know? Everywhere. It's everywhere. So, in fact, the highest density of dark matter is in the middle of the galaxy, but it's actually just in a more extended halo. So, even after you've run out of stars and gas, there's still dark matter. So, there's still mass increasing. And because there's kind of more area at large radi, more volume at large radi, there's actually a lot of mass out there. Even though the density is that bit lower, there's a whole load of space, so there's a whole load of mass out. It's always struck me as a little strange that people think
it's mysterious, right? But actually, I'm not sure it is in that there's no reason why the universe had to be laid on for the benefit of astronomers, right? There's no reason why everything has to glow in the dark. And so, why shouldn't there be material out there which just doesn't let its presence be known by emitting electromatic radiation that astronomers can detect? So, the only way we detect dark matter at the moment is by its gravitational influence. Yeah. And there are a lot, you know, so it could be that it's made up of massive particles and there's a lot of work going on of people trying to detect those massive particles because the dark matter is everywhere. So it
means it's passing through the earth at this minute. So there are a lot of people who've set up detectors to try and detect that dark matter and it interactions with material on Earth. None have yet detected anything at all. As someone who spent their career studying galaxies, galaxy evolution, galaxy formation, motion, how much of a thorn in your side is dark matter? It's not. I mean you know you take the universe you're given right and so actually it's not it's just one of those things we have to try to understand and in some level that's what makes astronomy interesting that we don't understand everything so actually you know the fact there are things that we yet to get our heads around what's
really going on is what makes the subject interesting in the first place this guy Seth Shostak's thesis couple more things to say about it so let me read I just read a bit from what he actually said about the detection or how he how close he got to the detection of dark matter so this is from the conclusions of his thesis it says the flat rotation curve of NGC 2403 seen in other late type spirals. So it's already a known thing right even then although he's the first person I think to really talk about it implies a mass density dropping as slowly as 1 / r squ. So that's this slow fall off of density with radius in the dark matter around this galaxy. Uncertain total masses
result just because if you actually keep adding up all that mass it actually diverges. You end up with more and more mass as you go to larger and larger radi. And for this reason is felt that only bounded masses limits by the last point on the measured rotation curve should be quoted. So he can't actually say what the mass of a galaxy is because it's still increasing as you go to larger radi beyond the last point he's been able to measure. This is the bit where he gets as close as possible to actually saying that there's dark matter there. The flat curve also requires high mass to light ratios in the outer galactic regions. So the mass to light ratio is how much mass there is for
every given bit of light. So a high mass to light ratio means there's a lot of mass and not much light. And so he's saying there's loads of mass here but not many stars. So we really don't know what's going on. Um so that's kind of as close as he got. He almost said there was dark matter there. He just couched it in the sort of the vocabulary of the day, which was to say that you have a high mass to light ratio. If I went back in time and asked him, "What do you think's going on there?" What would he have told me? I think he'd have said that he didn't know. Even I mean, you know, that's what we say now, right? So, it's not like things have changed that much. It would have been a bit of a leap
to say there's some mysterious matter that we don't know what it is, but he might well have said that. Were you speculating? Were you talking to your supervisor? Did you have any wild theories or was it just like a complete mystery to you? I think it was pretty much a mystery. I mean, you I knew that there had to be some mass out there to account for this fast rotation, but you know, you couldn't see it. I mean, it wasn't luminous. It wasn't stars, at least not luminous stars. I mean, it could have been, and as I said, I think at the time, it could have been footballs. you know, the galaxy could have been filled with footballs, which don't make a whole lot of light, but if you have enough of them, they
might, you know, provide enough mass to account for this peculiar phenomenon. I didn't know what it was. And uh I think that actually is still the case. We don't know what the dark matter really is, but we do, you know, it's generally accepted that it's out there. Is there any part of you that thinks there was like a missed opportunity for you to be Mr. dark matter for you to have been the first person to kind of pin down this mysterious substance that's now so famous. Well, yes, of course. I mean, you know, when you later in later I mean, it's like many things that uh you know, were discovered in that era when you thought gosh, you know, I could have written that paper, right? I had the data, but you know that's all the
benefit of hindsight. Seth Schuststak isn't famous for this. Cesak is famous for being one of the world leaders in SETI, the search for extraterrestrial intelligence and being a director of the SETI institute and getting very involved in searching for intelligence elsewhere in the universe. And I discovered if you look at the acknowledgements for his thesis, so acknowledgements to the thesis is where you kind of thank everybody and thank your mom and all that kind of stuff. But he also mentioned at the end of his thesis he gave a dedication and the dedication in his thesis is this thesis is dedicated to NGC 2403 and all its inhabitants to whom copies will be furnished at cost. So even then he was thinking about extraterrestrial
intelligence in NGC 2403. I think I was just being a wise guy. I don't think there was anything very deep in any of that. I mean I looked at the dedications of other thesis and you know they followed the pattern you would expect. I mean, people would d, you know, dedicate it to their wives or girlfriends or whatever or boyfriends. I mean, that was common. And I just decided to dedicated to NG2403. That was the result of about one minute's worth of thought. How much would it cost to send one of these papers to that galaxy, do you reckon? I think he's going to have a long while to wait for them even for the request to arrive, let alone to have to worry about the shipping cost to get it out there.
How likely do you think it is that there are inhabitants of NGC243? as you referred to. Yeah. Well, I would bet my next month's paycheck, which uh given I'm semi-retired, is not terribly large, but yeah, I'm sure there are. Look, the number of stars in NGC 2403 is, you know, it's on the order of maybe hundreds of billions. And yeah, most of those stars are kind of worthless for supporting life on a planet around them because they're too bright or they're too dim or something like that. But nonetheless, 10% of them are probably okay. So that's still a very large number of star systems. And unless you are convinced that the origin of life is some sort of miracle,
some sort of highly unlikely event, then life will have arise risen uh you know on many planets, you know, billions and billions as Carl Sean would occasionally say in that galaxy. And uh you know maybe most of that life is not terribly interesting. It's all single-sellled life or insects or something like that. But you know it's it's hard to believe that advanced life you know that has the capability to invent science and do science things like that is restricted only to this planet. We have a lot of history that suggests every time you think you're very special uh you're usually wrong. Hi there. Thanks for watching. If you'd like to see us make more and better Deep Sky videos, why not support us on Patreon? Like the peoples whose name you
see on the screen at the moment, thank you to them. And as Patreon supporters, they and you could go along and hear the answer when I asked Professor Marfield who he dedicated his thesis to. Did you put anything cute in your acknowledgements to your thesis? Were you Were you a smart Alec? You strike me as someone who would have done something like that. The answer may or may not surprise you. I'm guessing it won't surprise you, but Mike didn't want you to hear it. Don't put that in the video.
