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what do we got messier 61. gorgeous isn't that a pretty one we've saved up some of the good ones it's a bad spiral galaxy it's in the virgo cluster so it's about however far the virgo cluster is away 50 million light years that kind of distance away it's a bit unusual because the virgo cluster because it's a relatively rich cluster of galaxies that's quite good at stripping the gas out of a lot of its galaxies but as you can just actually see from the image here this is a galaxy which has got a lot of gas because there's lots of star formation going on in it and you only really get star formation where you know if you've got a gas-rich galaxy there's all sorts of features that you can see like these for
example these dust lanes are associated with the orbits of the material in the bar here where the orbits get distorted in such a way that you end up bringing lots of gas together in these kind of weird spirally features which where you see this dust obscuration where a whole load of gas has all been shot together there are two clear indicators of star formation in this picture one are the blue bits and the blue bits are just where there are bright young stars that are formed because the blue stars are the massive stars and massive stars have very short lifetimes which means wherever you see blue light that's telling you stars have formed very recently which almost certainly means stars are still forming there and then
the other indication of star formation are a lot of these kind of pink bits and these are these things called h2 regions where there is a very bright star which is actually ionizing the gas around it so it's very much exciting the gas around it and causing the hydrogen gas around it to glow with this kind of characteristic reddish color a couple of features here one is if you look at the bar itself it actually looks slightly redder in color and there aren't any of these big pink regions in it so it's not a huge amount of star formation going on in the bar and that's a common feature of bar galaxies in the bars themselves they tend not to actually have that much star formation and the other feature is
at the very ends of the bar you can see this kind of collection of pinky red bits which are these indications of very strong star formation going on there it's less clear at the other end maybe out here you can see something but you tend to see star formation it's sometimes enhanced at the ends of the bars and so one of the questions has been in astronomy is why is that the case why is it that within the bar you tend not to see very much star formation outside the bar lots of star formation and sometimes there's this enhancement right at the end of the bar as well and there's kind of two possibilities one is that maybe whatever the process is that drives star formation just doesn't work so well within a bar
so you know maybe it's you know you need to have a particular kind of orbit or a particular set of things going on and for whatever reason you don't find that in the barred regions of galaxies the second possibility is maybe there just isn't the raw material right that may be there's just loads of gas outside the bar and not very much gas inside the bar can't make stars if you haven't got the gas to make the stars out of in the first place and so i have a paper from a couple of years ago called ceo multi-line imaging of nearby galaxies blah star formation in the bar spiral galaxy ngc 4303 which is actually just another name for messier 61. so they're looking at and they're observing molecular gas so
carbon monoxide in this case to study how much material there is to see whether or not the reason that stars are or aren't forming in these different regions is because of the amount of raw material where there's something else going on professor why wouldn't they be looking at hydrogen i thought hydrogen was the key guess for star formation so if you want to form stars you need cold gas right because hot gas doesn't kind of collapse down so you need cold gas which probably means you need molecular gas so the molecule you'd really like to look at is molecular hydrogen because that's what most of the universe is made of that's what most of the stars are made of molecular hydrogen has this
irritating feature that it doesn't actually have any features or at least one not none that we can actually observe very easily it's to do with it you have to talk to a chemist about this not me but it's to do with the fact that it's it's a symmetric molecule because you've got two hydrogen atoms and that means that there are modes of excitement that don't happen in hydrogen because it has this symmetry about it whereas carbon monoxide because it's got a carbon and an oxygen is nicely asymmetric which means you get a whole load more features which means it's much easier to observe the hope is that if you observe carbon monoxide that's telling you about molecules and if there's carbon monoxide molecules there's probably hydrogen molecules as
well it's a slightly dodgy thing to do but it but nonetheless co is often used as kind of a tracer for hydrogen for molecular hydrogen because we can actually observe it where we can't observe the molecular hydrogen so they observed they made maps of this thing in co in carbon monoxide and again it turns out life is quite complicated in that if you look at normal co which is like normal carbon and normal oxygen there's so much of it that actually it becomes what's known as optically thick it's like looking at a star right when you look at a star you only see the surface layers you don't see right through it down to the middle it's the same with this if you're observing normal carbon monoxide you kind of only see the outer layers and that's not
helpful if you're trying to figure out how much of it there is because you don't know whether it's that much or that much right because you're only seeing the front bit so actually because it's optically thick you can't see through it so it's not terribly easy to translate from the observations to the amount of actual carbon monoxide and then hence how much molecular material it is fortunately there's another isotope of carbon 13 instead of carbon 12 which is much rarer and because it's a different isotope that means that the carbon atom weighs a slightly different amount that means that the lines the things that get excited as it jiggles around are at different wavelengths so you can see the
carbon 13 differently from the carbon-12 so you can see carbon monoxide which is made of carbon 13 and oxygen as well as carbon 12 and oxygen because it's so much rarer there's much less emission from it and you don't run into this problem of it being optically thick you can actually see right through so in principle carbon 13 and oxygen gives you a nice measure of how much actual molecular material there is there's one final complication that's if we didn't have enough in it already which is what you actually observe of course is the intense intensity of emission not the total mass of stuff so if for example supposing the transition that you're seeing wasn't being excited at all you wouldn't see anything so
there could be a whole load of carbon monoxide there but you wouldn't see a thing um and so you need to know okay so how much is this stuff being excited if the thing were in thermodynamic equilibrium in other words if it had a kind of a well-defined temperature turns out you can do that calculation we don't think these things are in thermodynamic equilibrium so it's kind of hard to figure out what kind of temperature it is how excited it will be so what the guys did in this paper is they did this stuff called non-lte non-local thermodynamic equilibrium calculations where they use both those 12 co observations so the normal carbon monoxide and the 13co observations
combine them together and it turns out that if you make a few assumptions about what's going on in terms of how far you are from thermodynamic equilibrium that's enough to translate it into the mass basically what they've been able to do by making these molecular observations of carbon monoxide are figuring out how much molecular material there is in this galaxy and what they found when they look at the different regions of the galaxy is in the middle region here where star formation is suppressed by about 30 percent relative to a bit further out there's 30 less molecular material at the ends of the bar where star formation is a bit enhanced by 20 or so percent there's about 20 percent more molecular
hydrogen so it looks like the thing that's really driving whether or not you see star formation isn't anything fancy about the star formation process it's actually whether you've got the raw material there in the first place just haven't got the gas yeah you can't if you haven't got the gas you can't make the stars well you know what my next question is what's that why is there not much gas in the bar so the other piece of information you get out of these observations because you're looking at an emission line not only does that tell you how bright it is and so far this very convoluted argument how much there is of the material but because there's a doppler shift in it depending on whether that material is
coming towards you or going away from you can actually figure out the motions of this gas and so they were able to see whether the motions of gas were systematically different in the regions where there's less molecular gas or whether there's more molecular gas and the quantity they measure is some measure of how much random motion there is in this gas what you find is where there's little random motion there's not much molecular material then as you start going up in random motions you get more molecular material then when you go up a bit further in random motions you get less again you're doing the goldilocks thing again right you need it just right if there's too little random
motion then actually you don't squeeze things together you don't make the gas so you're not going to make stars if there's too much random motion it turns out then probably the collisions are too energetic so that things don't merge together they just kind of bounce off each other and so actually you don't end up making the kind of big molecular clouds you need to make stars there either you need things just right somewhere in the middle so what's going on in the bar so there's probably quite a lot of gas in the bar but because the orbits are very distorted in a bar you have very some kind of shear so you have big differences between one bit of gas and the next bit of gas and that means that
they're actually moving at high speeds relative to each other and that generally means that actually that they're moving too fast to smash together to create these molecular clouds so the gas isn't even getting to the molecule phase right almost certainly too turbulent and sometimes it could be just not turbulent enough but i think in the bar it's probably because it's too turbulent because there's just so much of this kind of shearing motion m69 i think is 6200 light years from the galactic center in comparison to our solar system which is like 25 000 light years why
