Clusters Of Galaxies - Professor Carolin Crawford

Gresham College presents clusters of galaxies i professor Carol and Crawford Gresham professor of astronomy so good afternoon everyone delighted to see so many of you again so you back few next days of astronomy and today we're going to be looking on things on quite a grand scale I'm going to be discussing clusters of galaxies and you have to realize that a lot of the galaxies in the universe don't live into a wonderful isolation they tend to be tethered together by their mutual gravity into collections such as group or giant clusters of galaxies and that's going to be the topic of my talk today and here is a beautiful example of one of the rich clusters of galaxies in the nearby universe and I'm going to talk about the kind of galaxies that are inside clusters also about the properties of the clusters and as themselves and also just the way that then distributed around the sky and what they can tell us about the kind of contents of the universe in particular about dark matter but just to step back and talk about this whole idea of galaxies grouping together under gravity so for example here is the whole sky it's a survey of the sky so you've got the sky and it's been unwrapped and laying flat and every spot on here is the position of a fairly nearby galaxy this is taken in infrared light in each of these spots marks a source that's kind of extended so you don't get stars you don't get anything point like you just get extended galaxies and the near infrared and altogether there are 1.6 million galaxies in the nearby universe marked on this plot the blue band across all the sort of sash across the middle marks the position of our galaxy kind of in the way when you're doing extra galactic astronomy you can't see behind it very well because all the dust and gas clouds in it and so that's obscuring some of the counts of galaxies and is that blue lines just to give you an idea of where our galaxy is and as you look at this picture one thing is quite apparent which is that galaxies are not randomly distributed across the sky you can see that they're all knotted together into little groups and concentrations throughout the sky and these are the clusters so it's showing you that they're already even at this very simple level beginning to cluster together now three of these galaxies you can see with the unaided eye so for example for those of us in the northern hemisphere we might be used to seeing the Andromeda galaxy in in our autumn skies now this is a sister galaxy to our own Milky Way's a similar size is similar mass in the sized black hole in the middle it's very it's all expense and purposes we could be looking back to our own galaxy this is two-and-a-half million light years away from us and what you may notice about it and you can't see these so much with the unaided eye but it's got two little galaxies very clearly in orbit around it not sort of fluffy spirals but little sort of ball shaped galaxies the same is true about our galaxies we have dwarf galaxies in orbit around us and if you're ever lucky enough to go and see the southern hemisphere skies you may have seen or may yet see the Magellanic Clouds and they really look for all the world like little clouds in the sky we've got our own galaxy the centres down there a sort of edge on a band of our galaxy crossing the sky a gratuitous pretty calm it down here at button I know it's supposed to be clusters of galaxies I can never resist a comet if there is one show and here you've got two Magellanic Clouds this one the Large Magellanic Cloud is about 170,000 light-years away this one's slightly further at about two hundred thousand light-years away and very orbit our galaxy and those are the most obvious ones there are others that are nearer or more distorted so they're more kind of spread out on the sky and all together between our Milky Way and it's satellite Dwarfs and the Andromeda and their satellite doors and then also this giant spiral well it is slightly smaller than a drama in the Milky Way this is the Triangulum galaxy it's about three million light-years away so we're all kind of round each other and together we've formed something called the local group you have a group of about thirty galaxies you've got three big spirals and then all their satellites and we're all joined together we feel each other's mutual gravity and if we could fly it aside and look back at us would probably look something like this this obviously is another compact group of galaxies they're all over the sky groupings of you know tens of galaxies all contained within a volume of perhaps a few million light-years across and some of these are very beautiful systems that you see and a complete mix of different types of galaxies all together now this is the the thin end of the wedge this isn't really the kind of system I'm talking about today because this is only the very smallest scale clustering of galaxies grows and extends to much larger scales so for example we have something like the Coma Cluster of galaxies here now obviously some of these are stars you can see the Stars but I hope you can see two giant elliptical galaxies here and then it may not be so apparent from the back of the the lecture theater but there's a whole host of other little fuzzy orange blobs that in each of those is a galaxy all together you've got probably about 3000 galaxies contained in a volume space about 20 million light-years across the main thing to notice about to begin with is that these galaxies right at the core of the cluster here are truly enormous they would dwarf our Milky Way it's difficult to pick out a scale but for example you might be able to see a small kind of spiral-shaped galaxy about there that's probably about the size of our Milky Way in comparison to these giant objects in the centre and this is an example of a very rich cluster our nearest rich cluster about 250 million light-years and weighs the Perseus cluster it's one that I particularly studying I'm interested in and you can see this has different kind of shape where there's a long chain of orange looking galaxies and the central very sort of massive dominant galaxy is over here and you get a range of shapes you know the distribution the galaxies you get a range of types of galaxies and the thing that characterizes clusters is basically how badly dense they are when I'm talking about a group of galaxies I'm saying you've got tens of galaxies within a few million light years clusters only perhaps occupy maybe tens and you know a slightly bigger volume but you get many many more galaxies within that volume and we tend to characterize a cluster we call it whether it's rich or poor so if it's very sparsely populated it's a poor group or a poor cluster and going up to these kind of systems that we call a very rich cluster and you get the full span of cost clustering from tens to hundreds - as you can see solutely thousands of galaxies contained within this space and within this volume now clusters were detected fairly early on so as soon as Sirius astronomy started to be done by telescopes for example shall messier the French astronomer who was cataloguing that were fuzzy blobs in the sky things that he kept stumbling across he was looking for comet and these would confuse the issues so he started to draw a blissed of where these little fuzzy blobs were in the sky so that he didn't just keep rediscovering them and this has now become the very famous Messier catalog of nearby galaxies and nebulae but at that point he just saw their little fuzzy blobs in the sky didn't really know what they were but even he was surprised to find that there 11 of these nebulae all within the constellation of Virgo and William Herschel just shortly afterwards the same result started to notice this concentration of these fuzzy blobs in the sky in constellation Virgo now this is in fact our nearest cluster in fact it's a super cluster it's known as the the Virgo group of galaxies and again you've got one giant system down here and then others began to be studied so like the Coma Cluster and the Perseus cluster I just showed you they were both studied by the German astronomer max wolf and all of this was happening before we even really knew that these fuzzy blobs were galaxies outside our own that didn't changed the 1920s when Hubble showed that a lot of these nebulae especially the spiral shape one were other Island universes other galaxies and at that point began to recognize that some of these were clustered and it's only in the 1920s that Harlow Shapley an American astronomer discovered our nearest super cluster one of those fantastic super clusters I'm going to be showing you later so you can see the scale of clustering doesn't just end with groups two clusters you also get super clusters or clusters of clusters of galaxies and one was discovered back in the 1920s serious study or cataloguing of clusters of galaxies though had to wait quite a while had to wait probably till about the 1950s and pioneers in this respect with George Abel and fritz Vicki and what you have to realize is that when you're discovering a cluster of galaxies it's one thing to think yes kind of a lot of galaxies over there it's another thing to quantify that grouping and you have to remember the 1950s this work is done either through kind of it through your detecting stuff with a through telescope or through photographic plates or spectra and so the method of determining whether a galaxy is there is all done by pouring over photographic plates and you have to realize this is quite back-breaking work so here for example is the Coma Cluster on one of these photographic plates and it's fairly obvious so the first thing for every little speck on the photographic plate you're looking at through the microscope or the it's a magnifying glass you have to realize is it a start is it a galaxy does it look kind of slightly spatially extended and then maybe you have to guess its distance from its sides or its brightness and then you have to look for over densities so groups of galaxies that appear to be all crunched together on the plate and there's a kind of a background level of galaxies all the words you're looking for over densities over what you to expect normally and all of this is done by sort of counting out by eye across the plate very subjective because not only do you make decision about whether it's a star or a galaxy you need to make a decision about do you think this galaxy belongs to this grouping you get problems that you know there may be interlopers along the line of sight they could get confused or included in the cluster it start to get very complicated how big is the cluster you have to guess how far away it is from the brightness and the size of that giant galaxy in the center and then guess how far out you need to count to to incorporate galaxies within the cluster so you have to repeat the counts on different days and so this is quite a mammoth undertaking it may look obvious for something like the Coma Cluster that's nearby by the time you get to the Perseus cluster this is actually quite low down and you're seeing it through the kind of fringes of our galaxy so you've got lots of in the way as soon as you start getting further away so more distant clusters of galaxies the hot problem comes a lot harder first of all most of the galaxies in the cluster are faint and the further away they are the fainter they are you don't necessarily pick them up on your photograph the cluster is smaller on the sky so you only getting the brightest galaxies and it's smaller in the sky and you've got more interlopers along the line of sight so these clustered catalogs will only really survey the comparatively nearby universe compared to what we use now but nonetheless able for example able catalog is the one that we use he discovered over 2700 clusters just again by eye from examining photographic plates and indeed many of the clusters we the really rich big clusters that we use and we all study today or tend to be known by their Abel number so like Oh Percy's here is Abel 46 the Coma Cluster is Abel 16:56 they've all got you know catalogue numbers from this this this Abel catalogue of course today it's much easier to find clusters you can take the light with CCD cameras and get computers to evaluate relative brightnesses of galaxies to actually detect them decide whether they're spatially extended you can use spectra you can use colors to decide whether galaxies are real physical associations and you'll see later we can also use information from other wave bands you know other astronomies come on since the 1950s we can use other wave bands and other characteristics of clusters and so the x-ray wave band to detect get our clusters at progressively more distant progressive distances away from us but there's key thing about these catalogs is they started to have enough clusters that we could begin to compare one cluster with another in terms of its brightness in terms of the properties of the central course galaxies in terms of the number of galaxies within it and what was happening to the galaxies within it and so that's your first you know a key point where we start to understand some of the processes going on within clusters of galaxies so here is a beautiful rich cluster 16 Abell 1689 and if you look at the galaxies in that cluster one thing that really should leap out at you is that there are very few blue fluffy spirals most of the galaxies in fact nearly all the galaxies in the cluster are giant ellipticals they're yellow in color because they're made of old stars they don't have a lots of current massive star formation and in fact anywhere where you might see something that looks slightly bluer perhaps there's something here a sort of sputters foreground spiral here's a beautiful example I'm willing to bet that just lies along the side it's not actually part of the cluster and so this is very different from the distribution of galaxies out of clusters what we call the field there you've got sort of you know about 30% of them are spirals in the center of a cluster nearly everything in his elliptical galaxy and the more massive and the richer the cluster the more the tendency is for all the galaxies especially through the core to be in elliptical in shape something happens to galaxies in a cluster that perhaps means that spiral galaxies if they were there to begin with don't actually survive and indeed we think the process is going on within a cluster that can transform a nice fluffy you know a very active star formation spiral galaxy to something that's what we call anemic perhaps there's an echo of a disk structure but it's much more like this board shaped galaxies it's no longer blue there's no longer lots of active star formation going off it's lost its spiral arms and when you study individual galaxies in a cluster anything that's slightly disc shaped and look like it could be related to a spiral galaxy you find it no longer has an obviously it doesn't have the blue colors doesn't have much active star formation it's also lost a lot of the gas a spiral galaxy like this has a large disk of atomic so neutral atomic hydrogen or things like cold molecular gas all associated beyond the disk and with the spiral arms if you look at galaxies or sparrows the disk shaped galaxies I was in a cluster they lost all that they're depleted in this gas and as in fact gas that starts from and form from the fact that somehow this gas has been removed from the galaxy means that active star formation is shut down and you're left with more or less the core of the galaxy which is this kind of bulge component and there are couple of processes that produce what we call this stripping both at your stripping the spiral galaxy from all that vulnerable outer edge stuff and this happens if you have a spiral galaxy that falls into the cluster so you have a giant gravitational mass which is the cluster little spiral galaxy on the outskirts begins to fall come through an orbit right through the core of the cluster and what happens if it tends to get ripped to shreds so it shouldn't look so happy when I say that it's very sad very sad for the spiral galaxy now a couple of couple of processes that combine together to rip it apart the first thing it's got lots of tidal gravitational shears and forces pulling on it it's feeling the gravity of the whole cluster not just individual other galaxies but the combined effect of this and this sort of tears and tugs at the outer layers and here's just for example a simulation of starting off with a little spiral galaxy and what we reckon happens to it as it falls through and after about sort of you know maybe about a few Giga yeah maybe about a gear you find that all the outer extremities have got pulled off to leave behind something that's like these very anaemic disc shaped galaxies that we find in these clusters so that's one process we call it an tidal harrassment because basically you're just kind of pulling out this little spiral galaxy shredding off its outer layers this is one process but perhaps the dominant one is not just due to the gravity of the galaxies but it's because you don't just have the galaxies in a cluster now I alluded this to this last time when I talked about all the time for last sounds in the universe when you look to cluster galaxies again back to this one you're only seeing the tip of the iceberg of what's there in between all the galaxies and the clusters there's a very hot gasps and it's the temperatures of millions of degrees at those temperatures it doesn't give off optical light it gives off x-ray light so if I look at this same cluster with an x-ray telescope I no longer see individual galaxies they're all too cold to produce x-ray light instead I see something called the inter cluster medium it's just coded purple here in this picture and it's just like a giant puddle of a hot plasma that pervades the cluster it fills all the space between the galaxies there's probably about seven to ten times the mass in gas as there is galaxies and just two Super Bowls the optical and the x-rays together you can see how it just fills all the space so this is a huge hot atmosphere you can imagine what happens if you throw a little spiral galaxy into it it's sort of getting it we call this Ram pressure stripping it's falling through a medium and you know it can falter millions of kilometers an hour through into the cluster and this this atmosphere pushes back on the outer side of the galaxy and sort of pushes out and again attacked those vulnerable outer sides of the spiral galaxy now we do in some cases see this process occurring obviously the richer the more massive the cluster the more x-ray gas is in there and so and perhaps the hotter it is and the harsh of the environment that the spiral galaxy is falling into so if we go back to the okay this is April 26 to 667 again giant galaxies here what you may notice a little fluffy spiral out here and that's probably along our line of sight remember this is a picture of what's really a 3d situation so it's probably somewhere between you and me and it's just kind of falling in to the cluster like this here we're just getting a snapshot of this thing being torn to shreds if I blow it up here you can see that it's beginning to lose those knots of star formation you get the sense of stuff streaming off the galaxies almost like a comet and getting left behind and these little knots of star formation will just kind of live out there like and Iowa and all this matters being pulled off this galaxies it starts to fall into a cluster we go back to the Virgo cluster if I just look at this little galaxy here this is a disk spiral again falling in at maybe about ten million kilometers an hour into the cluster and into that inch cluster medium if i zoom in on that I hope you get the sense of stuff kind of boiling and getting left behind the galaxies moving through the cluster in this direction and you have these dusty wisps a matter being called up and stars being pulled up there's another spiral example in the Virgo cluster and again you get the sense of its moving this way and stuff is being stripped off and left behind and so this process over perhaps you know several hundreds of millions of years transmits a nice fluffy spiral into something that's much more board shape much more elliptical much more anemic and this is I mean this process has gone on is going on and it's part of the reason why we think that loss of the galaxies in the center of the cluster are you know not spirals a lot of them are genuine ellipticals ellipticals tend to be more massive they tend to sink more easily down to the center of the cluster in the center of the gravitational potential and in particular I've just remind you that right at the core of the cluster and in terms of where that x-ray gas was right in the center of the x-ray gas there is a giant elliptical this is m87 this is the the giant galaxies the dominant galaxies the Virgo cluster with an enormous mass of stars and sometimes these galaxies can have a very diffuse halo spreading much further out from the the body of the galaxies and these are in a fabulous position they sit right at the center of the cluster at the center the gravitational potential well they can just secrete any matter any of these stars any of the stuff that's been stripped off all the galaxies eventually might settle towards the core and it receives a lot of this matter is also have lots of other processes going on within the cluster and so at the center of these clusters you get the massive galaxies anywhere in the universe you do not find these in the field you only find them sitting squat in the center of the gravitational potential of the clusters and these are the biggest galaxies or the most massive and they have the most massive supermassive black holes at their core so these are some of the most exciting galaxies in the universe for my very biased viewpoints I study them and just to go back of course you can see that the x-rays give you a fabulous way of detecting more distant clusters of galaxies you're no longer having to count individual galaxies and decide whether they're members of a cluster or whether they're really associated with each other instead you just go and look for a big puddle of x-ray gas and so here for example from one of my colleagues are some x-ray images of clusters are relatively nearby one and then some that have been discovered through the x-ray emission at progressively further distances away from us you can immediately tell from the amount of x-ray light how massive the cluster is how big it is you can tell how rich it is so it's a very empowering way to find very distant structures and in particular this is how the most distant galaxy clusters galaxies are found this is one where confusingly x-ray is now color coded blue and the white are the stars from the optical you would struggle to see even without that blue murk in the way that there's a cluster of galaxies there but this x-ray light reveals that there's the intra cluster gas showing that there's a massive amount of gravity there pulling the galaxies and keeping it hot and that's in fact a ten point two billion light-years away this is about as early as we think you can actually build clusters of galaxies in the early universe and you know yesterday would have told you this is the most distant cluster of galaxies known about I've just got news this morning that one's been discovered ten point five billion light years away so so it's very competitive so that you get clusters of galaxies dimensionally throughout the universe so there's something very interesting about clusters the galaxies you have to realize are not stationary they are feeling each other's gravity they're moving around each other and I just want to be quite clear about we all know galaxies are moving apart and the University of expanding yes that's happening but only on the very biggest scales you have to think of groups or clusters of galaxies as isolated exceptions to that general expansion of the universe so within a cluster the gravity dominates on the scale of a cluster or a group of galaxies gravity the gravity or the galaxies feel to each other dominate and dominates their motions so they tend to respond to that and so you have a pocket of what we call local motions a galaxy's going around each other the whole cluster will be receding away from us but within that cluster on top of that recessional velocity you've got local motions you have liquid pockets of motion now if you study how the galaxies and a cluster of moving round each other you know once you're taken out that recessional velocity you find something very very curious now in the same way that when the earth moves around the Sun how fast it moves depends on you know how much mass there is in the Sun how far away we are from it for any one galaxies in a cluster how fast it's moving through the cluster what orbits it's on through the cluster depends on the combined mass of all the other galaxies in the cluster and if you sum all the motions you can work out what mass they are responding to how much gravity is there if you do this even so the Coma Cluster here you have a problem measure how fast all the galaxies are moving and you find they're moving too fast okay they shouldn't stay as a dynamically gravitationally bound system they should have all flown off into space they're going so fast that gravitation bother them they should have dispersed Gigot Giga years ago the fact that they're all still within one parent stable system tells us there's actually more gravity there than we realize there must be more gravitational attraction to keep them bound to the cluster that means there's more there than you realize and the problem is that if you work out the gravity they're responding to and you can't have all the stars and all the galaxies in the cluster you find there's nowhere near enough math and the discrepancy is huge it can clusters galaxies it can be up to like a factor of a hundred or more that means there's a large amount of mass within a cluster that is very very dim and could eight is well over sort of a hundred three hundred five hundred times dimmer than the Sun is and it's more mass in a cluster than we can actually see and this was discovered again back in the 1930s by the wonderful Fritz Vicki looking at the Coma Cluster he was the first one to raise this idea that there was missing mass I mean obviously now we know it's not the mass that's missing is just the light from the mass it's missing but the idea of what we now call dark matter and my rotation taught my last lecture I talked about how that gallic the Stars and the galaxies moved too fast to stay bound to the galaxy for therefore we think that's dark matter on the scale of a galaxy this is even more dark matter required on the scale of a cluster and the discovery of this predates anything done with the rotation curves the spiral galaxies so what you see is only the tip of the iceberg of what's actually there now are you might say you've just shown us there's all this x-ray gas you've told us that there's lots of x-ray gas there that's true an x-ray gas is much easier to map and extends out further than the galaxies so here's a garish false color image of the Coma Cluster so this is this cluster I've just shown you here in x-ray light with the German rosette satellite and it goes from purple is kind of the background level and it goes right up to red which is the peak a unit stronger secretary mission where the gas is densest and most concentrated just to give you a scale almost like within that central contour you could fit those giant galaxies okay so this is all this gas most of it is primordial leftover from when cluster form some of it's been reprocessed through the galaxies and if you count up and include all the mass in the gas you find yes there's probably about 10 times 5 to 10 times more matter in the gas than there is natural visible galaxies that's still not enough so it's there even if you can dub the light in all the wave bands you find there's not enough emitting matter to account for all the gravitational mass of the cluster a key thing to remember is the x-ray gas actually is the dominant emitter it this traces most of the emitting or the light-emitting matter within a cluster but it's still nowhere near enough so when's Vicki you know announced this idea of dark matter missing mass you can imagine it wasn't very well accepted and in fact it took several decades throughout the 20th century for this whole idea of dark matter to be reinforced by spiral rotation curves and other measurements of other galaxies it's a big step if you're loose if you're used to looking at optical images and of course our sense of astronomy grows up with optical Astronomy because that's a chin to our own eyes to realize that there's a whole lot more stuff out there that doesn't actually necessarily even bother with light and so this idea was challenged and you know if you have a really big idea that there's a huge amount of the mass you're missing you need to reinforce that interpretation with other ways you need to go out other ways to prove it's there and this has been done so obviously with other clusters of galaxies look at the motions of the galaxies within the cluster that all supports it similarly you can use the x-ray gas to show that there's a huge mass all of this gas is held in place by gravity this big puddle of gas you know it's responding to the gravity of the whole system in the same way the galaxies are moving around and that makes it hot so if you have your puddle of x-ray gas you can directly measure from x-ray measurements the temperature the density of the gas that's producing this light you got density and temperature you can work out the pressure so this gas is under pressure and that pressure is caused by all the outer layers of gas squeezing on it pushing in they're being pulled in by gravity and how much the outer layers being poured in depends on how much mass is there how much gravity is there and so from x-ray observations you can do an analysis where you look at the the way the pressure changes across a cluster how much gravity that implies is in the cluster and you find excellent agreement with the the estimates from the motions of the galaxies you need lots more mass than you actually see and even that isn't quite enough proof for some people something that happened on the same time scale is understanding the cluster emission and x-rays is this idea of gravitational lensing and this is the third method you could use to weigh a cluster of galaxies and what I want to stress is all of these different methods are employing different observational techniques different assumptions about physics involved in a cluster gravitational lensing appeals to einstein's idea of the gravity not as you know Newton's invisible force acting over distance but if you plunk a mass in space it bends and distorts the space around it it curves it so for example not to scale obviously we're here this is a very distant source in sky and phil is a big galaxy in between us and the source now if space is all curved around this galaxy any light that goes past it gets bent the light thinks it's traveling in a straight line unfortunately for the light and the space is curved so it ends up following a curved path and this creates mirages and illusions of where the light comes from so in the absence of this galaxy the light from this background source would sort of carry on and miss the earth because you've got the galaxies and the curved space around it it gets distorted and deflected towards us now as far as we're concerned we see light from that direction and so we see two mirages and of course this is a two-dimensional representation of a three-dimensional situation you also have light that's bent this way as well as that way and if you get a perfect alignment of source galaxy observer you can get perfect rings around the mass so here for example are some beautiful cases where you've got a giant galaxy and the blue ring around it is the light from the background source distorted into this Mirage this ring around the galaxy so the key thing here is the closer you are to the mass the more space is distorted the bigger the mass the more space is distorted so you can begin to see if you just know about the the optics of this this phenomenon you can use these distortions these mirages to work out how much mass is there now these are just examples when you have individual galaxies as soon as you have a whole cluster of galaxies it it comes a lot it's quite technically complicated but the dividends of actually mapping out the distribution of the mass within the cluster are great and again it was Fritz Viki who was the first one to come up with this idea Einstein came up with the idea of been spaced a random-ass but he only reapplied it to kind of point like matters like stars you may not think of a star as a point like maths but compared to these things it is it was Viki that came up with the idea that perhaps you could the mass in clusters would be so great that you would get much different lensing distortions however it was until the 1980s when you start getting ccd detectives and the next generation of large telescopes that people began to see the kind of distortions predicted to being clusters of galaxies now you may notice some of them in some of the pictures I've already shown you so oh sorry go to your bank explain physics just keep you in suspense a bit longer flashes and your edges what you have to realize is that mass in a cluster or even in a galaxy it's all kind of clumpy it's a little bit of a lumpy distribution especially got lots of different galaxies I use finger named gravitational lensing is misleading as we use the lenses that bring light to a perfect focus that suits us when you get a lumpy uneven lens it doesn't bring the light to a clear focus it doesn't necessarily bring light to a focus Neos it may be in front of us it might behind us what we get instead of something called core sticks where you get bands or envelopes where the light gets more concentrated and darker and here are just some examples of course sticks we see in everyday we know where light is reflect refracted through the surface of water say in a swimming pool or a lake or even in your coffee cup you've got a clean coffee cup and you've got a bright light coming down you go I'm sure you will recognize the shape that's a coal stick and again it's the light being reflected not to a perfect focus so the same is true with these clusters of galaxies now there's no cluster here this is a backdrop this is the wallpaper of the universe thousands of tiny galaxies here is a simulation showing what happens if you drop a cluster in front of them so here's your backdrop of all these little galaxies now I'm going to drop an invisible cluster into the left-hand corner and you begin to see these mirages that are created these distortions and you get the greatest distortion in the center where there's these are the most concentration of mass you get the most warping the space and you get the stronger distortion and then further out where the mass gets diluted you get much weaker distortion so if this in mind have a look at this cluster of galaxies now I don't know you can accurately see them but you might get a sense of small arcs all the way around there's a big one here and all together it's like looking through say the bottom of a tumbler you know a glass tumbler and you get this slight shearing all round the center of the cluster of galaxies so for example if i zoom in on some of those arcs you can see giant ones which are right close to the really big galaxies and then much further out you get finer sheering and even way out you get just a sense of a general general electricity of galaxies and so here is going back to this galaxy this cluster I showed you before you may have noticed this large banana round the largest edge of galaxies beautiful case of a giant arc and where the distortion is greatest in it you can see other arcs further out and again the cluster that I started my talk with here enormous arcs and you see these are blue usually because they're background galaxies they could be spiral galaxies they're nothing to do with a cluster they just happen to lie along the line of sight and because there are so many in the backdrop you get mirage's from each of them so if you can measure how far away the background sources you know how far away the lensing cluster is from you you can map out all these distortions and you can work out the mass of the cluster and not just that you can work out how the mass of the cluster is distributed and you find the results agree perfectly with both the motions and the x-ray maths there is far more mass in the cluster than you actually see in any wave band and remember you know this responds to the total masses that's there whether or not it happens to be emitting light or not so within a cluster again it varies a lot between different clusters but for any cluster you have the mass in galaxies is less than 10% all the stars and all the galaxies and the gas clouds in the galaxies the inter cluster medium can be somewhere between five and ten times as much mass but again that's always less than a quarter of the total mass that's there you've got up to three-quarters of the or even more actually at least three-quarters of a cluster is in the form of dark matter stuff that is there it's it's got gravity but it doesn't interact with light at all not only does it not emit light it doesn't absorb light and it doesn't seem to interact with light or anything except through the force of gravity and yet it is you know fundamental component not just of individual galaxies but also on the scale of clusters of galaxies and what dark matter is is a complete enigma you've really got two choices either it's something we understand or something it isn't okay a little bit trite but say we understand what we call baryonic matter when stunned electrons protons neutrons stuff like that planets galaxies stars it's you know that our occasionally ideas and folks that it could be made of ordinary matter in practice we can't work out how you can make enough quantities of already matter that don't you can get stuff that doesn't emit light like you know black holes or compact objects you can't necessarily get them distributed way out further than the galaxies and therefore there to be this huge imbalance of more of that than there is light emitting matter we can't make that to make sense and so most astronomers appeal to the idea of much more exotic forms of dark matter and again you've got two choices you've got one particles we know about and it tends to be particles we're talking about rather than large concentrations particles we know about and in particles which are many of which have just been theorized to actually account for the dark matter so for example in terms of what we know we know neutrinos exist so there are so many billions trillions of neutrinos through space imagine Li weren't completely massless imagine each one had the tiniest bit of math associated with it well maybe all of that can account from the dark matter now that's a very exciting idea doesn't quite match up to the observations because your problem is that neutrinos are relativistic they travel in huge speeds they tend to not what they kind of resists clumping together and if you put neutrino dominated dark matter into models that simulate galaxy growth and structure formation you find that you get too much structure on to bigger scale it doesn't match what we see it seems that it's going to have to be one of the other kind of exotic particles that we call them wimps or you know it's weakly interacting massive particles it doesn't tell you much it means like weakly interacting it doesn't actually interact with anything else very much and it's got a lot of mass attached and these are things that maybe like forty nose or accion's are neutrally noisy I've heard some of these names these tend to be particles that arise out of supersymmetry theories that like to unify gravity with the other forces and the idea behind this is that maybe these were really important in the early universe they helped shape the whole universe when gravity was still tied to the other forces that's why their existence comes out of ideas about supersymmetry and what-have-you about this unification they're really important then but as the forces disassociated with each other they are now really are very slow-moving unreactive to the rest of the universe in fact they just sit there in a litter space so if we can ever work out whether dark matter is it has profound implications being tied back to the hip early history of our universe and also the way that matter grows throughout the universe so as any astronomer says we need more data so you've got strands so you know obviously the particle physicists are working on detecting necessary particles other ways you can look at it well you can use all these studies combined or clusters of galaxies under extreme environments so for example when clusters collide doesn't happen very often clusters are enormous objects but every so often some are close enough together that they're neutral gravity pulls them together and they smash into each other and when it does it's one of the most energetic events in the whole universe here you have two clusters of galaxies the little one there little one there it's bit difficult to see the distribution of the galaxies but you can do a lensing analysis gravitational lensing analysis you find out where most of the dark matter is which is color-coded blue here and you find the galaxies in the dark matter live in two clumps now these look like they're fairly separate then in fact they've already smashed through each other because when we look in the x-rays so this is this hot gas atmosphere that comes from each cluster you can see it's actually collided and provided a very strange shape so when galaxies much when clusters of galaxies merge the galaxies are separated by huge huge they don't actually have head-on smashes they tend to pass by jet and just notice each other's gravity and perhaps be slightly slowed down the x-ray gasps it kind of it's pervasive it fills the cluster it can't they can't avoid each other the to inter custom media and that meaning to each other squeeze each other compress each other and slow each other down in particular you can see that the smaller cluster is kind of got this bullet shape this is known as the bullet cluster the there's been like a drag force similar to air resistance has slowed down the inter cluster medium and you find that the inter cluster medium has got dragged out and stuck in the middle the curious things the dark matters behave like the galaxies just sad past each other only notice the gravity of the other there's none of this kind of interaction that you see the x-ray into cluster media so here's a similar and simulation simulation indeed don't match in blue x-ray gas in pink small cluster going through big cluster the Dark Matter sails through unaffected the x-ray interest a medium severely distorted compressed and squeezed by the interaction to produce those structures that we see and so instead you know this is a way of modeling how dark matter behaves how it interacts on huge scales and there only handful of these galaxies studied but they all will orbit one or seem to show this kind of behavior so beginning to map out how dark matter reacts to other matter on enormous scales and how the distribution might vary the handful of these discovered nearly all behave like this apart from this one released last week you've got one cluster of galaxies here another one here or just code the galaxies distribution in orange if you look at the Dark Matter distribution from the lensing it's actually separated out and a factor seems to lie where the intraclass the gases and the x-rays and again you can see some compression structures within the inter cluster gas and so here this is a very strange one that bucks everything else was seen in mishmash is of you know cluster collisions it could be that this is just an oddball maybe it's a three-way collision of the galaxies or something or maybe this is something profound to tell us about dark matter the fact that is actually lagged behind the galaxies it seems to be having more about their gas so this is again you know this is stuff that's being done over these few years it's relatively recent discoveries and hopefully more detailed analysis of the distribution of dark matter and the relative components of the cluster may start to tell us more about it but as yet it's still a huge unknown so finally this brings me to the idea that if galaxies are close enough to collide with each other from time to time you get the sense things are still evolving things are still changing and clusters of galaxies are building up to form super clusters if I go back to the all-sky distribution of structure you can see these knots them concentrations of galaxies and this has got some depth for touching that blue to red you're going further out away from us you can see that these knots join up and link up to form two linear structures that we call walls or slabs and these are super clusters I told you about the Shapley supercluster that was discovered in the 1920s by Harlow Shapley that is our nearest big concentration in the sky now so here's some three-dimensional maps from which a pal beautiful demonstrations of the local geography of our universe you are down there you can probably barely see where it says Milky Way you're down in this corner this is the Virgo cluster our nearest rich cluster and you'll see the local group is kind of hanging along to the Virgo cluster and there's a link in each of these yellow things each of these names marks an able cluster there's a smattering of galaxies along a line and this is a true lining in three-dimensional space and over here you've got some live thirty able clusters six of those are amongst the brightest 46 known in the universe and they all sit within a volume of perhaps a few a few tens probably up to you know a couple of hundred million light years across if you look at the dimension of this superclass you've got 30 clusters in that size and down here you've got like the Virgo cluster in the same space the is truly a super cluster and you find they are endemic through the universe again just going back to perhaps this first bit and giving you much more of a three-dimensional distribution of galaxies you are now here at the center local group tagging onto the Virgo supercluster and you begin to get chains of clusters joining up and then knots where the filaments and the chains intersect to form super clusters and panning out a bit there's your Shapley supercluster in that chain of galaxies Perseus Pisces over here of the Purser's cluster you get a wonderful idea as we move further out with registered surveys the distribution of clusters and galaxies on the sky that this clusters are linked up and in long chains of super clusters and superclusters where there's changed in filaments intersect and they surround enormous voids of space when it's fairly any galaxies so you have sort of voids like bootys void Capricornus forward with barely any galaxies and on huge scales the universe has a cellular or kind of cobwebby structure about it so as you move out it's not like you get bigger and bigger clusters or anything you just get this repeating cellular structure within the universe and this is the large-scale distribution of galaxies the first thing it tells us is that super clusters are still forming galaxies are nice and stable it kind of sorted out where they are or the galaxies know where each other are super clusters Gallic clusters are still merging to form super cluster structures they're still evolving so even though they're the largest structures in the universe they're not true truly done you know gravitationally bound stable entities they're changing they're evolving and these structures this that a linearity starts to tell us something perhaps about how structure formation grows in the universe so for example one thing that I use find very intriguing about clusters is there's an alignment right from the larger scale right down to the central cluster galaxies I could show you any number of examples I'm just going to pick well here is some for example where you have this these contours or of the x-ray so that's the shape of the puddle of x-ray guess that's the shape of the cluster and I've kind of giving you an alignment by eye on that and these are contours of the brightest cluster galaxies same clusters up and down and you can see that they share the alignment I want to go into detail for example with the Coma Cluster here's one of those giant ellipticals and that's my rough axis about how that's it's kind of a bit more like that but how it's aligned right then with its neighboring galaxy lie along that axis if we span out a bit further and look at the distribution of all the galaxies in the sky perhaps it's tilted round a bit maybe the axes a bit more like that but it's still along the same sense moving then further out to the x-ray mission that I showed you earlier again has a sweet slightly long gauge this way this sub cluster here is merging in from that direction and then if we map out all the neighboring clusters of galaxies you'll find the local superstructure is aligned along that way and these are true alignments in three-dimensional space if you start putting depth into it and so and there's something shared about the alignment of the super cluster right down to the central cluster galaxies and it says this is some echo perhaps of how the clusters form and this gives us a constraint for cosmological theories that want to simulate the formation of clusters of galaxies the growth of structure right from the early universe to the present day can they reproduce clustering on the right scales can they reproduce these voids it is a huge challenge it assumes you know something about the Dark Matter distribution the visible matter distribution how clusters collide there's an enormous amount of physics that goes into it and there is this complete industry of producing cosmological simulations where you vary the cosmology do you get out this large-scale distribution that you see and here are just some steps in time so at the bottom here you have time since the beginning of the universe let's as 200 million years and this these are just snapshots from a fabulous simulation called the millenium simulation by Vox Pringle town and these are different scales large-scale middle scale very small scale and as we go up go forward in time you can see that things start to congregate along these filamentary structures and in fact we think the matter flows in and galaxies and clusters merge along preferential directions along these filamentary structures and that preserves the alignment both of the super cluster and the galaxies than the the cluster and then right down to the galaxy there's this coherent sense that matter cumulants again along the filaments and you grow large clusters of galaxies right down there deep in the core and this is one of the things one of the ways in which clusters of galaxies can provide very powerful cosmological constraints it's not the only way of course I've talked a lot more I have talked a lot about the contents of the clusters and the properties of the clusters I don't have time to go into other ways that we can use clusters as constraints for cosmological theory you can have a talk in that next year ok clusters are everywhere in the universe they provide very important constraints the whole idea of the whole flow of the universe and its expansion but that's a completely different area of cosmology but I will be revisiting that next year but nonetheless I hope you can see that even just like within the cluster itself the dynamics of the galaxies and also the the linking of clusters and the large scale structure we've still an awful lot to learn just both from the content and also about the much wider universe and I think these are really important structures to study so that's enough clusters of galaxies for those of you who might be interested I'll just put a quick flyer for my next talk so I'll be coming back in May and as I keep saying everybody wants bigger telescopes and more data well that's part of the reason but I talked about some of the challenges about building the very large telescopes we need to carry out these kinds of observations thank you very much you just mentioned dark matter before you're talking about dark energy on the edges are they one of the same thing no so dark net dark matter is mass and it has gravity and if you know so it's associated with you know a pulling force dark energy is associated with an absence of stuff and absence of mass and it's a pushing force and indeed that's what I'm alluding to when you map the distribution of clusters and galaxies and they're recessions from us that's yet where the evidence for dark energy comes in so if you like extending this idea of galaxies as only the thing were most used to talking about galaxies is just like n percent of a cluster dark matters somewhere between 70 and 90 percent of a cluster you then start averaging out the whole universe of voids and clusters and superclusters you will find all of that everything I've mentioned in today's talk so the the matter that's giving off light the matter that's dark that's less than a quarter of the whole universe in terms of you know mass energy density the whole universe and so actually the dominant component of the universe is dark energy and we know even less about that when we do about dark matter which is really exciting by the way it's a good thing I find it quite amazing when you have the collision of the super structures that one can pass through another yeah is it the dark energy that causes that to happen or I mean what is it that gives a difference between the two they don't coalesce one goes right through again it's gravity on the scales the things put you know gravity for pulling forces what's pulling those two clusters together and the thing is that in terms of the individual galaxies they don't clot you know there's so much space between the galaxies separation between the galaxies is much bigger than the size of a galaxy so they don't head-on smash they tend to go past each other maybe notice the gap the gravity of the other galaxies it depends how fast they're moving so really slowly then they'll slow down and notice the gravity and probably what happens is that you have to have several passes through before something actually merges so sometimes an x-ray guess we see things we call sloshing or like ripples where something's gone through and back and then it's kind of settles and we see the after-effects of the merger something like the Coma Cluster for example with the fact you've got this two giant ellipticals suggest that it's a merger a long time ago there was a merger of two fairly equal clusters of galaxies together to produce that each had their own brightest cluster galaxies coming together so it's gravity that's driving the interaction and it will eventually cause them to merge if they're going the small clusters going too fast then maybe it'll go on but if it's going relatively slowly it goes so far and then get pulled back together and so you'll oscillate and then they'll collide like that but the time you know actually finding them at that moment where they're kind of passing through each other and that first pass is quite rare it's quite a short there's only a few hundred million years for that you know and for that for that to see so that's always quite rare to actually catch them in the act of merging we tend to see clusters that may be sort of bimodal or something suggesting they're the remnants of a merger don't matter again I'm afraid when one does how much baryonic matter we're missing an taking in terms of objects been obscured by other objects and indeed the lensing effect we're inside the focus there must be other situations as well as they'd ever been calculated how much potential there is in that yes in fact some people very specifically target areas where there isn't a visible cluster galaxies in the way and they just look at that background field you know backdrop of galaxies and look for tiny shears that suggest there could be concentrations of dark matter that are not associated with galaxies or clusters along the way and it's very hard science to do and yet there's no evidence for large clumps of dark matter that are not you know associate in some way around the periphery of clusters of galaxies and also say on small scales between us and Andromeda the idea there could be small dark objects along the way it could cause what we call Micro lensing sort of one-off events they passed between us and a star and the alignments right again I'd say the evidence for that is very inconclusive and we're certainly not finding baryonic matter in the quantities we need to account for the dark matter that's needed on these scales so you know it could be there's some great observation yet to come but at the minute the best guess is it's something much more exotic okay thank you very much for all information please visit www.cannainsider.com/careers

Channel: Gresham College

Views: 122,054

Keywords: Galaxy, supercluster, superclusters, Galaxy Clusters, Clusters of Galaxies, Galaxies, Cosmology, Space, Astronomy, Dark energy, Planet, Sun, Universe, neutron star, dark matter, Collapsing Stars, Black Holes, Carolin Crawford, Institute of Astronomy, Gresham College, Gresham, Astronomy Lecture, Astronomy Talk, Free education, London Lecture, Free Lecture, Free public lecture, Public lecture

Id: LhVQ4nIgS_E

Length: 60min 8sec (3608 seconds)

Published: Wed Mar 21 2012