So this is picture of the core of Virgo cluster. This is so called Markarian's chain of galaxies. It's only like 15 to 20 megaparsecs away from us, and so clusters are certainly the, there's a little bit of an editing problem there. Clusters are very prominent parts of the large scale structure, right, and they're being the first one to be seen. And only small fraction of all galaxies are in clusters, most of them are in groups, but interesting things happen in clusters or galaxies. So how did it look like? This is Virgo Cluster, the closest one to us, center local supercluster. The picture on the left is X-ray image. You can see their individual bright galaxies superimposed on them overall plus their X-ray emission. On the right is map of where optical galaxies are. So you can follow the same structure. It is of the order of couple thousand galaxies, almost all of them are dwarfs. Only a few tens are really big galaxies. So Virgo is not a very interesting cluster, it's really poor cluster as clusters go. The Coma Cluster is more like a real thing and that's a hundred megaparsecs away and here you can see X-ray and optical pictures in their super position. And this turns out to be fairly typical of what clusters might look like. They're very prominent X-ray sources and that's important in finding them. And another important nearby one is called Perseus Cluster. This is somewhere in between Virgo and Coma, in terms of overall mass and density. The brightest galaxy there, and you see, 1265, is actually housing one of those obscure quasars. And if it wasn't for obscuration, it may be even visible by naked eye, but fortunately or unfortunately, it is obscure. And so the same thing carries on as you keep pushing out in red shift, the galaxies become harder and harder to see. But X-ray gases, easily detectable, there is very little else that can be confused with. I mean there are quasars but they're all point sources as opposed to clusters which are well resolved blobs of gas. And even the most distant clusters that we now know, which we know are like two thirds of look back time to the Big Bang, are following similar kind of behavior. I'm having a problem with the PowerPoint on this laptop. So there are several ways in which clusters can be found. In the optical, we can look just for overdensity of galaxies, and sometimes, we can use colors to look for elliptical galaxies. But then you're vulnerable to superposition of structure, different directions. In X-rays, it's much easier because it's very distinctive, there is very little confusion and as by far the best way now to find clusters. A new method so called Sunyaev-Zeldovich effect, I'll explain it in a moment. And also you can use gravitational distortions, or gravitational lensing to look for those, that's very hard to do. There are several important trends that you see, and one is that Clusters range from very symmetric, well relaxed looking, elliptical galaxy rich, to very loose, fluffy, irregular ones that are mostly spiral-galaxy rich. And the former more resources, it's all understood in terms of. Clusters just coming together, and galaxies plowing through the X-ray gas. Spiral galaxy disc would lose their gas, and become, S0 eventually merge as ellipticals. And so we see clusters in different dynamical stages of formation. The most relaxed ones, the ones full of elliptical galaxies are the oldest and those which are irregular are still full of spiral galaxies are the youngest. You may remember that threefold time scale is few billion years for cluster, and so you expect clusters to be still forming. So the X-ray gas is interesting and it's millions or tens of millions of degrees. Some of it came out of galaxies super nova they're pushed out. Some of it is just pristine gas that came in from the universe that was never in the galaxies. We know that some which was in galaxies because it's not full of hydrogen and helium. It's metallicity is about one-third Solar, so that is likely to come out of elliptical galaxies. And X-ray luminosity correlates with all other important properties, masses, and what have you. It also has substructure and some of it is due to clumps of galaxy, groups falling in. Some of it is due to active nuclei, those jets disturbing the gas. So they're not perfect, uniform X-ray sources, but they contain interesting signatures of past history. I mentioned Synyaev-Zeldovich effect and this is what this is. Here's a cluster, it's a blob of X-ray gas called gas electrons and behind it is cosmic microwave background. So those low energy photons for microwave background come through this hot gas and do inverse Compton scattering on those fast electrons. Which boosts their energies at the expense of the energy of electrons, and so now cosmic micro background looks a little hotter, right against the cluster. Not due to temperature cluster, due to this energy exchange process. And at the bottom, you see the radio map of one of those clusters that shows a bump in microwaves that corresponds to the same distributions of X-rays. Now, since you are not looking at source itself, source as the micro background, this is wretched independent. The source is always in the same wretched. So this effect, unlike everything else, does not depend on distance. Right, so you can find clusters using this technique very, very far away, and now this is a very popular way of finding clusters for cosmological purposes with blank satellites scenarios. And we talked already about dark matter in clusters. And just to refresh your memory, the idea here is that, well, this X-ray particles, this test particles and gravitational potential, if it's in virial equilibrium, temperature is related to kinetic energy. If you go through the numbers, you'll find out that clusters have hundreds of times more mass that can be accounted by the visible material. And this has been actually demonstrated very clearly with gravitational lensing signal. They can map amount of mass due to the lensing of the background galaxies and what's shown here, simple Bullet Cluster, where blue blobs are the dark matter distribution inferred from lensing, pure gravity, red stuff is called X-ray gas. Super pose in optical picture. So what happens here is you have two clusters, they collide. Dark matter clouds can not go through each other because they're collisionals. But gas collides, gas stays. So you have now dark matter has cast out and the gas remains in the middle, eventually those two blobs of dark matter will come back and they will settle into virial equilibrium, but you're encountering this merger of two clusters right after a first passage. Several other cases of this have been found now and that's seen as one of the important pieces of heaviness that you need. The reason why we think there's dark matter there is correct because it's confirmed there is gravitational lensing as well as this dynamical process. Mentioned that spiral galaxies, as they fall into these extra atmospheres or clusters, will get stripped away of their hydrogen. And therefore, star formation then will be depressed, so evolution of galaxies will be strongly dependent on where they live, spirals in dense cluster environments will get snuffed out very quickly. And those in field, like the Milky Way say, would just keep making stars at a different pace. Now a very nice illustration of this is, this is a combination of regular maps, neutral hydrogen of spiral gaseous in regular cluster. The center is M87, galaxy in Virgo Clusters. Central Virgo Cluster, you can see that spiral galaxies are smaller closer to the middle, and bigger on the outskirts. And that's exactly what you expect. Those on the outskirts haven't really been stripped yet. Those that fell through only have enemas parts left. And the same kind of thing than it is so called, intracluster light. As galaxies pass by, distort each other. By this stellar thing, it was the biggest splatter of stars all over it. And there will be certain defused lights of stars, that now belong to the cluster, not to every galaxy. That's sort of accumulated as a result of all these galaxy interactions, and in order to see them, you have to look at a very, very low surface brightness level. So people who have done that, did this for a Virgo Cluster. And the missing cause of the bright objects are, but you can see that there are galaxies but in very extended envelopes. There also some of this kind of tidal tail's and features, all of which are due to the past dynamical interactions. So stars now belong to the cluster potential, more than they belong to the host galaxy's themselves.
