1 00:00:00,012 --> 00:00:03,952 We now turn our attention to clusters of galaxies. 2 00:00:03,952 --> 00:00:09,445 Well, they're certainly the most conspicuous parts of the large scale 3 00:00:09,445 --> 00:00:15,201 structure and were the first to be recognized as somehow distinct units. 4 00:00:15,202 --> 00:00:20,564 Typically, clusters have sizes of few megaparsecs and contain hundreds or 5 00:00:20,564 --> 00:00:25,940 thousands of galaxies with masses of 10 to the 14th to a few times 10 to the 15th 6 00:00:25,940 --> 00:00:29,850 solar masses. They are all gravitationally bound but 7 00:00:29,850 --> 00:00:34,829 they may not be fully virialized. As you remember t he free fall time scale 8 00:00:34,829 --> 00:00:39,991 for clusters is measured in gigayears and so many clusters are still forming. 9 00:00:39,991 --> 00:00:45,067 In addition to the obvious stars and galaxies, clusters are also filled with a 10 00:00:45,067 --> 00:00:50,449 hot x-ray gas which was partly expelled from galaxies and partly accreted from the 11 00:00:50,449 --> 00:00:54,895 outside and it's kept on the x-ray temperatures because of the deep, 12 00:00:54,895 --> 00:00:58,594 potential well. But the dominant component, of course, is 13 00:00:58,594 --> 00:01:02,129 the dark matter. Of all galaxies, maybe 10 or 20% are found 14 00:01:02,129 --> 00:01:05,188 in clusters. The rest are discovering large scale 15 00:01:05,188 --> 00:01:10,300 structures in general, and the majority of galaxies are really just small groups just 16 00:01:10,300 --> 00:01:14,844 like the local group with Milky Way and Andromeda dominating a whole bunch of 17 00:01:14,844 --> 00:01:19,489 dwarf galaxies around it. There is one important distinction though, 18 00:01:19,489 --> 00:01:24,871 the early type galaxies, ellipticals and S0s tend to concentrate in clusters more 19 00:01:24,871 --> 00:01:29,317 than in general field and there is relatively few spiral galaxies in 20 00:01:29,317 --> 00:01:32,314 clusters. This is reflecting processes of the 21 00:01:32,314 --> 00:01:36,216 galaxies evolution and we'll talk about them a little later. 22 00:01:36,216 --> 00:01:40,535 So here's the picture of the nearest cluster to us, the Virgo cluster. 23 00:01:40,535 --> 00:01:43,233 It is the center of the local super cluster. 24 00:01:43,233 --> 00:01:47,867 It's about 15 to 20 megaparsecs away. The picture on the left is the x-ray 25 00:01:47,867 --> 00:01:51,833 image, in false color. The picture on the right shows where the 26 00:01:51,833 --> 00:01:54,757 galaxies are. And you can see there is a broadly 27 00:01:54,757 --> 00:01:59,248 speaking a good respondents. About 2000 galaxies have been cataloged in 28 00:01:59,248 --> 00:02:04,212 Virgo cluster, but most of them are tiny dwarfs that you have difficulty actually 29 00:02:04,212 --> 00:02:07,347 pointing out. The next famous cluster is the coma 30 00:02:07,347 --> 00:02:10,263 cluster, it is five to six times further away. 31 00:02:10,264 --> 00:02:15,324 And it is richer than Virgo by a considerable factor, and it's comparable 32 00:02:15,324 --> 00:02:19,500 to the kinds of clusters that we see at larger distances from us. 33 00:02:19,500 --> 00:02:23,378 Here again, I show you the optical image in the upper right. 34 00:02:23,378 --> 00:02:28,620 See, there are 2 dominant galaxies there, and the x-ray image in the lower part and 35 00:02:28,620 --> 00:02:33,856 you can see it's much smoother than the one for Virgo because Virgo is dynamically 36 00:02:33,856 --> 00:02:38,850 young cluster, still falling together. Coma is, they're utterly much more 37 00:02:38,850 --> 00:02:41,778 evolved. The picture on the, on the right just 38 00:02:41,778 --> 00:02:45,701 simply overlays the x-ray countermap on the optical image. 39 00:02:45,702 --> 00:02:50,203 And on the opposite side of the sky, roughly, is the Perseus Cluster. 40 00:02:50,203 --> 00:02:54,733 And that is only about 4 or 5 times further away than Virgo not as rich as 41 00:02:54,733 --> 00:02:57,877 Coma. But it has a nice little chain of galaxies 42 00:02:57,877 --> 00:03:03,084 in it, one of whom harbors a very powerful active galactic, nuclea, nucleus. 43 00:03:03,084 --> 00:03:08,718 As we look deeper, we find more clusters and by and large, some of them look pretty 44 00:03:08,718 --> 00:03:12,975 much like clusters near us. Again, this is because of cluster 45 00:03:12,975 --> 00:03:17,935 formation time scales are broadly comprable to the age of the universe, so 46 00:03:17,935 --> 00:03:23,135 we expect to see, more or less, a similar kind of Clusters at any given time, not 47 00:03:23,135 --> 00:03:26,652 very early because they don't have time to form it. 48 00:03:26,652 --> 00:03:30,411 This is intermediate distance cluster at direction 5.43. 49 00:03:30,411 --> 00:03:35,097 Picture on the left is the visible light image from Hubble space telescope and 50 00:03:35,097 --> 00:03:39,102 picture on the right is the x-ray image from the Rosat sattelite. 51 00:03:39,103 --> 00:03:44,579 Clusters are now known[UNKNOWN] of maybe 1 and a half or so, and there are hints of 52 00:03:44,579 --> 00:03:48,371 cluster like structures to much higher that you see it. 53 00:03:48,371 --> 00:03:52,145 . But this is one of the most distant 54 00:03:52,145 --> 00:03:56,682 clusters proper that we know. Ah,[unknown] 1.23 and again it looks 55 00:03:56,682 --> 00:04:00,471 reasonably well formed. So how do we look for clusters? 56 00:04:00,471 --> 00:04:06,189 The most traditional method is just looking for[UNKNOWN][UNKNOWN] galaxy's on 57 00:04:06,189 --> 00:04:09,547 this The sky. And this is how most of the initial 58 00:04:09,547 --> 00:04:13,709 catalogs were compiled. We in fact, we use every one of the 59 00:04:13,709 --> 00:04:17,031 cluster properties as means of finding them. 60 00:04:17,031 --> 00:04:21,621 So, in the optical, this would be simply condensed to galaxies. 61 00:04:21,621 --> 00:04:27,188 And nowadays, the way this is done is the galaxy map on the sky, projected density 62 00:04:27,188 --> 00:04:32,749 map of the sky is smoothed and Algorithm of some sort is used to identify the other 63 00:04:32,749 --> 00:04:36,352 densities. Because clusters have a large population 64 00:04:36,352 --> 00:04:41,292 of red, early type galaxies, and becuase of the red shift, it makes sense to use 65 00:04:41,292 --> 00:04:46,286 new infra red bands rather than the optical ones, as we go to higher[UNKNOWN]. 66 00:04:46,286 --> 00:04:52,014 This is fairly strightforward way but it's very vulnerable to chance super-positions. 67 00:04:52,014 --> 00:04:56,706 They can't sometimes tell whether it's a cluster or it's just super position of 68 00:04:56,706 --> 00:04:59,655 several smaller clumps along the line of sight. 69 00:04:59,656 --> 00:05:04,520 Probably the most famous of optical cluster catalogs is the Abell Catalog, 70 00:05:04,520 --> 00:05:09,384 which was initially compile by George Abell from first Polymer sky surveys, 71 00:05:09,384 --> 00:05:12,909 simply looking at the graphic plates with a magnifier. 72 00:05:12,909 --> 00:05:17,381 Later on, this was extended to the southern sky as well, 4000 ga-, 4000 73 00:05:17,381 --> 00:05:20,644 clusters. This catalog has been a mainstay of 74 00:05:20,644 --> 00:05:25,389 cluster studies for a long time. Abell defined an operational criterion, 75 00:05:25,389 --> 00:05:30,189 what is something if you will call a cluster, there has to be certain number of 76 00:05:30,189 --> 00:05:35,289 galaxies within certain magnitude range and radius, and it's a reasonable set of 77 00:05:35,289 --> 00:05:40,239 criteria, essentially a density contract He divided the clusters in richness 78 00:05:40,239 --> 00:05:44,502 classes, depending on how many galaxies he could count in them. 79 00:05:44,502 --> 00:05:49,404 He also had distance classes, because at that time they didn't have directions to 80 00:05:49,404 --> 00:05:52,363 all of them. Abell's catalog had some problems. 81 00:05:52,363 --> 00:05:57,051 I mean, it's not a statistical sample although people often forget that. 82 00:05:57,051 --> 00:06:01,244 Abell himself defined a small subset of it that he thought would be. 83 00:06:01,245 --> 00:06:06,336 Complete suitable for statistical studies and everything else was extra. 84 00:06:06,336 --> 00:06:11,702 So people complained about, subjective nature of the catalog and these days 85 00:06:11,702 --> 00:06:17,282 nobody's doing this by hand anymore. We deploy algorithms that look for galaxy 86 00:06:17,282 --> 00:06:20,996 or[UNKNOWN] using some. Well defined criteria. 87 00:06:20,996 --> 00:06:27,143 And typical modern catalogs of clusters of galaxies using sky survey plates would 88 00:06:27,143 --> 00:06:32,112 have 10s of 1,000s of clusters in. The next method is using x-rays. 89 00:06:32,112 --> 00:06:38,348 Clusters do contain large amounts of hot gas, and they're very conspicuous on x-ray 90 00:06:38,348 --> 00:06:41,402 sky. Basically in the X-ray sky, you see point 91 00:06:41,402 --> 00:06:46,202 sources which would be active nuclei or maybe stars of some sort and extended 92 00:06:46,202 --> 00:06:51,467 sources which are clusters of galaxies. Here, the superposition problem does not 93 00:06:51,467 --> 00:06:55,162 play a significant role. In part, this is because the x-ray 94 00:06:55,162 --> 00:06:58,701 emission is proportional to the square of the density. 95 00:06:58,701 --> 00:07:03,507 And that means you would have to have material in one place to have a 96 00:07:03,507 --> 00:07:09,411 significant brightness in x-rays. But even so, x-ray method does not select 97 00:07:09,411 --> 00:07:14,638 clusters by mass just like optical method does not select them by mass. 98 00:07:14,639 --> 00:07:20,212 Related to this is the Sunyaev-Zeldovich effect, where we use combination of extra 99 00:07:20,212 --> 00:07:24,101 measurements in micro background maps to find clusters. 100 00:07:24,101 --> 00:07:29,547 Now you may recall what Sunyaev-Zeldovich effect is, we talked about this when we 101 00:07:29,547 --> 00:07:34,292 talked about the distance scale. If you look through a cloud of hot gas 102 00:07:34,292 --> 00:07:39,376 towards the microwave background photosphere, those photons sometimes 103 00:07:39,376 --> 00:07:44,460 encounter a hot electron and get universally come to a scattered, meaning 104 00:07:44,460 --> 00:07:49,531 they get some of the electrons energy. As a net result, the sppectrum of the 105 00:07:49,531 --> 00:07:55,118 microbackground as seen through this cloud of hot gas shifts to higher energies. 106 00:07:55,118 --> 00:08:00,934 And now depending whether you're observing it in a radio [inaudible] or whether the 107 00:08:00,934 --> 00:08:05,433 peak of the black body you can either see a bump or a dip in the sky. 108 00:08:05,433 --> 00:08:10,021 And that bump or dip corresponds exactly to the x-ray signal. 109 00:08:10,021 --> 00:08:16,191 Finally, weak gravitational lensing provides, at least in principle, means of 110 00:08:16,191 --> 00:08:21,987 detecting clusters and this time by the mass because that's what lensing is 111 00:08:21,987 --> 00:08:27,212 actually sensitive to. As you will recall, weak lensing will show 112 00:08:27,212 --> 00:08:33,510 distorted images of galaxies due to some foreground mass concentration such as 113 00:08:33,510 --> 00:08:39,620 cluster, and, by looking for such signals in the shape pattersn in background 114 00:08:39,620 --> 00:08:43,742 galaxies, you could find Large masses would or not. 115 00:08:43,742 --> 00:08:47,087 They actually have any light or x-ray emission. 116 00:08:47,087 --> 00:08:51,824 As it turns out, to the best of my knowledge, there was never a cluster 117 00:08:51,824 --> 00:08:57,219 discovered using this method that wasn't already seen or easily seen through 118 00:08:57,219 --> 00:09:01,189 optical or x-ray. So, what can you do with clusters? 119 00:09:01,189 --> 00:09:07,336 We can measure some of their properties more about those later, but you can just 120 00:09:07,336 --> 00:09:11,255 look at them. Typically in any imperical science, you 121 00:09:11,255 --> 00:09:16,138 will begin with [unknown]. So, Abell initally classified clusters 122 00:09:16,138 --> 00:09:21,339 simply on the basis of their appearance. He noted that some are well relaxed 123 00:09:21,339 --> 00:09:25,166 looking symmetric and the others still look like raggedy. 124 00:09:25,166 --> 00:09:29,582 Now we know that, that corresponds to dynamically well formed versus still 125 00:09:29,582 --> 00:09:33,342 forming clusters. Now, in 1970s, Bautz and Morgan introduced 126 00:09:33,342 --> 00:09:37,009 a different classification. This one was not really based on 127 00:09:37,009 --> 00:09:42,049 properties of cluster per se, but whether or not it had a giant, dominant elliptical 128 00:09:42,049 --> 00:09:47,478 galaxy in the middle as many of them do. This turns out to have some significance 129 00:09:47,478 --> 00:09:53,029 for galaxy evolution in clusters, but again, it was relatively superficial. 130 00:09:53,029 --> 00:09:58,469 Then further shifting focus, not on cluster appearance, but on their galaxy 131 00:09:58,469 --> 00:10:04,164 population, Oemler also classfied them according to the Amont of spiral versus 132 00:10:04,164 --> 00:10:08,643 elliptical galaxies in them. That directly relates to the galaxy 133 00:10:08,643 --> 00:10:13,132 evoltion processes in clusters. And Rood and Sastry had one of these 134 00:10:13,132 --> 00:10:18,592 schemes at which point the meager amount of informaiton that's actually present in 135 00:10:18,592 --> 00:10:21,998 these optical appearances was really exhausted. 136 00:10:21,998 --> 00:10:27,311 However, we do now have good physical understanding where all this comes from. 137 00:10:27,312 --> 00:10:32,388 Just a not about those central dominant galaxies and clusters, or cD as they are 138 00:10:32,388 --> 00:10:35,407 called. They're giant ellipticals, but unlike 139 00:10:35,407 --> 00:10:40,146 other giant ellipticals, they have an extra fuzzy envelope at large radii. 140 00:10:40,146 --> 00:10:44,652 Now we think that those extra fuzzy envelopes do not really belong to the 141 00:10:44,652 --> 00:10:49,942 galaxies, but to the clusters themselves. That they are Piles of stars stripped away 142 00:10:49,942 --> 00:10:53,040 by tidal interactions of galaxies in the cluster. 143 00:10:53,040 --> 00:10:57,186 They accumulate at the bottom of the potential well and the B galaxy also 144 00:10:57,186 --> 00:11:01,292 happens to be cospatial with them at the bottom of the potential well. 145 00:11:01,292 --> 00:11:03,871 So it looks as if it has this fuzzy envelope. 146 00:11:03,871 --> 00:11:09,222 So there's some interesting trends. Well relaxed looking, symmetric round 147 00:11:09,222 --> 00:11:13,415 clusters tend to be dominated by elliptical galaxies. 148 00:11:13,415 --> 00:11:19,541 Ruggedy, flat-ish looking clusters are those where they're dominated by spiral 149 00:11:19,541 --> 00:11:23,018 galaxies. And even within given cluster, there is a 150 00:11:23,018 --> 00:11:28,568 segregation, that the early type galaxies, Elliptical,[INAUDIBLE] tend to congregate 151 00:11:28,568 --> 00:11:33,600 in the inner portion of the cluster, where spirals are predominately found at the 152 00:11:33,600 --> 00:11:36,376 outskirts. Now, we have good understanding 153 00:11:36,376 --> 00:11:39,329 explanation effects from galaxy revolution. 154 00:11:39,330 --> 00:11:45,518 As far as the appearance is concerned, the round, smooth ones, simply had enough time 155 00:11:45,518 --> 00:11:49,396 to relax into varial or almost varial distribution. 156 00:11:49,396 --> 00:11:54,743 Whereas the lumpy, flattish, nonuniform ones are still coming together. 157 00:11:54,743 --> 00:12:02,119 And those Central dominant galaxies, probably, are in part at least, product of 158 00:12:02,119 --> 00:12:06,981 mergers of many smaller galaxies that built them up. 159 00:12:06,981 --> 00:12:13,440 So next time, we will talk about contents of the clusters of galaxies.