1 00:00:00,012 --> 00:00:07,632 Now continuing our study of clusters of galaxies, let's consider their content. 2 00:00:07,632 --> 00:00:12,953 Here's the X-ray gas. The origin of it is free-free emission 3 00:00:12,953 --> 00:00:19,583 meaning, scattering of electrons on protons in fully ionized plasma, that is 4 00:00:19,583 --> 00:00:23,926 it has temperatures of tens of millions of kelvin. 5 00:00:23,926 --> 00:00:29,262 And in fact, x-ray emission from clusters is so conspicuous that now there are 6 00:00:29,262 --> 00:00:34,455 surveys for clusters, that are based exclusively on their x-ray emission. 7 00:00:34,455 --> 00:00:39,741 So whereas, we use approximation, vertical symmetry and such In reality we see 8 00:00:39,741 --> 00:00:44,941 features in the X-ray gas that show us that clusters are not yet fully formed or 9 00:00:44,941 --> 00:00:48,015 relaxed, to differ from cluster to cluster. 10 00:00:48,015 --> 00:00:53,381 So for example, if you had as, a group of galaxies fall into a large cluster, there 11 00:00:53,381 --> 00:00:58,461 would be a shock wave as the inter, intergalactic medium of both collides. 12 00:00:58,461 --> 00:01:03,907 In the centers of clusters there is a possible phenomenon called the cooling 13 00:01:03,907 --> 00:01:07,997 flow and it works like this. Gas emits radiation, cools. 14 00:01:07,997 --> 00:01:13,832 Because of that it will fall towards the middle of the cluster, where it will heat 15 00:01:13,832 --> 00:01:19,748 up again, because the x-ray emission is proportional to the square of the density 16 00:01:19,748 --> 00:01:23,956 and so on. And so early estimates people thought that 17 00:01:23,956 --> 00:01:29,632 their clusters are having extra gas flowing through the middle, not because of 18 00:01:29,632 --> 00:01:33,104 gravity because of temperature differential. 19 00:01:33,104 --> 00:01:38,654 At some rapid rate and is a real puzzle where this gas go, now we know that this 20 00:01:38,654 --> 00:01:42,804 is only partly correct. And there are feedback processes that 21 00:01:42,804 --> 00:01:46,072 reheat the gas. So that cooling flows are not quite as 22 00:01:46,072 --> 00:01:50,306 important as people once thought . So where does the gas come from? 23 00:01:50,306 --> 00:01:53,691 Some of it simply came from the intergalactic space. 24 00:01:53,691 --> 00:01:58,497 And some of it was expelled from galaxies, intergalactic winds and driven by 25 00:01:58,497 --> 00:02:02,717 supernova explosions. And the reason we know this is that the 26 00:02:02,717 --> 00:02:06,918 gas is not pristine. It's not just hydrogen and helium but it 27 00:02:06,918 --> 00:02:10,457 contains metals and roughly at about of 1 3rd solar. 28 00:02:10,457 --> 00:02:15,485 So we know that some of that gas came through chemical evolution in stars in 29 00:02:15,485 --> 00:02:19,004 galaxies. And the x-ray properties correlate very 30 00:02:19,004 --> 00:02:24,037 well among themselves and also other with other physical properties of. 31 00:02:24,037 --> 00:02:27,540 Clusters. Let's look at some of the X-ray pictures. 32 00:02:27,540 --> 00:02:32,444 The cluster on the left here shows a cometary like thing which is a bow shock 33 00:02:32,444 --> 00:02:37,582 from a clump of stuff falling through. The cluster on the right shows a strange 34 00:02:37,582 --> 00:02:42,902 filament and we think that that's due to the jet from an active galactic nucleus. 35 00:02:42,902 --> 00:02:48,092 So they're not Perfectly symmetric and homogeneous and so on but by and large 36 00:02:48,092 --> 00:02:51,511 they can be and so our approximations are not bad. 37 00:02:51,511 --> 00:02:56,896 And again, how do we determine masses? Simply applying the virial theorum that 38 00:02:56,896 --> 00:03:02,161 kinetic energy has to be one half potential to within Plus or minus sign and 39 00:03:02,161 --> 00:03:07,516 we can consider galaxies as test particles using their velocity dispersion. 40 00:03:07,516 --> 00:03:12,350 Or we can use X-ray gas as test particles, measuring it's temperature. 41 00:03:12,350 --> 00:03:16,248 Either way we measure kinetic energy precluding to mass. 42 00:03:16,248 --> 00:03:21,672 And as you would recall, both of these methods yield the conclusion that the 43 00:03:21,672 --> 00:03:27,176 amount of mass in clusters is much higher than can be accounted by stars or gas 44 00:03:27,176 --> 00:03:30,582 alone. So therefore, there has to be some dark 45 00:03:30,582 --> 00:03:33,723 matter. This is how strictly originally 46 00:03:33,723 --> 00:03:37,098 discovered. People actually do a little more 47 00:03:37,098 --> 00:03:43,204 sophisticated studies using x-ray gas and apply full blown hydronomical models and 48 00:03:43,204 --> 00:03:47,147 so on. And that yields essentially the same 49 00:03:47,147 --> 00:03:51,688 results. So, typically in a cluster 1 6th or so of 50 00:03:51,688 --> 00:03:57,057 the matter would be variance, the rest would be dark matter. 51 00:03:57,057 --> 00:04:03,663 And on the variance, gas and galaxies, meaning stars, would contribute to bulk 52 00:04:03,663 --> 00:04:07,958 equal mass. Cluster collisions have recently led to 53 00:04:07,958 --> 00:04:12,666 yet another piece of evidence for the existence of dark matter. 54 00:04:12,666 --> 00:04:18,216 Here's one of those clusters that seems to have resulted from a merger as two 55 00:04:18,216 --> 00:04:22,345 clusters of galaxies and what the picture here shows. 56 00:04:22,346 --> 00:04:29,601 X-ray emission as well as galaxies. But also dark matter as inferred from 57 00:04:29,601 --> 00:04:34,781 micro-lensing studies around this emerging cluster. 58 00:04:34,781 --> 00:04:42,245 So the blue shading indicates where the mass should be, and the pink one is where 59 00:04:42,245 --> 00:04:46,250 the gas is. You see that there is title shock but also 60 00:04:46,250 --> 00:04:50,480 the two are displaced. Now this is what you'd expect if the dark 61 00:04:50,480 --> 00:04:56,263 matter does not interact directly through electromagnetic radiation with the gas. 62 00:04:56,263 --> 00:05:01,696 The two clusters which get, just going to pass through each other and eventually 63 00:05:01,696 --> 00:05:06,601 settle together into a merger. But the gas cluster collides with one in 64 00:05:06,601 --> 00:05:10,648 the other and does not participate in this boxing action. 65 00:05:10,648 --> 00:05:16,476 So you see the gas is concentraded towards the middle, whereas the dark matter, halos 66 00:05:16,476 --> 00:05:22,134 of clusters are now seperated apart.There are now several cases of these known and 67 00:05:22,134 --> 00:05:26,607 They all yield the same results. That in the mass to light ratios of 68 00:05:26,607 --> 00:05:31,663 clusters at several hundred times the solar mass to luminosity ratio, which 69 00:05:31,663 --> 00:05:36,511 implies again there that must be a significant non-barionic component. 70 00:05:36,511 --> 00:05:41,899 When we talked about numerical simulations of some of the modern hydro-dynamical 71 00:05:41,899 --> 00:05:47,359 simulations, and these are now deployed to study evolution of clusters, including 72 00:05:47,359 --> 00:05:52,819 their mergers the effects of octogalactic nuclei, star formation, stellar winds, 73 00:05:52,819 --> 00:05:57,889 these are some snapshots of what those simulations showing various physical 74 00:05:57,889 --> 00:06:01,350 properties. I mentioned the properties of clusters 75 00:06:01,350 --> 00:06:06,663 correlated with each other and whats shown here is plot of mass infered from varial 76 00:06:06,663 --> 00:06:11,236 arguments versus temperature. Now this trans look too good and the 77 00:06:11,236 --> 00:06:15,577 reason for this is that we actually use temperature to for the mass. 78 00:06:15,577 --> 00:06:18,468 But intuitively this is exactly what you'd expect. 79 00:06:18,468 --> 00:06:23,097 The more massive clusters will have deeper potential levels, which will require a 80 00:06:23,097 --> 00:06:26,731 higher velocity of particles which means a higher temperature. 81 00:06:26,731 --> 00:06:31,060 And that's exactly heat we see. Likewise, X-Ray Luminosities itself is 82 00:06:31,060 --> 00:06:33,812 proportional to temperature[UNKNOWN] power. 83 00:06:33,812 --> 00:06:37,173 And X-Ray Luminosities also corelates with cluster mass. 84 00:06:37,173 --> 00:06:40,737 Now this is reassuring to know, it's not terebly surprising. 85 00:06:40,737 --> 00:06:45,780 It would be very surprising if they didn't correlate but it's good to actually know 86 00:06:45,780 --> 00:06:49,699 measure it. So how do we study properties of clusters 87 00:06:49,699 --> 00:06:54,268 as a population? Well we can form the luminosity function, 88 00:06:54,268 --> 00:07:00,220 or mass function, usually in x-rays and see how it changes as a function of red 89 00:07:00,220 --> 00:07:02,648 shift. And here is an example. 90 00:07:02,648 --> 00:07:07,752 And you goes in the sense that at larger red shifts there are fewer more massive 91 00:07:07,752 --> 00:07:13,134 clusters, which is exactly what you expect if the clusters are being built in time, 92 00:07:13,134 --> 00:07:15,919 takes a while to build up the biggest ones. 93 00:07:15,919 --> 00:07:21,017 This opens possibilities for use of clusters as chronological probes, because 94 00:07:21,017 --> 00:07:25,615 in models with lower density and or positive cosmological constant. 95 00:07:25,615 --> 00:07:30,890 There is more time, and so you expect to form more massive clusters earlier than 96 00:07:30,890 --> 00:07:35,000 you would in high density zero cosmological constant models. 97 00:07:35,000 --> 00:07:40,206 So simply by counting clusters, through some luminosity or mass function as a 98 00:07:40,206 --> 00:07:43,753 function of red shift, you can consider it cosmolog. 99 00:07:43,753 --> 00:07:48,667 And here are examples of cluster mass functions from x-ray measurements. 100 00:07:48,668 --> 00:07:53,055 In two different regid shelves. The one on the left, shows the concordance 101 00:07:53,055 --> 00:07:58,023 cosmology model, plotted against the data, as you can see it goes beautifully through 102 00:07:58,023 --> 00:08:02,439 the data points, which is in a sense another vindication for the concordance 103 00:08:02,439 --> 00:08:05,233 model. The one on the right has the same hotter 104 00:08:05,233 --> 00:08:09,923 density, suppose we got that right, but suppose there was no dark energy, and you 105 00:08:09,923 --> 00:08:14,154 can see that it fails. Completely in predicting the abundance of 106 00:08:14,154 --> 00:08:18,039 clusters at larger ranges. So, just as we did before, combining 107 00:08:18,039 --> 00:08:23,368 constraints from microarray background, from supernova, from verionica acoustical 108 00:08:23,368 --> 00:08:27,320 oscillation and so on. Cluster measurements can be folded into 109 00:08:27,320 --> 00:08:32,495 the same type of statistical analysis, and, beautiful enough, their air ellipsis 110 00:08:32,495 --> 00:08:35,596 cross exactly where all other methods cross. 111 00:08:35,596 --> 00:08:41,029 So they improve our knowledge of this logical parameters, but also because this 112 00:08:41,029 --> 00:08:45,403 is a completely different way of measuring, they give us an extra 113 00:08:45,403 --> 00:08:48,732 confidence that we've actually done a good job. 114 00:08:48,732 --> 00:08:54,228 Now let's turn to galaxies and clusters. One thing that will happen is a galaxy 115 00:08:54,228 --> 00:08:57,954 like the spiral galaxy plows through that X-ray gas. 116 00:08:57,955 --> 00:09:02,972 It will get stripped away, of it's neutral hydrogen just because of the ram pressure 117 00:09:02,972 --> 00:09:05,776 sweeping, and that's indeed what is observed. 118 00:09:05,776 --> 00:09:10,472 The most gas deficient spirals are found in near a cores of clusters of galaxies, 119 00:09:10,472 --> 00:09:15,162 where as those on the outskirts look like pretty much like spiral galaxies in the 120 00:09:15,162 --> 00:09:18,712 field. The strength of this effect, the intensity 121 00:09:18,712 --> 00:09:22,899 of stripping, if you will. It's proportional to cluster x-ray 122 00:09:22,899 --> 00:09:28,259 luminosity, which is, again what you expect, because it's the x-ray gas that 123 00:09:28,259 --> 00:09:33,078 strips away the neutral hydrogen in those galaxies around through it. 124 00:09:33,078 --> 00:09:38,414 And if you look carefully, within individual spiral galaxies, the stripping 125 00:09:38,414 --> 00:09:43,826 occurs primarily in their outer parts which are less likely bound to the galaxy 126 00:09:43,826 --> 00:09:46,731 itself. So here is the plot of the hydrogen 127 00:09:46,731 --> 00:09:52,068 deficiency versus X-ray luminosity. And deficiency goes from, no hydrogen 128 00:09:52,068 --> 00:09:57,864 removed, to all hydrogen gone, and that correlates beautifully with x luminosity 129 00:09:57,864 --> 00:10:03,720 of clusters which is measured about their mass, and simply the density of the gas. 130 00:10:03,720 --> 00:10:08,911 So here is a very instructive map of vertical cluster galaxies in neutral 131 00:10:08,911 --> 00:10:12,376 hydrogen produce by [unknown] collaborators. 132 00:10:12,376 --> 00:10:16,208 These are [unknown] maps of the neutral hydrogen. 133 00:10:16,208 --> 00:10:22,557 Density in galaxies and M87 is the big galaxy in the middle essentially the 134 00:10:22,557 --> 00:10:28,017 center in argo cluster. And it's a obvious effect, that closer you 135 00:10:28,017 --> 00:10:33,731 get to the cluster core, the smaller disks look in neutral hydrogen. 136 00:10:33,732 --> 00:10:39,138 So this is exactly what you expect if they're being stripped by falling through 137 00:10:39,138 --> 00:10:44,062 a cluster intergalactic meteor . So gas collisions will strip away gas. 138 00:10:44,062 --> 00:10:48,097 What happens to stars? Gas will not effect them, but you may 139 00:10:48,097 --> 00:10:53,919 remember in numerical simulations, say, these galaxies merge or just pass by each 140 00:10:53,919 --> 00:10:59,249 other, their title features in some number of stars in these galaxies will get 141 00:10:59,249 --> 00:11:03,532 unbound [unknown], but they're still bound to the cluster. 142 00:11:03,532 --> 00:11:07,676 So they, they don't want, they no longer belong to a galaxy now. 143 00:11:07,676 --> 00:11:12,085 They belong to a whole cluster. And they'll tend to accumulate in the 144 00:11:12,085 --> 00:11:15,498 middle, but because galaxies interact everywhere. 145 00:11:15,498 --> 00:11:21,352 You expect some defused light, and in fact this is what we see first in coma cluster 146 00:11:21,352 --> 00:11:27,544 and here in the contours of very subtle, so low surface brightness distribution of 147 00:11:27,544 --> 00:11:31,941 stars with galaxies and they're very high contrast break up. 148 00:11:31,941 --> 00:11:37,031 And another one for Virgo cluster and the big holes are where many stars and 149 00:11:37,031 --> 00:11:42,525 galaxies are, but you can see their features and in the slides which probably 150 00:11:42,525 --> 00:11:48,335 corresponds to the title tails, streamers, and those galaxies and throughout. 151 00:11:48,335 --> 00:11:51,977 So the whole picture holds together very well. 152 00:11:51,977 --> 00:11:56,368 So let's recap what we learned about clusters of galaxies. 153 00:11:56,368 --> 00:12:02,358 They're the most conspicuous point of the logical structure, and they represent 154 00:12:02,358 --> 00:12:08,406 stage where structure becomes virialized. Gradually, typically a few mega parsecs 155 00:12:08,406 --> 00:12:13,802 across, they have hundreds of thousands of galaxies in them, the mixture of luminous 156 00:12:13,802 --> 00:12:18,818 and dark matter in them is exactly what is the overall mix from the cosmological 157 00:12:18,818 --> 00:12:23,018 measurements, baryonic to non baryonic dark matter component. 158 00:12:23,019 --> 00:12:27,777 Presence of the hot x-ray gas opens ways of finding them such as simply x-ray 159 00:12:27,777 --> 00:12:32,802 emission or [unknown] effect, which is popular because, as you may remember, it 160 00:12:32,802 --> 00:12:37,827 does not depend on Richert, the source is not the cluster, the source is the micro 161 00:12:37,827 --> 00:12:42,657 array background and clusters just provide x-ray shadow for a background. 162 00:12:42,658 --> 00:12:48,762 And those galaxies have by and large fully formed except for an occasional merger or 163 00:12:48,762 --> 00:12:52,041 so. Clusters are very much still forming and 164 00:12:52,041 --> 00:12:57,087 this is a process we now understand very well, thanks to the miracle of 165 00:12:57,087 --> 00:13:01,027 simulations. Because of the dense environment both 166 00:13:01,027 --> 00:13:07,187 proximity of many other galaxies to zip by or merge with and the dense intergalactic 167 00:13:07,187 --> 00:13:11,927 medium of x-ray gas. Galaxy evolution is effected in clusters. 168 00:13:11,927 --> 00:13:17,848 It proceeds a little differently from galaxy evolution in the general field, 169 00:13:17,848 --> 00:13:23,531 because there are more galaxy encounters some of which result in merging. 170 00:13:23,531 --> 00:13:29,213 There is certain stripping of gas which removes few of our star formation and so 171 00:13:29,213 --> 00:13:32,589 on. So we do expect and we actually observe 172 00:13:32,589 --> 00:13:38,681 differences in galaxy evolution between clusters and the general field. 173 00:13:38,681 --> 00:13:46,985 So next, we will start talking about properties of galaxies themselves.