1 00:00:11,090 --> 00:00:16,610 Okay. See, here's my outline. I'm going to go about. And John's already touched on the subject, 2 00:00:16,610 --> 00:00:23,030 but I'm going to talk more completely about the connection between quasars and the growth of these black holes at the census of galaxies. 3 00:00:23,360 --> 00:00:28,910 And that's going to lead me on to a quite a long discussion of the physics of the accretion disk, which is the mech, 4 00:00:29,090 --> 00:00:37,750 which is the machine which takes in material dropped into a black hole and processes into into a. 5 00:00:38,740 --> 00:00:44,170 Impact since we're all very interested in impact these days research council driven. 6 00:00:44,500 --> 00:00:52,960 And then I'm going to talk about black hole the connection between black of briefly black holes and stuff formation through cosmic history. 7 00:00:54,730 --> 00:01:00,250 So black hole growth. So as John said, we understand where supernova works, right? 8 00:01:00,250 --> 00:01:03,580 Where black holes of of stellar mass come from. 9 00:01:03,820 --> 00:01:09,340 They're formed in milliseconds when the centre of a star goes unstable and collapses to a black hole. 10 00:01:10,570 --> 00:01:16,030 But we think that the black holes that we see in the middle of galaxies have grown to their present masses. 11 00:01:16,030 --> 00:01:21,910 We don't know what they quite have grown from. Perhaps they've grown from stellar mass black holes, but there are difficulties with that. 12 00:01:22,720 --> 00:01:29,590 But we are pretty sure well, you'll see that there's overwhelming evidence that they have grown gradually to their present large masses. 13 00:01:31,030 --> 00:01:34,240 So they grew. They started small and they grew by feeding. 14 00:01:34,960 --> 00:01:37,960 Now a black hole can swallow us. 15 00:01:38,590 --> 00:01:42,069 The black hole of the middle of the galaxy could swallow the sun just like that hole. 16 00:01:42,070 --> 00:01:47,170 Gulp it. John showed it. Swallowing a star like the sun. 17 00:01:49,210 --> 00:01:52,420 Relatively slowly over a period of a month or so. 18 00:01:53,800 --> 00:01:58,810 But he also explained the rate at which the galaxy can feed stars to a black hole at the centre, 19 00:01:58,810 --> 00:02:06,820 at its centre is simply too insignificant, is too small to allow for significant growth of black holes. 20 00:02:06,820 --> 00:02:12,070 And so we will. We have talked about black holes with masses in excess of ten to the eight solar 21 00:02:12,070 --> 00:02:16,630 masses that are black holes known with masses like ten to the nine solar masses. 22 00:02:16,810 --> 00:02:24,360 And we believe these black holes we have reason to think these black holes existed when the universe was a 15th or so of its present age. 23 00:02:24,370 --> 00:02:27,370 These things have been around for a very long time. They must have formed rather rapidly. 24 00:02:27,370 --> 00:02:33,760 They cannot be formed by eating stars. They must have. They must have grown to their present weight by accreting gas. 25 00:02:35,600 --> 00:02:45,410 So there's a really beautiful argument that a Polish astronomer gave in 1980 to Zoltan to show the connection between quasars and black hole growth. 26 00:02:46,280 --> 00:02:51,410 So what you start by saying is supposing you drop a mass little m into a black hole. 27 00:02:52,850 --> 00:03:00,709 And obviously you were going to get energy M.c squared is then in play in play the rest mass energy 28 00:03:00,710 --> 00:03:08,240 of what you've thrown in is under discussion and suppose you get an amount cappa an efficiency. 29 00:03:08,240 --> 00:03:13,040 So Kepler is dimensionless efficiency. You get an amount of energy Kappa M.c squared out. 30 00:03:14,080 --> 00:03:16,630 We have reason to think. We haven't talked about it today, 31 00:03:16,870 --> 00:03:24,969 but we have great reason to think that copper is a number that varies between something like 0.1 and maybe something like point three, 32 00:03:24,970 --> 00:03:31,240 depending on the spin of the black hole. So it's, it's tense but, but we'll see what value we get for Kepa. 33 00:03:32,500 --> 00:03:42,070 So then what Zoltan showed was that if you ask how much energy is is produced over all of cosmic time in a unit volume. 34 00:03:43,920 --> 00:03:51,370 Then you can obtain that from, from, uh, from just this simple thing here. 35 00:03:51,390 --> 00:03:54,510 So n is the number of sources, 36 00:03:54,510 --> 00:04:05,760 quasars that you see with a flux density is as is measured in something like watts per square metre radian at a redshift z. 37 00:04:06,090 --> 00:04:14,400 So this is the number of this is this is the number of sources per unit volume with this kind of flux density, 38 00:04:14,610 --> 00:04:18,990 with this with this kind of with this kind of redshift. 39 00:04:19,860 --> 00:04:29,759 So any something you can you can you can get simply by counting objects at different flux levels known with a known and measured redshifts, 40 00:04:29,760 --> 00:04:38,640 etc., etc., etc. So then you can do this integral over your data and then you can do this other integral over the redshifts of your data. 41 00:04:38,940 --> 00:04:49,890 And what you end up with is the energy per unit volume that has been emitted by by all these sources over all of cosmic time. 42 00:04:50,670 --> 00:04:54,870 So it's remarkable that just from the observations, without knowing anything much about cosmology, 43 00:04:55,710 --> 00:04:59,190 the absolute bare minimum of cosmology, you can write this down. 44 00:05:02,420 --> 00:05:11,809 So if this energy has per unit, volume has come by this kind of efficiency by dropping by accreting this this mass m that tells 45 00:05:11,810 --> 00:05:19,010 you that there should be a mass density of of the black holes that are built up by this process. 46 00:05:19,020 --> 00:05:24,650 So we're assuming that all the mass in the black hole has been accreted by this process over cosmic time. 47 00:05:24,890 --> 00:05:30,200 You should have a mass density, which is just the energy density that you've measured divided by Kappa C Squared. 48 00:05:30,560 --> 00:05:32,900 And Zoltan, back in this remote time, 49 00:05:32,900 --> 00:05:42,350 1982 predicted what the mass of a black hole should be in the middle of the galaxy by supposing that this this black hole, 50 00:05:42,350 --> 00:05:51,200 mass density, which is so many solar masses per cubic per cubic volume, is divided up amongst the number amongst the galaxies. 51 00:05:51,200 --> 00:05:54,169 So you have a number density of galaxies, of sort of luminosity, 52 00:05:54,170 --> 00:05:57,680 something like the luminosity of our own galaxy, which is very characteristic luminosity. 53 00:05:58,160 --> 00:06:04,190 So you have a number of galaxies per unit volume, you have a mass per unit volume that gives you a mass per galaxy. 54 00:06:04,430 --> 00:06:11,600 That should be the ash obtained by the result of accreting all this material in order to make the luminosity. 55 00:06:11,600 --> 00:06:18,980 So the assumption here is that every galaxies black hole has shone brilliantly as a quasar or whatever is was accumulating this material. 56 00:06:19,190 --> 00:06:23,900 And if it's not shining now this just because it's stopped accumulating, it's stopped accumulating the material. 57 00:06:24,410 --> 00:06:28,550 You look inside it and you should see this residue. That's evidence of what of what is achieved. 58 00:06:29,180 --> 00:06:36,829 So you and Trymaine revisited this this idea 21 years after it was originally done, 59 00:06:36,830 --> 00:06:44,450 when the observational data was spectacularly improved by counting from counting the number of quasars that you see in optical bands. 60 00:06:45,380 --> 00:06:49,220 The that led to an estimate. 61 00:06:49,340 --> 00:06:56,600 So that's so that's that's doing the end of S and Z count by counting quasars using optical radiation. 62 00:06:58,310 --> 00:07:02,090 You arrive at a number density of 2.1 by ten to the fifth. 63 00:07:03,050 --> 00:07:07,340 This is point one of Kappa one one whatever. 64 00:07:07,340 --> 00:07:10,730 10% for Kappa Solar masses per cubic parsec. 65 00:07:12,140 --> 00:07:15,320 And for the correlation. So this is an estimate obtained from quasars. 66 00:07:15,350 --> 00:07:20,630 Now let's look at an estimate in the number in the mass density in galactic centre. 67 00:07:20,900 --> 00:07:27,830 Black holes that you get from that correlation between the mass of the black hole and the velocity dispersion of the bulge, 68 00:07:27,830 --> 00:07:32,210 you know how many galaxies there are with which. 69 00:07:32,430 --> 00:07:38,239 And so the Sloan Digital Sky survey measured for millions of galaxies, 70 00:07:38,240 --> 00:07:41,719 the velocity estimates, the velocity dispersion and counted the number of galaxies. 71 00:07:41,720 --> 00:07:44,690 So one knows how many galaxies there are with each velocity dispersion. 72 00:07:44,930 --> 00:07:50,149 And you know from this correlation what size of black hole this thing, this galaxy should contain. 73 00:07:50,150 --> 00:07:53,690 And so that gives you a separate estimate of the mass density in black holes. 74 00:07:53,690 --> 00:07:58,460 And that turned out to be this number here, which is astonishingly in agreement with that number up there. 75 00:07:59,210 --> 00:08:02,660 So that implies that agreement, which is better. 76 00:08:03,200 --> 00:08:06,889 The agreement seems better than it really is because there are some there are 77 00:08:06,890 --> 00:08:10,700 some skeletons in the closet to do with the existence of obscured quasars. 78 00:08:10,700 --> 00:08:18,589 A very large number of quasars are believed to be hidden behind dust, and you can see them only through their X-ray radiation or infrared radiation. 79 00:08:18,590 --> 00:08:25,850 But we'll see. We'll see that these these issues these are technicalities which which means that this agreement is a little bit fortuitous, 80 00:08:25,850 --> 00:08:30,830 but within certainly within factors of of two or so, 81 00:08:31,040 --> 00:08:39,410 there is astonishingly good agreement between the mass density in galactic centre black holes and the radiation emitted by quasars, 82 00:08:39,590 --> 00:08:50,149 which implies that these these black holes formed through rather efficient accretion by material with an efficiency of that sort, 83 00:08:50,150 --> 00:08:51,650 which is sort of efficient ish. 84 00:08:52,520 --> 00:09:01,940 So let's talk now about the physics which leads to the production of radiation, quasar light, etc., when stuff is dropped into a black hole. 85 00:09:01,940 --> 00:09:09,110 So a black hole is just a gigantic drain. And when you tip the washing up water down the sink when you finished with it, 86 00:09:09,980 --> 00:09:16,850 most of the time as it moves towards the axis of the of the plug hole, it has some angular momentum. 87 00:09:16,850 --> 00:09:19,760 So it starts spinning and you get this characteristic vortex. 88 00:09:21,280 --> 00:09:27,490 As the water goes down, it goes down the hole because the water is spinning around the axis of the pipe as it goes down. 89 00:09:28,030 --> 00:09:32,530 And hurricanes and tornadoes depend on exactly the same physics. 90 00:09:32,530 --> 00:09:42,189 The air is drawn inwards because because moist air is moving upwards and has to be replaced by other air coming in horizontally as it moves inwards. 91 00:09:42,190 --> 00:09:45,880 It has some angular momentum, which it conserves, largely conserves. 92 00:09:46,090 --> 00:09:53,440 So it starts to spin fast. So the same happens when gas is dropped on to any compact object, a star or a black hole. 93 00:09:53,680 --> 00:09:54,790 It starts to spin. 94 00:09:54,790 --> 00:10:04,540 And we have here this artist's impression of gas spinning around of what this is supposed to be, a forming a protostar, a forming star. 95 00:10:04,540 --> 00:10:13,179 So this is the solar system at some very early time. So let's just think a little bit about the last thing I want to talk about. 96 00:10:13,180 --> 00:10:17,530 The simplest conceivable model is kind of basic baseline model of an accretion disk. 97 00:10:18,040 --> 00:10:25,989 We imagine that we have a steady state in which mass is flowing inwards towards it's being acquired by the black hole, 98 00:10:25,990 --> 00:10:28,980 the forming star of the neutron stars, 99 00:10:28,990 --> 00:10:34,630 but it's being acquired by some compact object in the middle at a rate down by d t that's that's constant in time. 100 00:10:35,350 --> 00:10:38,200 And as the and as each parcel of gas moves inwards, 101 00:10:38,530 --> 00:10:44,499 it orbits at all times more or less circular because it's not orbiting exactly circular because it's spiralling inwards. 102 00:10:44,500 --> 00:10:48,760 But it's but the inward motion is very small compared to the going in a around motion. 103 00:10:48,970 --> 00:10:56,440 So at all times it's more or less on a circular orbit. And so you can from high school mathematics compute what the energy is of that gas. 104 00:10:56,440 --> 00:11:03,069 It's kinetic and its potential energy. And as it moves inwards it its energy gets more and more negative. 105 00:11:03,070 --> 00:11:07,149 You can also compute from high school mathematics what its ranking of momentum is. 106 00:11:07,150 --> 00:11:10,240 And as it moves in with this angle, momentum j gets smaller and smaller. 107 00:11:10,780 --> 00:11:14,139 So the characteristic view is that here's the black hole. 108 00:11:14,140 --> 00:11:22,930 This is sort of this is cartoon. The matter moves in at some steady rate down by d t this sort of radius here some concept of radius. 109 00:11:24,430 --> 00:11:32,170 And as it moves in it, each parcel of gas has to dump some mangled momentum and some energy. 110 00:11:32,380 --> 00:11:39,280 And the energy that it dumps is going to be, we imagine, is radiated because the disk becomes very hot from this energy being dumped on it. 111 00:11:39,280 --> 00:11:43,240 So it shines and it radiates photons carrying away the energy. 112 00:11:43,450 --> 00:11:46,030 But the photons don't carry away much of the angle of momentum. 113 00:11:46,270 --> 00:11:55,420 The angle momentum has to be laboriously carried outwards by viscous stress, by friction within the disk, which we'll talk about in greater detail. 114 00:11:56,140 --> 00:12:00,280 So Fisker's torque carries angular momentum out. It's some steady, some rate. 115 00:12:00,280 --> 00:12:03,370 There's a small flux of angle momentum here and a large flux of angle momentum there. 116 00:12:04,210 --> 00:12:08,560 And the surface of the disk radiates the energy. This is this is our baseline model. 117 00:12:10,110 --> 00:12:18,599 Let's maybe put this on some kind of quantitative basis. The the reason that there is a fiscal tool is because the velocity of the rings. 118 00:12:18,600 --> 00:12:25,040 So we mentally divide the. This is just mentally divided into a series of annually circular annually the interannual. 119 00:12:25,770 --> 00:12:32,460 A moving fast because the Kepler the speed of Kepler orbits goes like one over the square root of radius. 120 00:12:32,760 --> 00:12:37,169 So the inner ones are going fast, the outer ones are going slow, which means that this ring, 121 00:12:37,170 --> 00:12:42,360 as it spins around here, is rubbing on this ring is which is spinning more slowly. 122 00:12:42,540 --> 00:12:46,500 But this and this ring is therefore doing viscous work on this ring. 123 00:12:46,710 --> 00:12:50,100 And this ring is doing viscous work on that ring and so on and so forth. 124 00:12:50,100 --> 00:12:57,659 And that that work is also associated with this ring is twisting this ring and giving angular momentum to it. 125 00:12:57,660 --> 00:13:00,989 So that's why the material in this ring is able to move inwards because it sheds angular 126 00:13:00,990 --> 00:13:04,860 momentum by passing it on to this one and this one passes on to this one and this one, 127 00:13:04,860 --> 00:13:09,160 so on and so forth. But let's think about the energy first. 128 00:13:10,160 --> 00:13:20,240 We have that within within any annulus there is and there is there is the matter is dropping is decreasing its energy. 129 00:13:20,540 --> 00:13:25,700 So when it's that hour plus deal, when it's so the outer edge of one of these rings, 130 00:13:25,940 --> 00:13:37,010 the energy of the mass is minus g m overall plus deal with the factor to the this number here is its gravitational potential energy. 131 00:13:37,160 --> 00:13:44,090 It has it has kinetic energy which is of opposite sine is positive but only of half the size. 132 00:13:44,100 --> 00:13:47,720 So the total energy is is minus a half of this. 133 00:13:47,900 --> 00:13:54,890 So this is the energy it has at the outer edge of an annulus and then the inner edge of the annulus, it has minus a half GM overall. 134 00:13:55,130 --> 00:14:01,760 So the energy that's dropped, that's deposited by a parcel of gas is of mass DM as it passes through. 135 00:14:01,760 --> 00:14:10,160 Here is the difference between these two and it's being dropped at a rate down by d t the rate at which mass is flowing through this annulus. 136 00:14:10,550 --> 00:14:18,080 So that means there is a power rate of a rate of dropping of energy in this annulus which is 137 00:14:18,080 --> 00:14:22,780 given by this where g m overall squared is of course the difference between these two times. 138 00:14:22,790 --> 00:14:27,439 Times delta is the difference between these two. So that's the rate at which matter. 139 00:14:27,440 --> 00:14:28,880 Moving inwards drops energy. 140 00:14:29,780 --> 00:14:37,879 But also there is this business of friction that the each ring has work done on it by the ring that's just interior to it. 141 00:14:37,880 --> 00:14:41,750 And it's it itself does work on the ring that's just outside it. 142 00:14:41,750 --> 00:14:47,899 This is a fundamental this work done is a fundamental part of the of the transfer of an element m outwards, 143 00:14:47,900 --> 00:14:51,620 which is a zeroth order, you know, key, a key part of what's happening. 144 00:14:53,350 --> 00:14:58,660 And when you work out the difference between the work done by this ring and the work that this ring does on that, 145 00:14:58,900 --> 00:15:04,660 which is the which is the sort of surplus this this this this ring here is engaged in a trade. 146 00:15:04,810 --> 00:15:09,820 In some sense, it's picking up energy here, and it's and it's passing it on there. 147 00:15:09,820 --> 00:15:13,180 And the profit, the surplus of one worker over the other work is a profit. 148 00:15:13,280 --> 00:15:23,440 So it's an energy dumped in there. It turns out it's not it's a slightly intricate calculation, but it's done with high school, high school physics. 149 00:15:23,740 --> 00:15:30,969 It turns out that the discus power is three times precisely three times the power that we've just worked out from the matter, 150 00:15:30,970 --> 00:15:36,190 dropping energy as it passes through, directly, dropping this Fisker's work surplus, 151 00:15:36,190 --> 00:15:40,329 this profit on the profits on the handling of a packet of Anglo momentum, 152 00:15:40,330 --> 00:15:45,190 because that's what's happening with we're taking we're buying Anglo momentum here and we're selling it here. 153 00:15:45,520 --> 00:15:52,509 And the energy that comes with it is different. In the two cases we rip off that that surplus and that's three times what we get 154 00:15:52,510 --> 00:15:56,710 from doing a trade in mass where we take in mass here and we jump to dump it up. 155 00:15:57,980 --> 00:16:04,850 Okay. So we have this we have a total power, which is four times the one we just worked out. 156 00:16:05,270 --> 00:16:14,690 And if we equate this total power to the energy that this annulus can radiate per unit time, 157 00:16:14,690 --> 00:16:18,740 so we're going to assume that each annulus radiates like a blackbody. 158 00:16:18,770 --> 00:16:20,989 So it's the Stephan Boltzmann constant times. 159 00:16:20,990 --> 00:16:27,559 T Each of the four is the energy per unit area per unit time, and this is the area of the annulus two pi. 160 00:16:27,560 --> 00:16:32,690 Our delta r is the area of the top surface and there's the bottom surface of the same area. 161 00:16:32,690 --> 00:16:39,589 So this is the total area. So this is the this if it's a temperature t that material and radiating like a black body. 162 00:16:39,590 --> 00:16:50,540 This is the luminous power of an annulus. And we equate this luminous power to the two four times the power worked out on the previous page, 163 00:16:50,720 --> 00:16:55,250 which is then this where the two was on the bottom before is now in the top because of the fall. 164 00:16:56,030 --> 00:17:06,440 We can then for any chosen value of the accretion rate, we can talk about what the what the luminosity is of the disk. 165 00:17:06,890 --> 00:17:12,860 As a function of radius, we can talk about what the temperature of the disk is as a function of radius, but to work this out, I need to, 166 00:17:12,980 --> 00:17:17,690 I need to adopt some value for this and I'm going to adopt values for those stellar mass 167 00:17:17,690 --> 00:17:22,639 black holes and galactic centre black holes which imply that they have a growth time. 168 00:17:22,640 --> 00:17:28,310 They're accumulating material 100 mega year time scales so that they over 100 years 169 00:17:28,310 --> 00:17:34,219 they can double mass because we have reason to think that's a reasonable estimate. 170 00:17:34,220 --> 00:17:38,780 So here is here is a solar mass of black hole of just exactly one solar mass. 171 00:17:38,780 --> 00:17:43,430 So the mass black holes are typically more massive than this by a factor of a few, but not many. 172 00:17:44,240 --> 00:17:49,940 And so here is the radius in astronomical units, one astronomical unit way out here. 173 00:17:50,150 --> 00:17:53,150 So we're talking about mostly what happens on very small radii. 174 00:17:53,180 --> 00:17:56,540 Here is the radius of the sun. Here is the radius of a white dwarf. 175 00:17:56,750 --> 00:18:00,740 So if when the sun had an accretion disk, which it did when the solar system was forming, 176 00:18:00,890 --> 00:18:06,440 only this part of the diagram was relevant and the characteristic temperatures were less than a thousand degrees. 177 00:18:06,440 --> 00:18:10,250 Kelvin, when the sun becomes a white dwarf, 178 00:18:10,520 --> 00:18:16,970 it will it will have an accretion disk where this bit is relevant and the temperature rise is something like ten to the fifth Kelvin. 179 00:18:17,180 --> 00:18:22,370 And the thing it will be radiating as a in ultraviolet light. 180 00:18:22,850 --> 00:18:29,210 But if it is if it is a black hole, then the here is the actual radius, three kilometres of the sun. 181 00:18:29,300 --> 00:18:34,459 Its whole diagram becomes relevant and it can radiate in hard X-rays up here. 182 00:18:34,460 --> 00:18:41,150 Here is the temperature at the very middle of the sun. Here is the temperature on the surface of the sun to give you some sense of scale. 183 00:18:42,070 --> 00:18:46,930 So that's that encapsulates the characteristics of accreting stellar mass black holes. 184 00:18:47,440 --> 00:18:52,209 If you now change the mass scale to ten to the eighth, but you leave the time of accretion, 185 00:18:52,210 --> 00:18:57,190 the scale, the luck, the timescale of accretion at 100 mega years, 100 million years. 186 00:18:57,520 --> 00:19:01,480 Then the diagram looks like here now we have radius in AEW. 187 00:19:01,510 --> 00:19:06,280 Here is one AEW and the radius is about to AEW here. 188 00:19:07,400 --> 00:19:13,900 Here is. So the crucial thing about this diagram, this is incredibly similar to this diagram, the really boring straight line eye Apollo. 189 00:19:14,920 --> 00:19:20,590 The key thing is that this temperature scale is lower by a factor of 100 than that temperature scale. 190 00:19:20,590 --> 00:19:22,720 So here is the surface temperature of the sun. 191 00:19:23,380 --> 00:19:30,220 So that if you have a galactic centre, a black hole which accretes all the way down to the to the actual radius, 192 00:19:30,220 --> 00:19:35,200 it radiates at temperatures of ten to the 50 K ten to the fifth K. 193 00:19:35,500 --> 00:19:39,100 So only ultraviolet radiation, not x rays. 194 00:19:39,940 --> 00:19:43,030 I mean not hard x rays, soft x rays, ultraviolet radiation. 195 00:19:43,330 --> 00:19:47,650 And there will be lots of and there will be lots of radiation coming out in the optical bands. 196 00:19:48,730 --> 00:19:52,520 Okay. So that's that's an important thing to know. 197 00:19:53,810 --> 00:20:01,640 What about luminosity is this is just this is a plot showing the luminosity for us galactic centre type black hole. 198 00:20:01,940 --> 00:20:07,219 The luminosity that's radiate outside radius are really all we need to know is that that 199 00:20:07,220 --> 00:20:12,260 now the luminosity is that you can achieve if you go all the way to the actual radius, 200 00:20:12,500 --> 00:20:18,620 you can achieve maybe 100 times the luminosity or a few hundred times the luminosity of a whole galaxy. 201 00:20:19,040 --> 00:20:23,060 And, and that to achieve the kind of luminosity, as the John mentioned, ten to the 12. 202 00:20:23,190 --> 00:20:29,480 So the solar luminosity is which which are characteristic of bright quasars and which staggered people in the 1960s. 203 00:20:29,750 --> 00:20:36,080 You have to go in to you have to go into maybe ten A.U., maybe five structural radii. 204 00:20:36,230 --> 00:20:42,580 So you are talking about accretion to the bitter end. I think that's what we need to say there. 205 00:20:43,150 --> 00:20:53,110 Now, you may be impressed by a black hole being able to radiate tens of luminosity of an entire galaxy of 10 to 11 stars. 206 00:20:54,160 --> 00:21:01,040 And that you can see these these things, these quasars are bright enough that you can see them even when they right across the universe. 207 00:21:01,060 --> 00:21:05,889 So as I say, these quasars are seen which have redshifts of six or so. 208 00:21:05,890 --> 00:21:16,540 And when the universe was to more than 200 times denser than it is now, so really is an early time. 209 00:21:16,840 --> 00:21:21,910 This is, in a certain sense, very impressive. But frankly, it's only histrionics. 210 00:21:22,120 --> 00:21:28,160 It's not what makes black holes important. That's what I want to ask you next. 211 00:21:30,860 --> 00:21:37,639 So black holes do have a big impact. These these black holes do, I believe, have a very big impact on the structure of the universe. 212 00:21:37,640 --> 00:21:42,410 But it is not through this histrionics and to understand how they have an impact. 213 00:21:42,920 --> 00:21:46,370 We want to look we need to think bit more carefully about what happens in an accretion disk. 214 00:21:48,480 --> 00:21:57,750 So what provides the all important viscosity? What the accretion disk does is shuffle angular momentum outwards to its angular momentum. 215 00:21:58,020 --> 00:22:02,910 The surface of angle momentum that prevents material disappearing down the structural throat. 216 00:22:04,240 --> 00:22:07,240 And you have to get rid of this angular momentum if the mass is going to move inwards. 217 00:22:07,240 --> 00:22:14,560 And that's what the accretion disk does. It's a machine for shuffling, for carrying an element of out of material to somewhere where it's harmless. 218 00:22:15,130 --> 00:22:19,480 And what actually does this what we're pretty convinced now that it's it's a magnetic 219 00:22:19,480 --> 00:22:24,580 field which does this the thing is that magnetic field lines carry tension. 220 00:22:24,670 --> 00:22:29,130 They're like elastic bands. And when a field line is stretched. 221 00:22:30,240 --> 00:22:37,290 Work is done on the magnetic field and the magnetic field energy goes up, which means that the magnetic field is amplified. 222 00:22:37,790 --> 00:22:44,760 The strength of the magnetic field has to increase, reflecting the work that you've done on the magnetic field and stretching the field line. 223 00:22:46,650 --> 00:22:52,180 A gas in astrophysics is usually can usually be considered to be a perfect conductor. 224 00:22:52,180 --> 00:22:58,719 The scales are very large and the availability of free electrons is always relatively free. 225 00:22:58,720 --> 00:23:02,410 Electrons are usually pretty available. They're very available in an accretion disk. 226 00:23:02,680 --> 00:23:06,520 So you can treat the accretion disk is made up of a perfect conductor and it's 227 00:23:06,520 --> 00:23:10,780 easy to show that when you have a magnetic field line inside a perfect conductor, 228 00:23:11,050 --> 00:23:17,470 the field line is trapped with respect to the fluid or the fluid is trapped with respect to the magnetic field line, 229 00:23:17,470 --> 00:23:25,480 depending on who you consider to be the boss. So if the feel, if the fluid stretches in some way, the field line within it has to stretch too. 230 00:23:26,680 --> 00:23:28,150 So here we have, I hope, a. 231 00:23:31,140 --> 00:23:39,330 A baby film which shows so what we have here is a black hole and here we have around is an accretion disk that you can't see. 232 00:23:39,540 --> 00:23:42,540 What you can see is a series of. 233 00:23:43,880 --> 00:23:47,330 Magnetic field lines. Which I've just drawn. I've just drawn. 234 00:23:47,930 --> 00:23:51,680 Going out radially. And then. 235 00:23:53,590 --> 00:24:00,550 If you animate this thing, each each point on the field line is moving around here with capillary and speed, 236 00:24:00,880 --> 00:24:04,270 and the inner points go around faster than the outer points. 237 00:24:04,510 --> 00:24:11,229 And so what was a radial arrangement of field lines becomes a tangential, a more and more tangential array of field lines. 238 00:24:11,230 --> 00:24:14,950 And also these field lines are getting longer and longer. They're being stretched. 239 00:24:19,190 --> 00:24:23,820 So. The field lines, the trap. 240 00:24:23,910 --> 00:24:28,560 Yeah. Okay. So. So the field lines are constantly stretched, and that means the field is amplified. 241 00:24:29,820 --> 00:24:35,560 So. It doesn't matter how weak the magnetic field was originally. 242 00:24:35,860 --> 00:24:43,060 Through this stretching, this endless stretching that's inherent in the sheer of the flow within the accretion disk, 243 00:24:43,450 --> 00:24:49,929 the field is going to be amplified until it is strong enough for its back reaction. 244 00:24:49,930 --> 00:24:53,620 The tension that it provides to modify the flow. 245 00:24:55,080 --> 00:25:01,559 And that means that the field will inevitably be amplified until it becomes capable of transporting angular momentum outwards. 246 00:25:01,560 --> 00:25:08,310 And I hope for that figure. You can imagine that it is going to transform Anglicanism outwards because the magnetic tension is pulling backwards 247 00:25:08,610 --> 00:25:15,680 on the on the particles that were a small radius and pulling forwards on the particles which are at large radius. 248 00:25:15,690 --> 00:25:18,020 So it's clearly transporting angry momentum outwards. 249 00:25:20,230 --> 00:25:28,720 Now field lines provide tension along there along their length, but they exert pressure perpendicular to their length. 250 00:25:31,220 --> 00:25:37,520 So. So here's a field line on detention, but and here's another field line on detention. 251 00:25:37,520 --> 00:25:43,760 And there's a pressure that that provided by the magnetic field that operates between these two. 252 00:25:45,020 --> 00:25:50,839 So here we have a field line. Here's here's the here we have a field line that's near the mid plane of the accretion disk. 253 00:25:50,840 --> 00:25:55,130 And I've drawn this. So it has a very slight a very slight upward curving this. 254 00:25:57,510 --> 00:26:01,830 This very slight upward curving this enables the plasma, encourages the plasma. 255 00:26:02,010 --> 00:26:08,309 So there's a sort of gravitational field down here towards the mid plane of the disk that the yellow is supposed to represent, 256 00:26:08,310 --> 00:26:11,320 the plasma which is surrounding the field line. 257 00:26:11,320 --> 00:26:16,770 And this plasma can't move perpendicular to the field line, but it can slide along the field line. 258 00:26:18,450 --> 00:26:23,940 So this because of this slight arching of the field line, the plasma begins to roll gently. 259 00:26:24,210 --> 00:26:33,030 Downhill here, downhill here. And the amount of plasma in this region here decreases and the amount of plasma here in here increases. 260 00:26:33,480 --> 00:26:39,780 But then that makes that that means so here we have this have been a thinning of the plasma here. 261 00:26:40,920 --> 00:26:49,110 But that encourages this. It means there's less weight acting holding down the field line because the plasma contains the mass, 262 00:26:49,110 --> 00:26:52,950 the field is providing force, but not any significant mass. 263 00:26:53,610 --> 00:26:59,190 So if you take away some of the mass here, this magnetic pressure will cause this to arch up. 264 00:26:59,430 --> 00:27:03,870 But then that encourages this flow downhill towards the wings. 265 00:27:04,410 --> 00:27:12,810 So that's shown here where almost all the plasma has flown away from flowed away from here down to down to these foot points here. 266 00:27:14,070 --> 00:27:20,880 So you have an instability here. This is the Parker instability that leads a straight field line to become a looped field line. 267 00:27:21,990 --> 00:27:27,629 We can't see this in an accretion disk, but you can see it most beautifully in action in the sun. 268 00:27:27,630 --> 00:27:32,190 This is an ultraviolet picture of the sun. And you see these loops here. 269 00:27:32,190 --> 00:27:38,190 So these these loops are field lines. They're illuminated by relativistic electrons which are trapped along the field lines. 270 00:27:38,190 --> 00:27:46,380 So it's kind of a it's kind of a fortunate chance that you can actually see the field lines, but you can see that they form these loops. 271 00:27:51,000 --> 00:27:59,700 Now, another key piece of magnetic physics now plays a role, which is that when you have a field line going this way and a field line going that way, 272 00:27:59,700 --> 00:28:04,440 and if they come close, then you can have a rearrangement here, 273 00:28:04,680 --> 00:28:08,129 which so that this field line which used to go to that South Pole, 274 00:28:08,130 --> 00:28:13,380 starts to go to this South Pole and corresponding the field line that used to go to this pole goes to that pole. 275 00:28:14,300 --> 00:28:24,050 So you have this reconnect. But this is reconnection, which is a sort of discontinuous process occurs at this at this moment here leading to this. 276 00:28:24,560 --> 00:28:29,450 But when you compare the geometry from here to here to here to here, 277 00:28:30,710 --> 00:28:36,650 you find that this the field lines have grown shorter as a result of going from here to here. 278 00:28:38,230 --> 00:28:42,670 And I said that when field lines are stretched, the magnetic energy is increased. 279 00:28:42,670 --> 00:28:45,850 Correspondingly, when they shorten, the magnetic energy is decreased. 280 00:28:46,360 --> 00:28:50,700 So the magnetic energy over here is less than the magnetic energy over here. 281 00:28:50,710 --> 00:28:57,070 And where is the energy go? All the energy has gone in the violent heating of the of particles, 282 00:28:57,070 --> 00:29:01,390 the raising to the production of energetic particles at this moment of reconnection. 283 00:29:03,080 --> 00:29:06,620 So when feels. When? When, when. So the the. 284 00:29:06,630 --> 00:29:13,370 The. The constant movement of the foot lines of the easily we've got these loops forming 285 00:29:14,090 --> 00:29:19,999 as as the Parker instability pushes field lines above the the plane of the disc and 286 00:29:20,000 --> 00:29:24,829 then the the differential rotation within the disc is bringing is bringing oppositely 287 00:29:24,830 --> 00:29:29,510 directed field lines ODIs or oppositely directed field lines close to one another. 288 00:29:29,750 --> 00:29:40,010 Then this reconnection process happens which accelerates particles, the accelerated particles, then bang into nearby other particles of the plasma, 289 00:29:40,010 --> 00:29:41,480 and the plasma becomes heated, 290 00:29:41,900 --> 00:29:49,550 and soon the region above and below the disc becomes too hot to be confined by the gravitational field of the disc in the black hole. 291 00:29:49,670 --> 00:29:59,030 So you get hot regions generating in the in the you get very hot plasma forming in the region above and below the disc. 292 00:29:59,360 --> 00:30:07,459 Just as in the sun. In the sun the temperature rises by orders of magnitude from the temperature underneath here, 293 00:30:07,460 --> 00:30:09,950 which is rather dark because we're looking at ultraviolet radiation, 294 00:30:10,670 --> 00:30:17,030 which is actually the photosphere the the part of the what what we with with our naked eye see of the sun. 295 00:30:18,520 --> 00:30:23,050 The temperature rises to about 100,000 degrees significantly above the surface of the sun. 296 00:30:23,320 --> 00:30:27,790 The sun can't contain that hot plasma and it flows away from the sun in the solar wind. 297 00:30:28,060 --> 00:30:31,600 The same thing must happen off of the two surfaces of an accretion disk. 298 00:30:32,170 --> 00:30:43,510 The next thing the magnetism does for you is arrange that this flow perpendicular away from the accretion disk is culminated. 299 00:30:43,540 --> 00:30:51,939 Bye bye. Physics that we don't fully understand, but is certainly to do with magnetic pressures of this magnetic field contained in this material, 300 00:30:51,940 --> 00:30:55,930 which is which is too hot, too hot, too hot to handle, too hot to contain. 301 00:30:55,990 --> 00:31:01,690 It flows away from the accretion disk and as it flows away from the accretion disk, the field lines tend to be stretched. 302 00:31:02,620 --> 00:31:05,620 And also this material, of course, is rotating. 303 00:31:05,620 --> 00:31:12,600 Everything is rotating with the accretion disk. So this stuff gets the field lines get pulled out somehow like this into some kind of a spiral. 304 00:31:12,610 --> 00:31:16,839 This is not properly understood. I mean, it's we can't exactly model it, 305 00:31:16,840 --> 00:31:24,639 but we're pretty sure this is what happens so that the flow off the disk becomes not a sort of quasi spherical flow, 306 00:31:24,640 --> 00:31:26,680 like the flow out of the sun to the solar wind. 307 00:31:26,860 --> 00:31:33,460 It becomes a strongly correlated wind that runs along the spin axis, the two along the spin axis of the disk in the two directions. 308 00:31:35,260 --> 00:31:39,550 So it's an amazing fact. It's a it's an empirical fact that this. 309 00:31:40,710 --> 00:31:47,790 I mean, this qualitative discussion illustrates the decreasing objects are actually detectable not by evidence of inflow. 310 00:31:47,790 --> 00:31:51,120 It's extremely hard to find any evidence of inflow in any accreting object. 311 00:31:51,300 --> 00:31:54,270 What is always very evident is outflow. 312 00:31:54,600 --> 00:32:01,860 So here is here is a protostar sitting inside here you can't see it because it's obscured by the accretion disk. 313 00:32:01,860 --> 00:32:05,250 There's a protostar, a young star, so as the sun once was, 314 00:32:05,910 --> 00:32:11,760 which is accreting material from a disk which you don't see properly, what you see is its illuminated surfaces here. 315 00:32:11,760 --> 00:32:15,659 So the protostar light coming out of the protostar, which doesn't come directly to us, 316 00:32:15,660 --> 00:32:19,710 it scatters off material above and below the accretion disk here and comes to us. 317 00:32:20,460 --> 00:32:28,980 So here is this accreting protostar is this Protostar with this accretion disk and here is here is a jet of material that's flowing at something. 318 00:32:29,610 --> 00:32:34,440 What's typical this is this is a hub here. Objects. That's what these things are, these accreting protostars. 319 00:32:35,070 --> 00:32:38,100 This stuff is coming off at maybe a couple of hundred kilometres a second. 320 00:32:38,100 --> 00:32:41,100 It's quite cold. This is the ordered outflow from the system. 321 00:32:41,100 --> 00:32:44,280 So this is just a boring protostar nothing relativistic. 322 00:32:45,580 --> 00:32:56,500 Here is this is SS four, three, three, which is an accreting stellar mass black hole, which is in a binary star. 323 00:32:56,500 --> 00:32:59,740 So here's the binary companion, here's the black hole, here's the accretion disk. 324 00:33:00,010 --> 00:33:07,180 And this thing is shooting out at about the third of the speed of light gas, which is quite cold. 325 00:33:07,180 --> 00:33:13,809 It's it's cold enough to form to contain hydrogen atoms which emit HL for radiation. 326 00:33:13,810 --> 00:33:15,700 That's how this thing was detected originally. 327 00:33:16,690 --> 00:33:26,350 So it's shooting out these jets and then this disk is processing in the because it senses the gravitational field of this neighbour. 328 00:33:26,590 --> 00:33:32,110 So the spin axis of this system is, is constantly processing, moving around the cone. 329 00:33:32,320 --> 00:33:37,030 And as it moves around a cone, you get this, you get this pattern form. 330 00:33:37,030 --> 00:33:43,240 This is a radio picture. This is this is radio radiation from relativistic electrons. 331 00:33:43,540 --> 00:33:51,760 But the point is that this is this the at the time that material was shot out, the jet was looking in this direction. 332 00:33:51,970 --> 00:33:55,540 At the time that this was shot out, the jet was looking in this direction. 333 00:33:55,540 --> 00:34:03,040 As the as the jet moves around and the material moves away from the jet, you get this characteristic corkscrew effect. 334 00:34:03,040 --> 00:34:08,560 So this is this is called a emitted emission from a accreting soda mass black hole. 335 00:34:08,770 --> 00:34:13,239 And this is column is emission from an accreting galactic mass black hole. 336 00:34:13,240 --> 00:34:19,180 This is this is in the radio. So Cygnus A the scale of this thing is bigger than the scale of a galaxy. 337 00:34:19,180 --> 00:34:22,629 It's something like 50 killer parsecs somewhere inside here, 338 00:34:22,630 --> 00:34:28,300 way too small to be seen to be strictly speaking, seen there is the there is the accreting black hole. 339 00:34:28,300 --> 00:34:36,010 And then there are these two two jets which have different intensities, largely because they're moving mildly relativistic. 340 00:34:37,520 --> 00:34:42,620 So. So these accreting objects manifest themselves by. 341 00:34:43,970 --> 00:34:48,140 To a large extent. I mean, what makes them really clear is that they're spewing stuff out in a jet. 342 00:34:48,710 --> 00:34:54,440 Once you recognise that these jets exist, you have to completely revisit your picture of how an accretion disk works. 343 00:34:55,100 --> 00:35:01,790 Because the picture I was the baseline picture I was peddling was that you had mass going in, 344 00:35:02,330 --> 00:35:06,830 angular momentum going out and radiation only going perpendicularly to the disk. 345 00:35:07,100 --> 00:35:08,780 But now that's not at all what happens, 346 00:35:08,780 --> 00:35:16,819 what your picture has to be that at every radius there is material boiled off the surface of the accretion disk. 347 00:35:16,820 --> 00:35:25,130 And as that material goes out, it's carrying mass, it's carrying energy in the flow, mechanical energy, kinetic energy of the flow. 348 00:35:25,580 --> 00:35:32,809 And it is carrying angular momentum because this stuff isn't flowing purely away from the it's not only moving down the spin axis, 349 00:35:32,810 --> 00:35:34,850 it's also moving around the spin axis, 350 00:35:35,350 --> 00:35:41,990 not moving away from the spin axis because the magnetic field is is wrapping around it and forcing it back onto the spin axis. 351 00:35:42,110 --> 00:35:46,130 So it's a it's a it's a twisting it's a corkscrew thing. 352 00:35:46,610 --> 00:35:52,220 So we're losing mass energy and angular momentum from the surface of the disc at every radius. 353 00:35:52,820 --> 00:35:57,020 So this so so there's an advection of mass and angular momentum inwards. 354 00:35:57,740 --> 00:36:04,550 There is a viscous transport of angular momentum outwards, and there is sort of a loss of angle momentum here. 355 00:36:05,840 --> 00:36:13,969 So what's the consequence of all this? Well, one consequence is that what makes it into the black hole is only can be can 356 00:36:13,970 --> 00:36:18,140 potentially be only a small fraction of what's dumped on the disc at a large radius, 357 00:36:18,650 --> 00:36:22,640 because much of what's dumped at large radius can be shot out in the jet. 358 00:36:24,110 --> 00:36:32,690 So since it's sort of a piece of self sacrifice, a small amount of mass sacrifices itself in all the order that many much mass maybe free. 359 00:36:34,890 --> 00:36:40,890 And there's a flow of there's still a vicious flow of angular momentum outwards, but there's also this loss of angular momentum here. 360 00:36:42,000 --> 00:36:53,730 The luminosity of the disk is going to be smaller than previously advertised because because the for the mass that 361 00:36:53,940 --> 00:37:00,690 joins the energy released by the mass that joins the black hole is not only radiated from the surface of the disk, 362 00:37:00,930 --> 00:37:06,240 it's also pumped into the, uh, into the jet. 363 00:37:06,480 --> 00:37:12,240 So the system has acquired a mechanical power output in addition to its luminous power output. 364 00:37:13,380 --> 00:37:22,020 So the luminous power output will be smaller per unit accretion per unit mass added to the black hole than we previously estimated. 365 00:37:24,720 --> 00:37:30,630 Okay. So we're now very much guided by observations because our ability to compute this stuff is very limited. 366 00:37:31,650 --> 00:37:39,440 The observations indicate that the mechanical power can can insert and can quite often significantly exceed the radiative power, 367 00:37:39,450 --> 00:37:45,300 so less energy can be radiated from the disk surface than is pumped into the jet. 368 00:37:45,510 --> 00:37:48,090 This jet production process could be very, very efficient. 369 00:37:48,720 --> 00:37:57,030 Also very interesting is that systems can switch rapidly between modes in which most of the power goes into mechanical output. 370 00:37:57,030 --> 00:38:00,660 And a lot of the power is is is relatively. 371 00:38:02,020 --> 00:38:07,020 Produced. And you can you can study this. 372 00:38:07,020 --> 00:38:14,759 What this sort of switching process has to be studied mostly has to be studied in the context of Sotomayor's black holes, 373 00:38:14,760 --> 00:38:16,480 objects like SS four, three, three. 374 00:38:17,010 --> 00:38:25,979 Because the time, the characteristic timescale of a black hole is proportional to its mass right it schwarzschild radius is proportional to its mass. 375 00:38:25,980 --> 00:38:31,680 The speed of light is always the same. The characteristic timescale is the structural radius divided by the speed of light. 376 00:38:32,190 --> 00:38:37,019 So an object like sigma say that I showed you a picture of an accreting galactic 377 00:38:37,020 --> 00:38:47,009 nucleus has a achieves or changes in 100 mega years by as much as we expect this four, 378 00:38:47,010 --> 00:38:50,940 three, three to change by in maybe a couple of years. 379 00:38:52,750 --> 00:39:01,260 So if you want to study, if you want to understand the time variability of black holes, you want to focus on these micro quasars. 380 00:39:01,270 --> 00:39:04,810 They're called objects like this four through three solar mass accreting black holes. 381 00:39:05,080 --> 00:39:10,270 And they tell us that that can be this rapid switching between the relatively in the mechanically efficient mode. 382 00:39:11,910 --> 00:39:16,810 So finally, very finally, black holes and star formation. 383 00:39:18,480 --> 00:39:24,540 This is a plot of the estimates of the rate at which stars have formed in logarithmic scale. 384 00:39:24,540 --> 00:39:27,540 The rate at which stars are formed is a function of redshift in the past. 385 00:39:27,540 --> 00:39:34,200 So it rose until redshift of two and has fallen steeply since then by a factor of more than ten. 386 00:39:35,650 --> 00:39:42,160 This down here is a plot which reproduces this is this curve here is fitted to the data points. 387 00:39:42,400 --> 00:39:49,520 This black curve is the same is the same curve shown on the same plot of redshift versus a rate. 388 00:39:50,140 --> 00:39:54,740 So the masses per year sort of thing per megaparsec. Per cubic megaparsec. 389 00:39:55,280 --> 00:39:57,770 And then also on this diagram on here, 390 00:39:57,950 --> 00:40:06,940 MADDOW and Dickinson have added the an estimate of the rate at which black holes have been accreting obtained from X-ray radiation. 391 00:40:06,950 --> 00:40:14,360 That's the black curve, except that they've multiplied they've scaled the actual measurements by a factor of a thousand. 392 00:40:14,750 --> 00:40:23,960 Remember, John said that the masses of black holes in the middle of galaxies are characteristically 1000/1000 of the mass of the stars in the galaxy. 393 00:40:24,470 --> 00:40:30,500 So here we see a very concrete indication that black hole, well, 394 00:40:30,710 --> 00:40:40,160 this this rate of accretion by black holes to within the errors is pretty much the same as the as the estimate of the rate of the formation of stars. 395 00:40:40,160 --> 00:40:43,250 So this suggests that black holes, these black holes have grown. 396 00:40:45,640 --> 00:40:52,050 Uh, uh, on the same time trajectory as the stars are formed. 397 00:40:53,290 --> 00:41:03,099 First rising, then falling. And that's presumably because the the the black hole accretion in the radiative li efficient mode that we see 398 00:41:03,100 --> 00:41:09,370 with the quasars is what's being plotted here depends upon heating having a high availability of cold gas. 399 00:41:09,580 --> 00:41:12,760 Star formation depends on the high availability of cold gas. 400 00:41:13,480 --> 00:41:22,060 And what we're looking here is a plot of the availability of cold gas suitable for either making stars or making a nuclear black hole. 401 00:41:22,270 --> 00:41:26,900 Both things happening simultaneously. But why? 402 00:41:26,960 --> 00:41:32,000 So why has this fallen? It's fallen because the availability of cold gas has decreased. 403 00:41:32,330 --> 00:41:35,900 Is that because the availability of gas is decreased? Not a bit of it. 404 00:41:36,320 --> 00:41:42,920 The universe has made very little progress at turning gas into stars and galaxies. 405 00:41:43,130 --> 00:41:47,300 Most of the universe's ordinary matter lies in the intergalactic medium, 406 00:41:47,420 --> 00:41:54,470 between galaxies and in clusters of galaxies like this one here, the Virgo cluster imaged in X-rays. 407 00:41:54,650 --> 00:41:57,770 We see this. This intergalactic gas is dense enough. 408 00:41:57,770 --> 00:42:02,030 We can actually see it by the thermal X-ray emission that it produces. 409 00:42:04,070 --> 00:42:08,270 And what you find here is that in these so-called cool, cool clusters, 410 00:42:08,270 --> 00:42:12,770 which are the majority of clusters of galaxies, is the temperature of the X-rays falls, 411 00:42:12,770 --> 00:42:18,919 the temperature of the gases emitting the X-rays falls by about a factor three between sort of this zone here typical zone. 412 00:42:18,920 --> 00:42:24,770 And, and as you approach the, uh, the black hole which sits in the middle of m87 in the middle there. 413 00:42:24,770 --> 00:42:28,820 So as you, as you move inwards, the temperature of the gas falls by about a factor of three. 414 00:42:29,060 --> 00:42:34,250 The density of the gas zooms up enormously because it's compressed by extra gravitational field. 415 00:42:35,630 --> 00:42:44,480 And the time in the time required for the gas in here to cool to absolute zero by radiating X-rays and then optical UV light. 416 00:42:44,480 --> 00:42:48,610 And the optical light is much shorter than the age of the universe. You might say so. 417 00:42:48,690 --> 00:42:57,979 So when people first saw this in the 1970s, late 1980s, around 1980, they assumed that there was a steady flow of material from the outside in. 418 00:42:57,980 --> 00:43:02,960 It got cooler and cooler and cooler and then eventually formed stars or something, or maybe was accreted by the black hole. 419 00:43:03,650 --> 00:43:07,400 Well, we now know that's not the case because the temperature drops by about a factor of three. 420 00:43:07,760 --> 00:43:14,300 But there are there's an absence of emission lines coming from material that's cooler than, say, a factor of three. 421 00:43:14,540 --> 00:43:19,160 So you don't see loads of material at temperatures of a million degrees K or less. 422 00:43:20,510 --> 00:43:24,950 So the material is not succeeding in cooling. It is radiating, but it is not cooling. 423 00:43:24,950 --> 00:43:30,049 Why is it not cooling? This is a radio map of the same galaxy. 424 00:43:30,050 --> 00:43:36,560 So this is what? This is what the Virgo cluster of galaxies looks like in synchrotron radiation produced by relativistic electrons. 425 00:43:36,560 --> 00:43:37,820 This is a large scale picture. 426 00:43:38,450 --> 00:43:47,929 There's a jet coming out, it's banging into the intergalactic medium, it at some 1020 killer parsec distances and and dumping energy in it. 427 00:43:47,930 --> 00:43:52,760 This is here we have a series of beautiful blow up. So what you can do with radio interferometry. 428 00:43:53,030 --> 00:43:59,990 So you you see on extremely small scales this is the jet coming out of the black hole at the centre of Messier 87, 429 00:44:00,170 --> 00:44:06,970 which has a mass of nearly 5 billion solar masses. It's a really it's a really beefy black hole in it is draw. 430 00:44:06,980 --> 00:44:10,910 It is what what is this? It's not a very luminous source. 431 00:44:10,910 --> 00:44:19,290 It is not a quasar. So it's this black hole has switched into into its mechanical mode. 432 00:44:19,770 --> 00:44:24,149 It's putting out energy mechanically, which we can trace by looking at this synchrotron radiation. 433 00:44:24,150 --> 00:44:29,370 This mechanical energy is being carried to regions of relatively low density by the jets. 434 00:44:29,580 --> 00:44:36,090 And there is is heating up the surrounding plasma and preventing that surrounding plasma cooling below 435 00:44:36,090 --> 00:44:44,610 a million degrees C because he can't cool below a million degrees C it can't form stars and it's. 436 00:44:46,090 --> 00:44:53,980 The rate at which this black hole accretes material should increase steeply with declining temperature at the centres. 437 00:44:54,010 --> 00:44:57,670 This is a very stable process if the gas in the middle gets a little bit cooler. 438 00:44:58,090 --> 00:45:01,959 The rate of of accretion by the black hole should go up strongly. 439 00:45:01,960 --> 00:45:04,630 It should go like the minus five halves power of the temperature. 440 00:45:05,520 --> 00:45:13,329 The the that means that the jets will ramp up to greater power, keep the gas in the middle. 441 00:45:13,330 --> 00:45:20,530 And so you have a stabilisation process. I made the black hole in the middle of of m87 is stabilising this intergalactic gas in 442 00:45:20,530 --> 00:45:24,660 exactly the same way that the nuclear reactor at the middle of the sun stabilises the sun. 443 00:45:24,670 --> 00:45:31,510 If the middle of the sun is a bit denser, it heats up and the rate of the rate of nuclear reaction zooms up. 444 00:45:31,810 --> 00:45:35,940 That releases energy, which causes the middle of the sun to expand a bit. 445 00:45:35,950 --> 00:45:39,130 And so that sort of that's why the middle of the sun is a stable thing. 446 00:45:39,580 --> 00:45:47,739 So what's that? What's the bottom line on this? The bottom line on this is that is that it's these galactic centre black holes 447 00:45:47,740 --> 00:45:53,170 which are responsible for setting the upper limits on the masses of galaxies. 448 00:45:53,860 --> 00:46:03,729 So as the universe clustering within the universe is preceded, the the distributed, the number of dark matter halos is a function of mass. 449 00:46:03,730 --> 00:46:08,559 This is log masses is the number of logarithm of the number of dark matter halos has 450 00:46:08,560 --> 00:46:14,590 been a curve like this which has had which is moved to bigger and bigger masses. 451 00:46:14,590 --> 00:46:18,160 So at earlier times it was looking something like this and then something like this. 452 00:46:18,490 --> 00:46:22,770 So it's moved to bigger and bigger masses as the clustering has proceeded. 453 00:46:23,500 --> 00:46:29,680 And for a while galaxies tagged along, mirroring this growth of clustering. 454 00:46:30,040 --> 00:46:34,449 But then these black holes switch from their readily efficient mode to their to 455 00:46:34,450 --> 00:46:39,510 their readily radioactively inefficient and mechanically efficient mode and, 456 00:46:39,770 --> 00:46:47,230 and prevented the formation of galaxies with masses of ten to the 13, ten to the 14th, and ten to the 15 solar masses. 457 00:46:47,500 --> 00:46:55,900 So the so they have they have set their imprint on the universe as a whole through this mechanical process that we don't entirely understand. 458 00:46:56,440 --> 00:46:56,800 Thank you.