1 00:00:03,430 --> 00:00:07,752 We'll now talk about unification models for active galactic nuclei. 2 00:00:07,752 --> 00:00:12,985 They unify a lot of the observed [inaudible] in what's li-, what's somewhat 3 00:00:12,985 --> 00:00:19,028 simpler picture of what's going on. The, the basic idea is that the structure 4 00:00:19,028 --> 00:00:24,500 of the active nucleus is as follows. There is of course the big, black hole in 5 00:00:24,500 --> 00:00:29,210 the middle that provides binding energy. There is an accretion disc of material 6 00:00:29,210 --> 00:00:33,332 that falls in from which a lot of continuum luminosity will come from. 7 00:00:33,332 --> 00:00:39,650 The ultraviolet radiation from the accretion disc will ionize the gas clouds 8 00:00:39,650 --> 00:00:43,637 near the black hole. The broad emmision line clouds which are 9 00:00:43,637 --> 00:00:47,992 close to it and moving fast, and then the narrow emmision line clouds which are 10 00:00:47,992 --> 00:00:52,515 little further away. But somewhere in plain of the accretion 11 00:00:52,515 --> 00:00:58,851 disc or close to it, there should be a torus of dust or maybe a warped disk which 12 00:00:58,851 --> 00:01:05,200 obscures the central engine itself from the equatorial plane, more or less. 13 00:01:05,200 --> 00:01:11,341 And does if you look perpendicular to it, you will see right into the central engine 14 00:01:11,341 --> 00:01:15,630 and you'll see broad line clouds, the continuum source. 15 00:01:15,630 --> 00:01:20,454 But if you look from the side you may only see the narrow emission line region. 16 00:01:20,455 --> 00:01:26,090 Because it's much bigger physically than the other two, and it's only partly hidden 17 00:01:26,090 --> 00:01:30,272 by the dust. This picture explains a remarkable number 18 00:01:30,272 --> 00:01:34,808 of observations. And in this regard, if you'll look closer 19 00:01:34,808 --> 00:01:39,670 to the axis of the system, you'll see broad lines in continuum. 20 00:01:39,670 --> 00:01:43,894 So those would be Seyfer type on our traditional quasars or broad emission line 21 00:01:43,894 --> 00:01:48,080 galaxies. If you look more from the torus side, 22 00:01:48,080 --> 00:01:53,775 you'll see the Seyfert 2's, or narrow emission line objects, or narrow emission 23 00:01:53,775 --> 00:01:58,622 line radio galaxies. So at some level, this is almost certainly 24 00:01:58,622 --> 00:02:06,420 correct, with a, in a broad brush sense. There could be, however, some genuine 25 00:02:06,420 --> 00:02:12,354 differences, even within a given orientation. 26 00:02:12,354 --> 00:02:17,540 And that's subject of some research today, but big picture seems to be pretty much in 27 00:02:17,540 --> 00:02:21,410 place. So let's look at this in a little more 28 00:02:21,410 --> 00:02:24,870 detail. The, probably the most secure unification 29 00:02:24,870 --> 00:02:29,848 is for the Seyfert galaxies themselves. Recall that Seyfert ones are those where 30 00:02:29,848 --> 00:02:32,610 we see the bright continuum and broad emission line. 31 00:02:32,610 --> 00:02:36,227 Seyfert twos you only see the narrow emission lines. 32 00:02:36,228 --> 00:02:41,654 So look at the picture like this. If you look from the side. 33 00:02:41,655 --> 00:02:44,700 You will only see the narrow emission line clouds. 34 00:02:44,700 --> 00:02:47,576 You will, the broad line region will be hidden. 35 00:02:47,576 --> 00:02:53,350 The contiuum will be hidden. But the light from the continuum and broad 36 00:02:53,350 --> 00:02:58,919 emission lines will be scattered from free electrons or dust clouds. 37 00:03:00,730 --> 00:03:06,558 Above or below the accretion disk and some of that would be scattered in the 38 00:03:06,558 --> 00:03:11,372 direction of the torus. That is, if we're looking from the side 39 00:03:11,372 --> 00:03:17,196 we'll still see some of that scattered light and so that opens up a severity of 40 00:03:17,196 --> 00:03:21,679 detecting the hidden emission from reflected radiation. 41 00:03:21,680 --> 00:03:26,976 But how can we tell? The key feature here is that, that 42 00:03:26,976 --> 00:03:34,195 reflected radiation would be polarized. And if you take spectrum of a Seyfert 2 43 00:03:34,195 --> 00:03:40,723 galaxy but using polarization filter, you will see that in polarized light broad 44 00:03:40,723 --> 00:03:47,053 emission lines make an appearance. So that's the reflected broad emission 45 00:03:47,053 --> 00:03:52,876 line light from electrons and or dust. This directly pretty much shows that the 46 00:03:52,876 --> 00:03:56,980 unification picture is. Largely correct, at least for Seyfert 47 00:03:56,980 --> 00:04:00,356 galaxies. But by extension you also apply to more 48 00:04:00,356 --> 00:04:06,940 luminous active nuclei like quasars. It took little while to discover type two 49 00:04:06,940 --> 00:04:13,412 quasars but we do have them now. Here is the same case of nearby Seyfert 50 00:04:13,412 --> 00:04:20,516 galaxy and you see 1068, the picture on the right shows polarization vectors 51 00:04:20,516 --> 00:04:24,797 superposed on Hubble Space Telescope image. 52 00:04:24,798 --> 00:04:29,298 One could take images through polarizing filters and by rotating them find the 53 00:04:29,298 --> 00:04:33,922 orientation polarization in their length is the relative fraction of polarized 54 00:04:33,922 --> 00:04:38,833 light compared to the total. And you can see that there is very nice 55 00:04:38,833 --> 00:04:45,631 systematic behavior of like polarized orthagonal to what likes the principle 56 00:04:45,631 --> 00:04:51,242 axis of the system. In addition to polarization we should be 57 00:04:51,242 --> 00:04:58,430 able to see dual cones of ionized gas. The gas along the torus plane will be 58 00:04:58,430 --> 00:05:04,060 shielded from ionizing radiation. Accretion disk. 59 00:05:04,060 --> 00:05:09,734 But the gas in the opening angles will see it, and it will become more ionized. 60 00:05:09,734 --> 00:05:15,410 And indeed Hubble Space Telescope images of some of the nearby Seyfert galaxies 61 00:05:15,410 --> 00:05:20,369 show ionization cones of ionized gas, just like this picture predicts. 62 00:05:23,140 --> 00:05:29,278 Here is a more detailed portrait of the Seyfert galaxy, NGCN68, which shows the 63 00:05:29,278 --> 00:05:35,416 ionization cones as well as the radio observations, so there is a radio jet that 64 00:05:35,416 --> 00:05:41,073 goes in the same direction as the opening angle of the obscuring torus. 65 00:05:41,074 --> 00:05:46,672 And so in the simplest sense the AGN unification scheme works as follows. 66 00:05:46,673 --> 00:05:53,184 The obscured, active nuclei, Seyfert 2's or type 2 quasars or narrow lined radio 67 00:05:53,184 --> 00:05:59,264 galaxies, in the un-obscured one of Seyfert 1's, regular quasars, or but are 68 00:05:59,264 --> 00:06:03,721 not that radio loud. And blazars are a special type where you 69 00:06:03,721 --> 00:06:09,481 look straight down the radio jet and that creates lot of the observed phenomena. 70 00:06:09,482 --> 00:06:14,482 There are, however, some very low luminosity active nuclei where there is no 71 00:06:14,482 --> 00:06:18,430 obscuration and yet there is no broad emission line region. 72 00:06:18,430 --> 00:06:25,910 So there, more complex things are going on, and there are theoretical models that 73 00:06:25,910 --> 00:06:29,842 explain those. One big question is, why are some of them 74 00:06:29,842 --> 00:06:35,228 radio loud and others are not? And the explanation almost certainly has 75 00:06:35,228 --> 00:06:41,728 to do with the spin of the black hole, and the basic picture is that the rotating 76 00:06:41,728 --> 00:06:48,228 ones have preferred axis which can serve as axis along which magnetic field is 77 00:06:48,228 --> 00:06:53,912 wound, from which radio jet can emerge. So the radio louds would have relatively 78 00:06:53,912 --> 00:06:57,400 low spin parameters, defined here in the lower right corner. 79 00:06:57,400 --> 00:07:00,812 We'll talk a little more about this in the next module. 80 00:07:00,813 --> 00:07:07,357 And the radio quiet ones have a spin that's much less than a unit is required 81 00:07:07,357 --> 00:07:14,737 for a, for a highly rotating black hole. This does explain a lot observed 82 00:07:14,737 --> 00:07:18,814 phenomenon. But then the question then is also how did 83 00:07:18,814 --> 00:07:22,607 black holes actual acquire angular momentum. 84 00:07:22,608 --> 00:07:27,952 And that's not as simple as it seems because the material that gets acreted 85 00:07:27,952 --> 00:07:32,890 that causes black holes to grow Has to be at almost the radial orbits. 86 00:07:32,890 --> 00:07:38,074 Otherwise it will simply miss the black hole, be in some elliptical orbit and 87 00:07:38,074 --> 00:07:41,832 never get in. So most of the material has to fall right 88 00:07:41,832 --> 00:07:47,292 on a radial orbit because rip black hole is such a small target and therefore it 89 00:07:47,292 --> 00:07:50,834 will have no angular momentum to speak of so. 90 00:07:50,835 --> 00:07:55,858 Black holes cannot acquire much if any angular momentum through accretion. 91 00:07:55,858 --> 00:08:01,222 So, then how did they get it? Well, one possibility is mergers of black 92 00:08:01,222 --> 00:08:04,054 holes. And we think that, that does happen 93 00:08:04,054 --> 00:08:08,852 through hierarchical structure formation. And then two black holes would orbit each 94 00:08:08,852 --> 00:08:12,721 other, eventually they would merge because of the loss of energy through gravitaional 95 00:08:12,721 --> 00:08:15,920 waves. But their orbital angular momentum will 96 00:08:15,920 --> 00:08:21,414 then be absorbed by the resulting black hole, and that one will be spinning in the 97 00:08:21,414 --> 00:08:25,447 same direction of the two were going around each other. 98 00:08:25,448 --> 00:08:32,498 Next we will address the question of how does the, energy really gets generated and 99 00:08:32,498 --> 00:08:34,133 how does it get out?