1 00:00:03,350 --> 00:00:08,327 Let us now address the empirical Classification of Active Galactic Nuclei 2 00:00:08,327 --> 00:00:13,095 based under observed properties. In case you're wondering, the background 3 00:00:13,095 --> 00:00:17,587 image here are Darvin's finches. So we can classify active nuclei according 4 00:00:17,587 --> 00:00:21,020 to their information. First of all whether or not they're Radio 5 00:00:21,020 --> 00:00:23,170 loud. This was how first quasars were 6 00:00:23,170 --> 00:00:27,232 discovered. Within the Radio loud category they come 7 00:00:27,232 --> 00:00:34,442 in two varieties, so-called Fanaroff-Riley types one and two which I'll define in a 8 00:00:34,442 --> 00:00:37,600 bit. And many of the Radio quiet ones are not 9 00:00:37,600 --> 00:00:41,264 really Radio silent. They're just not as Radio loud as those 10 00:00:41,264 --> 00:00:46,050 that we call Radio loud. Now according to the optical spectrum we 11 00:00:46,050 --> 00:00:51,562 can divide them by whether or not they show broad emission lines. 12 00:00:51,562 --> 00:00:57,021 They always do show narrow emission lines and those that have broad emission lines 13 00:00:57,021 --> 00:01:02,156 are called type 1 versus those that are called, with narrow emission lines are 14 00:01:02,156 --> 00:01:05,825 called type 2. They all does we talk about Seyfert 15 00:01:05,825 --> 00:01:10,538 galaxies of type 1 or type 2 and also quasars of type 1 and type 2. 16 00:01:10,538 --> 00:01:16,719 Luminosity is an obvious physical property to look at and there we can follow from 17 00:01:16,719 --> 00:01:22,374 the lower luminosity objects such as Seyfert galaxies to the higher ones like 18 00:01:22,374 --> 00:01:26,539 quasars. There are also special subtypes, for 19 00:01:26,539 --> 00:01:32,069 example, the so called Blazars or BL Lac's which are, Radio loud quasars where we're 20 00:01:32,069 --> 00:01:37,357 looking right down the relativistic jet and that causes strong variability. 21 00:01:37,358 --> 00:01:42,810 So these classification schemes are all very emperical and largely parallel and 22 00:01:42,810 --> 00:01:48,330 they do reflect some of the, intrinsic physical characteristics of these sources 23 00:01:48,330 --> 00:01:52,196 but not necessarily always. And, some depend on the orientation from 24 00:01:52,196 --> 00:01:56,507 which we're looking at. Seyferts first they are essentially low 25 00:01:56,507 --> 00:02:02,287 luminosity equivalent of quasars and they've been noted over 100 years ago as 26 00:02:02,287 --> 00:02:07,892 luminous nuclei of some nearby galaxies. And even had spectra that nobody 27 00:02:07,892 --> 00:02:12,002 understood at the time. This was largely neglected until the 28 00:02:12,002 --> 00:02:16,754 1940's where Carl Seyfert, after whom these galaxies are now named, really did 29 00:02:16,754 --> 00:02:21,919 the first systematic study, but even then, it wasn't clear what was going on. 30 00:02:21,920 --> 00:02:27,680 And somehow that was neglected by the time Quasars came on the scene and, even though 31 00:02:27,680 --> 00:02:31,819 quasar spectra look like Seyfert galaxy spectra only more so. 32 00:02:31,819 --> 00:02:35,507 Somehow that connection was not made at first. 33 00:02:35,508 --> 00:02:39,900 Here are a couple pictures of Seyfert galaxy seem based on they tend to be 34 00:02:39,900 --> 00:02:44,084 Spiral galaxies. And they have a very luminous, bright 35 00:02:44,084 --> 00:02:49,628 nucleus which in saturates look like a really bright star smack in the middle of 36 00:02:49,628 --> 00:02:53,742 a spiral galaxy. Nearby maybe 10% of early type spirals 37 00:02:53,742 --> 00:02:59,087 contain these nuclei, a smaller fraction for the later type spirals. 38 00:02:59,087 --> 00:03:03,880 They tend not to have much in terms of radio emission but they can have some. 39 00:03:03,880 --> 00:03:08,845 And they also have moderate x-ray emission just like luminous quasars do. 40 00:03:08,845 --> 00:03:15,483 Quasar themselves out shine their horse galaxies by great factor, maybe by a 41 00:03:15,483 --> 00:03:19,553 factor of 1000. And this is why they're called Quasars. 42 00:03:19,553 --> 00:03:23,765 Quasi stellar objects. Even with the Hubble Space Telescope it's 43 00:03:23,765 --> 00:03:28,948 sometimes difficult to discern their host galaxies because the central source is so 44 00:03:28,948 --> 00:03:31,468 bright. But nevertheless sometimes we can see 45 00:03:31,468 --> 00:03:33,765 that. There are sometimes Spiral galaxy hosts, 46 00:03:33,765 --> 00:03:36,412 but more often they tend to be in Elliptical galaxies. 47 00:03:36,412 --> 00:03:41,669 Also these images often show morphology that's characteristic of galaxy mergers 48 00:03:41,669 --> 00:03:45,150 that we've seen earlier. Tidal bridges, and tails and so on. 49 00:03:45,150 --> 00:03:50,482 And we believe that this is not an accident, that Quasar activity is fueled 50 00:03:50,482 --> 00:03:56,304 by galaxy encounters in a similiar way as the starbursts can be triggered. 51 00:03:56,305 --> 00:04:02,500 So here are the spectra of Seyfert one Galaxies and Quasars are very similar to 52 00:04:02,500 --> 00:04:05,560 this. They have both Narrow and Broad emission 53 00:04:05,560 --> 00:04:08,838 lines. Broad emission lines tend to correspond to 54 00:04:08,838 --> 00:04:12,727 more standard transitions like balmer lines of hydrogen. 55 00:04:12,727 --> 00:04:18,945 Narrow emission lines tend to correspond to ionized gas that is not easily observed 56 00:04:18,945 --> 00:04:24,725 in laboratory conditions, for example doubly ionized oxygen lines, 5007 axtron 57 00:04:24,725 --> 00:04:30,420 are very prominent and almost impossible to do the lab, and you can see that there 58 00:04:30,420 --> 00:04:35,252 are other prominent elements neon carbon, magnesium, and so on. 59 00:04:35,253 --> 00:04:41,503 Type 1 Seyfert Galaxies also have bright continuum we can see right down towards 60 00:04:41,503 --> 00:04:45,695 the Accretion disc. In the Narrow line of type 2 Seyfert 61 00:04:45,695 --> 00:04:51,420 Galaxies the continuum is almost invisible but we still see emission lines. 62 00:04:51,420 --> 00:04:56,670 Now it is interesting to think like you can see emission lines from star forming 63 00:04:56,670 --> 00:04:59,920 regions as well, where young stars ionize the gas. 64 00:04:59,920 --> 00:05:05,938 So how can we tell them apart? And the answers from atomic physics that 65 00:05:05,938 --> 00:05:11,357 you can form line ratios that correspond to low and high ionized states. 66 00:05:11,357 --> 00:05:17,714 For some transitions, you have to ionize gas very heavily, a lot of high-energy 67 00:05:17,714 --> 00:05:20,511 photons. And sometimes stars, even the most 68 00:05:20,511 --> 00:05:23,705 luminating stars don't make photons of that energy. 69 00:05:23,706 --> 00:05:29,087 But yet, active nuclei do. And so in, in diagrams that show these 70 00:05:29,087 --> 00:05:33,258 kinds of Line ratios. You can draw a boundary that separates 71 00:05:33,258 --> 00:05:37,772 those separate, those powered by star formation versus those powered by 72 00:05:37,772 --> 00:05:41,880 Accretion to black holes. And this is how we can tell them apart. 73 00:05:41,880 --> 00:05:45,656 Radio galaxies were the first active nuclei that were really discovered and 74 00:05:45,656 --> 00:05:48,164 from the very beginning of radio astronomy. 75 00:05:48,165 --> 00:05:53,314 But first, he wasn't clear what his radio sources are, but then soon enough in 76 00:05:53,314 --> 00:05:58,710 1950's thanks to Baden, Glovsky and others optical counterparts have been found for 77 00:05:58,710 --> 00:06:04,070 some of the more luminous radio sources. And some of them corresponded to peculiar 78 00:06:04,070 --> 00:06:10,883 looking galaxies like Centaurus A. And the vertical of A which is now Fornax 79 00:06:10,883 --> 00:06:14,840 M87. Sometimes they're just some NGC Elliptical 80 00:06:14,840 --> 00:06:20,600 galaxy and what's shown here in color is the contrast of the Radio emission or 81 00:06:20,600 --> 00:06:25,939 intensity image of Radio emission superimposed on an optical image. 82 00:06:27,170 --> 00:06:32,798 You can see that often times there are two lobes and there is a host galaxy right in 83 00:06:32,798 --> 00:06:36,290 the middle. Now we know that this comes because a 84 00:06:36,290 --> 00:06:41,708 central, the nucleus of the galaxy itself contains a central engine and has 85 00:06:41,708 --> 00:06:47,201 relativistic jet that pumps these lobes. The lobes are powered by synchrotron 86 00:06:47,201 --> 00:06:52,505 emission and from the measurements of their brightness and some simple physics, 87 00:06:52,505 --> 00:06:57,653 it was inferred very early on that they contained phenomenal amounts of energy 88 00:06:57,653 --> 00:07:02,802 something like10 to the 60th or 10 to the 61 ergs in highly ionized plasma. 89 00:07:02,802 --> 00:07:06,680 So the question was, what deposited that energy in the Radio lobes? 90 00:07:06,680 --> 00:07:10,874 What's powering them? And it was understood very early on that 91 00:07:10,874 --> 00:07:15,120 black holes can be a viable mechanism to do that. 92 00:07:15,120 --> 00:07:20,190 Here is a more Modern Radio Map of Cygnus A, one of the classical powerful Radio 93 00:07:20,190 --> 00:07:25,416 sources, and you can see there is a Point like nucleus that does coincide with the 94 00:07:25,416 --> 00:07:29,617 center of the galaxy. There is a very clear Jet on both sides 95 00:07:29,617 --> 00:07:34,168 powering the Lobes. And these instabilities as the gas is 96 00:07:34,168 --> 00:07:39,544 being accelerated out of the active nucleus region encounters. 97 00:07:39,544 --> 00:07:43,860 Say, gas in the cluster that, surrounds this galaxy. 98 00:07:43,860 --> 00:07:48,932 So, many instabilities and so on. So we can study the morphology of Radio 99 00:07:48,932 --> 00:07:52,417 emission and learn more about physics of what goes on. 100 00:07:52,418 --> 00:07:57,470 There is a simple morphological Classification for Radio Sources, and 101 00:07:57,470 --> 00:08:02,930 those are done by two radio astronomers, called Fanaroff and Riley, and type 1 102 00:08:02,930 --> 00:08:08,474 sources tend to have prominent jets, they're brighter in the middle than to the 103 00:08:08,474 --> 00:08:14,102 sides and type 2 are low dominated sources and sometimes you don't see the Jets at 104 00:08:14,102 --> 00:08:19,657 all order central nucleus at all. They do correspond to physically distinct 105 00:08:19,658 --> 00:08:25,611 states FR1 types are probably younger radius sources, they're just beginning to 106 00:08:25,611 --> 00:08:29,577 pump the Radio lobes. On the other hand they can be also older 107 00:08:29,577 --> 00:08:34,007 but they couldn't develop Radio-louds because of their environment. 108 00:08:34,008 --> 00:08:40,162 Bl Lacs or Blazars are a subtype of radio-loud quasars. 109 00:08:40,163 --> 00:08:45,575 Bl Lac is a name for a variable star in fact quasars or other Blasars have been 110 00:08:45,575 --> 00:08:50,059 spotted in 1920s and they were given names of variable stars. 111 00:08:50,060 --> 00:08:53,576 Because nobody knew that they were a new phenomenon of nature. 112 00:08:53,577 --> 00:08:58,603 There's about a dozen of them that have variable star names but BL Lacertae is the 113 00:08:58,603 --> 00:09:03,553 proper type and, so, the whole subclass is now called Blazars which is BL Lac and 114 00:09:03,553 --> 00:09:06,894 quasar. Sometimes you can say look you could have 115 00:09:06,894 --> 00:09:11,395 gone the other way called them clocks but that's not how didn't catch. 116 00:09:11,395 --> 00:09:15,030 While the spectra are dominated by strong blue continuum. 117 00:09:15,030 --> 00:09:20,263 And this is the continuum that's from the accretion disk and the jet itself and, 118 00:09:20,263 --> 00:09:25,801 it's been Doppler-boosted, since the jet is moving at relativistic speed towards us 119 00:09:25,801 --> 00:09:31,261 and completely outshines the rest of the central engine, the broad line region, and 120 00:09:31,261 --> 00:09:36,140 so on. Sometimes the continuum goes down and then 121 00:09:36,140 --> 00:09:39,412 we can suddenly see the usual quazar spectra. 122 00:09:39,413 --> 00:09:43,639 So Blazars are essentially cosmic accelorators and we are in their beam, and 123 00:09:43,639 --> 00:09:47,639 that makes them useful for variety of studies that we'll mention later. 124 00:09:47,640 --> 00:09:53,426 They're also very variable. This is why they're confused with variable 125 00:09:53,426 --> 00:09:56,862 stars. Variability in their case comes from 126 00:09:56,862 --> 00:10:02,144 instabilities in the jet. Shock waves or the jet is encountering gas 127 00:10:02,144 --> 00:10:06,874 around the host galaxy and so on. And they're Doppler boosted, exe- 128 00:10:06,874 --> 00:10:11,462 amplified. Was a relativistic motion of plasma 129 00:10:11,462 --> 00:10:15,939 towards us. Our quasars in general do vary and most of 130 00:10:15,939 --> 00:10:22,237 that variation is due to the changes in the accretion as the fuel is dropping down 131 00:10:22,237 --> 00:10:26,230 onto the black hole. That's a differnt type of variablilty. 132 00:10:26,231 --> 00:10:30,891 In addition to that there's variability that's due to the relativisitc jets. 133 00:10:30,892 --> 00:10:34,821 Some. Some types of quasars have mixed or bolt 134 00:10:34,821 --> 00:10:40,563 usually those we call optically violent variables, changed by more than 10% in 135 00:10:40,563 --> 00:10:45,696 scope of, of a night or less and can change by factors of tens depending on 136 00:10:45,696 --> 00:10:49,750 which frequency. Sometimes those variations are correlated 137 00:10:49,750 --> 00:10:54,930 between different wavelengths, say optical and radio and gamma, and sometimes they're 138 00:10:54,930 --> 00:10:57,460 not. These correlations or absence thereof, 139 00:10:57,460 --> 00:11:01,320 tell us something about physics of what goes on inside these objects. 140 00:11:01,320 --> 00:11:09,068 Now what we'd really like to have is a pure Physical Classification of active 141 00:11:09,068 --> 00:11:11,727 nuclei. And the things that we can think about 142 00:11:11,727 --> 00:11:16,640 would be the black hole mass itself. The Accretion rate which is what gives the 143 00:11:16,640 --> 00:11:19,813 luminosity and the Angular momentum of the black hole. 144 00:11:19,813 --> 00:11:24,816 These things are not directly observable, but they can be inferred from various 145 00:11:24,816 --> 00:11:29,196 observations and there will be a luminosity sequence in this space with 146 00:11:29,196 --> 00:11:34,306 quasars and powerful radio sources being high luminosity end, Seyferts being a low 147 00:11:34,306 --> 00:11:38,880 luminosity end. The high luminosity sources certainly 148 00:11:38,880 --> 00:11:44,320 corresponding to objects with higher Accretion rates and probably larger black 149 00:11:44,320 --> 00:11:47,570 hole masses. And there is also this distinction between 150 00:11:47,570 --> 00:11:51,925 Radio-quiet and Radio-loud, and this is probably where Angular Momentum of the 151 00:11:51,925 --> 00:11:54,385 black hole comes in. But more about that later. 152 00:11:54,386 --> 00:12:02,968 Next we will talk about Unification schemes for active nuclei that put lot of 153 00:12:02,968 --> 00:12:11,558 these phenomenological observations into a fairly well-unified picture.