1 00:00:03,260 --> 00:00:09,137 So finally, we turn to the study of quasars and other active galactic nuclei. 2 00:00:09,138 --> 00:00:13,050 They represent very interesting phenomena and play important role in modern 3 00:00:13,050 --> 00:00:18,326 cosmology. To define them, these are energetic events 4 00:00:18,326 --> 00:00:24,728 in the course of certain galaxies. And we believe that energy is being 5 00:00:24,728 --> 00:00:30,848 released because of the accretion on black holes that sit there and the loss of the 6 00:00:30,848 --> 00:00:37,148 binding energy for material to get there produces the great release of energy from, 7 00:00:37,148 --> 00:00:41,044 that is observed. There are empirical classification schemes 8 00:00:41,044 --> 00:00:44,950 that describe the properties of active nuclei, and we'll go through these 9 00:00:44,950 --> 00:00:49,035 shortly. And more recently, there have been an 10 00:00:49,035 --> 00:00:54,985 invocation scheme design to explain how various aspects of observe phenomenology 11 00:00:54,985 --> 00:00:59,700 may be really the same thing but observed from different angles. 12 00:00:59,700 --> 00:01:04,710 One interesting about quasars is that they evolved very strongly in time. 13 00:01:04,710 --> 00:01:10,670 As we go deeper in redshift, the numbers increase dramatically between here now and 14 00:01:10,670 --> 00:01:15,277 say redshift of 2 by about a factor of thousand per pummeling volume. 15 00:01:15,278 --> 00:01:21,478 Today, maybe 1 3rd of all galaxies show some sort of non-thermal activity in their 16 00:01:21,478 --> 00:01:24,770 cores due to the creation onto a black hole. 17 00:01:24,771 --> 00:01:30,785 Most of that is fairly luminosity. Maybe one galaxy in 10 million locally has 18 00:01:30,785 --> 00:01:37,267 really a quasar like luminosity, but many have much weaker but still active nuclei. 19 00:01:37,268 --> 00:01:41,952 And as we already hinted at, when we talked about elliptical galaxies, we now 20 00:01:41,952 --> 00:01:46,843 think that most large galaxies that is non-[UNKNOWN] galaxies do harbor a super 21 00:01:46,843 --> 00:01:51,442 massive black hole in their centers, and their formation is closely tied to 22 00:01:51,442 --> 00:01:56,040 formation galaxies themselves. So, here is a cartoon version of what we 23 00:01:56,040 --> 00:02:01,180 think an active nucleus looks like. In the middle, there is a massive black 24 00:02:01,180 --> 00:02:03,866 hole. Surrounding it is an accretion disk of 25 00:02:03,866 --> 00:02:07,946 material that has fallen down the potential well and it settle into this 26 00:02:07,946 --> 00:02:12,570 because it still has some angular momentum even though its lost much energy, from 27 00:02:12,570 --> 00:02:15,388 which it little bit slowly into the black hole. 28 00:02:15,388 --> 00:02:21,352 The energy that's been released comes from binding energy of material that came from 29 00:02:21,352 --> 00:02:25,653 large distances to the nearest proximity of the black hole. 30 00:02:25,653 --> 00:02:29,204 Usually, the black holes seem to be rotating. 31 00:02:29,204 --> 00:02:34,562 And if there is any magnetic field involved, that will create a jet of 32 00:02:34,562 --> 00:02:38,363 material perpendicular to the accretion disk. 33 00:02:38,363 --> 00:02:44,014 The jet comes because the magnetic field gets wound tight and accelerates the 34 00:02:44,014 --> 00:02:49,409 ionized gas and electrons through relativistic speeds, essentially as a 35 00:02:49,409 --> 00:02:54,186 cosmic accelerator. Surrounding the central engine, from which 36 00:02:54,186 --> 00:02:59,310 we see continuum emission, is a region of broad emission line clouds. 37 00:02:59,310 --> 00:03:04,315 Those are clouds of gas that they're moving at speeds of maybe the order of 1 38 00:03:04,315 --> 00:03:09,474 10th of the speed of light because of the depotential well near the black hole and 39 00:03:09,474 --> 00:03:14,993 they have characteristic emission lines. And larger yet is a narrow line emission 40 00:03:14,993 --> 00:03:20,258 region of gas clouds that are being ionized by emssion from the central engine 41 00:03:20,258 --> 00:03:25,199 and move with speeds that are more characteristic of galaxian potential 42 00:03:25,199 --> 00:03:29,441 wells. And finally, we think that there is an 43 00:03:29,441 --> 00:03:35,447 obscuring torus or disk of dust hiding the central engine from the view from the 44 00:03:35,447 --> 00:03:37,940 side. Probably the most important global 45 00:03:37,940 --> 00:03:42,428 characteristic of active nuclei is that they seem to emit energy at all different 46 00:03:42,428 --> 00:03:48,270 energies from radio through gamma rays. And unlike stars that tend to have thermal 47 00:03:48,270 --> 00:03:54,204 emission, usually focused over more or less one octave of frequency space, active 48 00:03:54,204 --> 00:03:57,840 nucleus span a much broader range of frequencies. 49 00:03:57,840 --> 00:04:03,604 Also, they tend to emit a higher energy than most stars. 50 00:04:03,605 --> 00:04:07,870 So, ultraviolet and hard X, and X-rays are really good ways to find quasars since 51 00:04:07,870 --> 00:04:11,099 stars generally do not emit much in these violent regions. 52 00:04:11,099 --> 00:04:17,347 Because of that, the colors of quasars which will be logs of the flux ratio of 53 00:04:17,347 --> 00:04:23,584 different bond passes tend to be very different from those of stars and that 54 00:04:23,584 --> 00:04:28,998 provides a means of finding. This is also very useful when we look at 55 00:04:28,998 --> 00:04:34,857 very high redshifts where the blue light in the rest frame is absorbed by the 56 00:04:34,857 --> 00:04:39,366 intergalactic hydrogen. One characteristic thing about active 57 00:04:39,366 --> 00:04:43,830 nuclei is that they do have emission lines, strong emission lines in their 58 00:04:43,830 --> 00:04:49,083 spectra which comes from ionized gas. The gas is ionized by ultraviolet and 59 00:04:49,083 --> 00:04:54,360 X-ray emission from the central AGN. These objects can reach phenomenal 60 00:04:54,360 --> 00:04:59,416 luminosities up to maybe 10 to the 15th solar luminosities, a galaxy like the 61 00:04:59,416 --> 00:05:04,472 Milky Way might have a luminosity of the order of 10 to the 10th or 10 to the 11th 62 00:05:04,472 --> 00:05:08,592 solar luminosities. Quasars can be thousands of times more 63 00:05:08,592 --> 00:05:13,674 luminous and yet that luminosity comes from, from a region that's smaller than 64 00:05:13,674 --> 00:05:18,087 the solar system. They also show a strong variability of all 65 00:05:18,087 --> 00:05:24,597 different wave lengths and the size of the small region dictates the times scales in 66 00:05:24,597 --> 00:05:29,989 which it's going to happen. And if a regency is, of the order of light 67 00:05:29,989 --> 00:05:34,467 hours across, then emission can vary on scales of hours. 68 00:05:34,468 --> 00:05:40,336 And even with the most modern state-of-the-art techniques, the central 69 00:05:40,336 --> 00:05:45,340 engines are still unresolved. Even for the nearest active galaxies. 70 00:05:45,340 --> 00:05:50,443 So, lot of what we know about these objects is inferred from variety of their 71 00:05:50,443 --> 00:05:54,984 physical properties that we observe that come from larger radii. 72 00:05:54,984 --> 00:06:00,165 And because they tend to be far away, with high redshifts, they do not move very much 73 00:06:00,165 --> 00:06:03,164 in time. Whereas, if you have a long enough 74 00:06:03,164 --> 00:06:09,148 baseline observations, you can see most start in the Milky Way having some proper 75 00:06:09,148 --> 00:06:12,920 motion. So, all of these characteristics have been 76 00:06:12,920 --> 00:06:16,567 used as means to discover active galactic nuclei. 77 00:06:16,568 --> 00:06:20,738 Most notably, it was the, the properties of the broad-band spectral energy 78 00:06:20,738 --> 00:06:25,300 distribution. Here, we have, on a log, log diagram, what 79 00:06:25,300 --> 00:06:29,769 spectra of quasars might look like from radio into the gamma rays. 80 00:06:29,769 --> 00:06:34,592 And the first thing you notice is that they seem to be more or less flat. 81 00:06:34,592 --> 00:06:39,808 There is a roughly equal amount of energy being contributed on broad range 82 00:06:39,808 --> 00:06:43,794 frequencies. Very much unlike stars with black body 83 00:06:43,794 --> 00:06:49,107 emission that seems to usually span about factor of two in wavelength as opposed to 84 00:06:49,107 --> 00:06:52,062 many orders of magnitude that is shown here. 85 00:06:52,062 --> 00:06:57,460 The different components are labeled here. They come from different sources of 86 00:06:57,460 --> 00:07:00,420 emission. Some are thermal from say, hot accretion 87 00:07:00,420 --> 00:07:04,820 disks, some are non-thermal from synchrotron electrons, or from the jet. 88 00:07:04,820 --> 00:07:07,820 We will talk about those in more detail later. 89 00:07:07,820 --> 00:07:12,980 And this is what the typical quasar spectrum looks like there is a strong 90 00:07:12,980 --> 00:07:17,720 broad continuum and superposed on it are strong emission lines. 91 00:07:17,720 --> 00:07:22,538 They're broad emissions lines which are Doppler broadened by the motion of the 92 00:07:22,538 --> 00:07:26,052 clouds, which can be thousands of kilometers per second. 93 00:07:26,052 --> 00:07:31,084 But also, narrow emission lines, which come from clouds moving further away from 94 00:07:31,084 --> 00:07:36,042 the black hole, and mostly is probing the potential of the host galaxy and not the 95 00:07:36,042 --> 00:07:40,272 black hole itself. We will also notice that many of these 96 00:07:40,272 --> 00:07:44,149 lines come from highly ionized species of ions. 97 00:07:44,149 --> 00:07:50,729 The notation of roman numeral after the elements name indicates how many electrons 98 00:07:50,729 --> 00:07:55,658 have been removed, minus 1. So, Carbon I will be at, at, will be 99 00:07:55,658 --> 00:08:01,550 neutral carbon, and Carbon IV is carbon that has lost three electrons. 100 00:08:01,550 --> 00:08:06,222 Magnesium II is magnesium that's lost one electron and so on. 101 00:08:06,222 --> 00:08:11,759 The square brackets indicates so called forbidden or semi-forbidden transitions. 102 00:08:11,759 --> 00:08:16,054 They're called that way because they're never observed in terrestrial lab 103 00:08:16,054 --> 00:08:21,704 conditions but they are observed in space. And that means that they usually require 104 00:08:21,704 --> 00:08:27,100 very low pressure and low density of the gas, and very high temperatures which are 105 00:08:27,100 --> 00:08:31,858 easily obtained in cosmic plasma's. But they're very hard to achieve in 106 00:08:31,858 --> 00:08:35,460 laboratory. So, since the, their discovery in early 107 00:08:35,460 --> 00:08:40,740 1960s, there have been surveys to find quasars in active, active, other active 108 00:08:40,740 --> 00:08:46,340 nuclei in a systematic fashion in order to study them and to use them as cosmological 109 00:08:46,340 --> 00:08:49,343 probes. At first, radiation was used this is how 110 00:08:49,343 --> 00:08:53,906 quasars were discovered. More recently, optical surveys based on 111 00:08:53,906 --> 00:08:57,346 colors are the dominant way of finding quasars. 112 00:08:57,346 --> 00:09:03,131 The one thing to keep in mind that each of these methods has its own power and its 113 00:09:03,131 --> 00:09:06,803 own limitation, and it's own selection effects. 114 00:09:06,804 --> 00:09:12,318 And to really get a complete picture of active galactic nucleus population in the 115 00:09:12,318 --> 00:09:17,664 universe, we have to actually approach this as a very panchromatic problem and 116 00:09:17,664 --> 00:09:22,025 try to complete surveys on a variety of different wavelengths. 117 00:09:22,026 --> 00:09:26,120 X-rays are also a very productive way to find them. 118 00:09:26,120 --> 00:09:32,326 Stars are sometimes X-ray sources, but not nearly as luminous as these active nuclei. 119 00:09:32,326 --> 00:09:38,042 And the great majority of X-ray sources in the sky outside of plane of the Milky Way 120 00:09:38,042 --> 00:09:41,685 are active nuclei at some larger redshifts. 121 00:09:41,685 --> 00:09:46,525 Nowadays, we have sky surveys that span full range of frequencies, from radio 122 00:09:46,525 --> 00:09:50,845 through gamma rays. And combining them together provides means 123 00:09:50,845 --> 00:09:56,540 of separating hese objects which have spectral energy distributiosn very much 124 00:09:56,540 --> 00:10:00,529 unlike stars from stars. Whereas, pictures that just look like 125 00:10:00,529 --> 00:10:03,394 stars but they're something very different. 126 00:10:03,395 --> 00:10:08,345 Today, there are well over 100,000 quasars that have been confirmed by actual 127 00:10:08,345 --> 00:10:12,500 spectroscopy, most of them coming from the Sloan Digital Sky Survey. 128 00:10:12,500 --> 00:10:17,765 But there is over a million of additional highly likely quasars that are being 129 00:10:17,765 --> 00:10:23,084 selected through their colors and haven't just had spectra taken yet. 130 00:10:23,085 --> 00:10:26,380 These come from large modern digital sky surveys. 131 00:10:26,380 --> 00:10:32,075 Early on, surveys were not nearly as large and they might have discovered tens or 132 00:10:32,075 --> 00:10:36,040 hundreds of quasars, and they still serve the purpose. 133 00:10:36,040 --> 00:10:42,026 The first surveys tend to be the tend to be done from Palomar Observatory and they 134 00:10:42,026 --> 00:10:48,012 usually denoted with some acronym like P, something where P stands for Palomar, for 135 00:10:48,012 --> 00:10:53,424 example, PG is for Palomar Green, after the astronomer who did the survey, PC is 136 00:10:53,424 --> 00:10:57,952 for Palomar CCD based survey, PSS for Palomar Sky Survey and so on. 137 00:10:57,953 --> 00:11:04,508 Many other observatories have participated in, in those enterprise, notably the 138 00:11:04,508 --> 00:11:10,241 University of Michigan and, and Case Western Reserve University have done 139 00:11:10,241 --> 00:11:16,611 extensive surveys for emission line objects as well as Byurakan Observatory in 140 00:11:16,611 --> 00:11:20,358 Armenia. There have been many compilations of 141 00:11:20,358 --> 00:11:25,255 quasars discovered in any which way, from radio through gamma rays, through X-rays. 142 00:11:25,255 --> 00:11:30,780 And two catalogs that have been used through literature from Hewitt and 143 00:11:30,780 --> 00:11:36,576 Burbidge and Veron and Veron-Cetty. However today, those heterogeneous 144 00:11:36,576 --> 00:11:42,624 compilations are superseded by much more complete homogeneous, statistically well 145 00:11:42,624 --> 00:11:47,481 understood examples. In addition to that, added partial overlap 146 00:11:47,481 --> 00:11:51,627 there are about 1 million radio sources catalog so far. 147 00:11:51,628 --> 00:11:57,160 Most of those radio sources in the sky are active galactic nuclei, not all some of it 148 00:11:57,160 --> 00:12:01,459 comes from star formation. And those in our galaxy, of course, come 149 00:12:01,459 --> 00:12:04,917 from things like Super Nova and unsourced star forming regions. 150 00:12:04,918 --> 00:12:10,960 But outside the plane of the Milky Way, the great majority of them are from active 151 00:12:10,960 --> 00:12:13,956 nuclei. Radio sources may or may not have other 152 00:12:13,956 --> 00:12:18,263 visible manifestations of central activity, sometimes only radio is 153 00:12:18,263 --> 00:12:22,896 detected. But no obvious signatures invisible or 154 00:12:22,896 --> 00:12:27,352 even accurate. Some of the more famous radius surveys 155 00:12:27,352 --> 00:12:33,509 include those from the Green Bank Observatory in US, from parks in Australia 156 00:12:33,509 --> 00:12:38,919 from the VLA which are called NVSS and FIRST, and many others. 157 00:12:38,920 --> 00:12:42,450 An interesting question to ask is, how many quasars are there? 158 00:12:42,450 --> 00:12:47,086 And so, if you count them per unit area in the sky going down to the limits of 159 00:12:47,086 --> 00:12:51,874 traditional sky series of the order of 20th magnitude, maybe going a little 160 00:12:51,874 --> 00:12:56,851 deeper, 22nd magnitude, there are of the order 100 quasars per square degree. 161 00:12:56,852 --> 00:13:02,374 And if you go to the very limit of observations, which of course we only do 162 00:13:02,374 --> 00:13:08,446 on very small areas, we can infer that there are some tens of millions of quasars 163 00:13:08,446 --> 00:13:12,516 observable in the in the, within our horizon. 164 00:13:12,516 --> 00:13:18,450 Or roughly speaking, there is one 1 uasar for every 1,000 galaxies integrated over 165 00:13:18,450 --> 00:13:23,811 all different directions. I mentioned the Sloan Quasar Survey and 166 00:13:23,811 --> 00:13:27,613 that was component of the larger Sloan Digital Sky Survey. 167 00:13:27,614 --> 00:13:35,024 They imaged sky in five filters that covered all the order of one half of the 168 00:13:35,024 --> 00:13:39,726 sky, maybe a little less. And using ratios of fluxes in these 169 00:13:39,726 --> 00:13:42,880 filters, they can separate quasars from stars. 170 00:13:42,880 --> 00:13:47,010 Quasars don't look like stars that's why they are called quasars. 171 00:13:47,011 --> 00:13:53,379 That stands for quasi-stellar object. But, of course, they look very different 172 00:13:53,379 --> 00:13:58,174 once you take the spectra. This shows couple of the color, color 173 00:13:58,174 --> 00:14:02,212 plots of star-like objects found in Sloan Sky Survey. 174 00:14:02,212 --> 00:14:10,757 And the big locus that you see in the middle are mostly stars, galactic stars. 175 00:14:10,758 --> 00:14:14,796 But all the dots standing away from that locus tend to be quasars that are a 176 00:14:14,796 --> 00:14:19,255 different redshift. So, by putting boundaries in a color space 177 00:14:19,255 --> 00:14:24,649 like that and then following up spectroscopically, one can find numerous 178 00:14:24,649 --> 00:14:28,305 quasars. And that's how most of them are actually 179 00:14:28,305 --> 00:14:31,291 done. Here are some additional plots. 180 00:14:31,292 --> 00:14:37,900 Here, in order to avoid crowding by individual dots the density contours in 181 00:14:37,900 --> 00:14:45,002 color space apply for most of the stars. And then the outlaying stars are plotted 182 00:14:45,002 --> 00:14:47,972 as dots. Quasars that are being conformed 183 00:14:47,972 --> 00:14:53,818 spectroscopically are indicated as colored points, and their color is representative 184 00:14:53,818 --> 00:14:58,862 of the redshift as you can see. Now, colors are always defined as blue 185 00:14:58,862 --> 00:15:05,322 minus red magnitude, and so the lower left corner corresponds to bluer colors the 186 00:15:05,322 --> 00:15:10,495 upper right to the redder colors. You can see that quasars tend to be bluer 187 00:15:10,495 --> 00:15:14,700 than most stars but sometimes they overlap with some kinds of stars. 188 00:15:14,700 --> 00:15:19,824 The other major survey was done by Anglo-Australian telescope as part of 189 00:15:19,824 --> 00:15:23,440 their redshift survey and is now called 2QZ survey. 190 00:15:23,440 --> 00:15:29,374 That one detected couple tens of thousands of new quasars in smaller areas than the 191 00:15:29,374 --> 00:15:32,860 Sloan. But nevertheless, provided another useful 192 00:15:32,860 --> 00:15:38,419 sample for statistical studies. And here is ih, an interesting diagram. 193 00:15:38,419 --> 00:15:44,995 What they have done here is they plotted all quasar spectra as individual rows in 194 00:15:44,995 --> 00:15:49,881 this image, sorted in redshift. And so, the luminous ridges that you see 195 00:15:49,881 --> 00:15:54,471 correspond to the strong emission lines. And as you increase the redshift, they 196 00:15:54,471 --> 00:15:57,772 move to the red, as they should, because of the redshift. 197 00:15:57,772 --> 00:16:02,316 And the new lines come in that are normally ultra-violet and not observable 198 00:16:02,316 --> 00:16:05,804 and divisible, such as Lyman-Alpha or Carbon 4 and so on. 199 00:16:05,804 --> 00:16:10,748 So, it's a nice illustration of how that there's a smooth distribution and how 200 00:16:10,748 --> 00:16:14,553 redshift actually changes the appearance of the spectra. 201 00:16:14,554 --> 00:16:25,125 Next, we will talk about classification of the active galactic nuclei.