1 00:00:00,025 --> 00:00:06,605 We'll now talk about high energy diffused backgrounds, which are in x-rays or gamma 2 00:00:06,605 --> 00:00:12,308 rays, and their origins, which are mostly from active galactic nuclei. 3 00:00:12,308 --> 00:00:17,752 This picture is an x-ray image of the moon from the ROSAT satellite, and you can see 4 00:00:17,752 --> 00:00:22,602 that the illuminated side of the moon is reflecting x-rays from the sun. 5 00:00:22,602 --> 00:00:27,321 Sun doesn't need sun. And there is a dark side of the moon, with 6 00:00:27,321 --> 00:00:31,475 some x-ray events even from there, they're, could be in the foreground of a 7 00:00:31,475 --> 00:00:34,370 world between the Earth's atmosphere and the moon. 8 00:00:34,370 --> 00:00:38,824 But there is also a notable excess background. 9 00:00:38,825 --> 00:00:44,316 Behind the dark side of the moon, which shows clearly that their origin is beyond 10 00:00:44,316 --> 00:00:49,355 Earth moon system. And those are x-ray photos of the diffused 11 00:00:49,355 --> 00:00:54,259 x-ray background. The cosmic x-ray background was actually 12 00:00:54,259 --> 00:00:59,810 discovered before the cosmic microwave background, with the very first 13 00:00:59,810 --> 00:01:06,271 astronomical x-ray experiment, which was rocket born test led by Ricardo Giacconi, 14 00:01:06,271 --> 00:01:09,937 who many years later got Nobel Prize for this. 15 00:01:09,938 --> 00:01:15,620 And they detected first souces, such as Scorpius X1, but also diffuse x-ray 16 00:01:15,620 --> 00:01:19,819 emission that seemed to be coming from everywhere. 17 00:01:19,820 --> 00:01:24,107 And at that time it simply wasn't known, where does it come from. 18 00:01:24,108 --> 00:01:29,548 For comparison it is about a percent of the diffused optical infrared background 19 00:01:29,548 --> 00:01:35,068 that we talked about when we talked about galaxy evolution, which essentially, is 20 00:01:35,068 --> 00:01:40,348 integrated starlight ever emitted from galaxies, and that itself is a percent 21 00:01:40,348 --> 00:01:44,304 level relative to the cosmic microwave background. 22 00:01:44,305 --> 00:01:49,860 So in terms of, sheer energy density, these backgrounds are relatively 23 00:01:49,860 --> 00:01:53,746 negligible. But, their astrophysical interest is, very 24 00:01:53,746 --> 00:01:57,280 considerable. We now, believe, and, actually very, 25 00:01:57,280 --> 00:02:02,480 certain of this, that most of the, X-ray background, originates from, active 26 00:02:02,480 --> 00:02:06,740 galactic nuclei. It's been resolved individual sources. 27 00:02:06,740 --> 00:02:10,820 Some of it comes from star formation in galaxies, say, Super Nova or [unknown] and 28 00:02:10,820 --> 00:02:15,237 such. But most of it comes from active nuclei. 29 00:02:15,238 --> 00:02:19,714 There was however, one major puzzle, and that was the x-ray spectrum of the 30 00:02:19,714 --> 00:02:24,736 background. So, here is, a very wide bandwidth plot of 31 00:02:24,736 --> 00:02:31,770 various cosmic backgrounds, from radio through almost gamma rays. 32 00:02:31,770 --> 00:02:37,440 And you can see that microwave background, the black body curve on the left, 33 00:02:37,440 --> 00:02:41,022 dominates the picture. This is log-log plot. 34 00:02:41,022 --> 00:02:44,959 And so then there is optical, near infrared back, or far infrared background. 35 00:02:44,960 --> 00:02:49,466 And then there is x-ray background, schematically illustrated with that wiggly 36 00:02:49,466 --> 00:02:52,342 line. The black dot in the lower right is the 37 00:02:52,342 --> 00:02:56,434 cosmic gamma ray background. So what about the spectrum, the cosmic 38 00:02:56,434 --> 00:02:59,750 x-ray background? Here is a plot of measurements from a 39 00:02:59,750 --> 00:03:03,717 variety of different satellites that have been doing that. 40 00:03:03,717 --> 00:03:06,600 Sometimes overlapping in energy, sometimes not. 41 00:03:06,600 --> 00:03:10,695 You can see that there's a little bit of spread between them, and that's just 42 00:03:10,695 --> 00:03:14,707 issues of cross calibration. It's not so easy to calibrate these 43 00:03:14,707 --> 00:03:18,546 things. Now this shape, of an x-ray spectrum is 44 00:03:18,546 --> 00:03:23,817 typical of the very hot gas. So thermal Bremsstrahlung radiation. 45 00:03:23,817 --> 00:03:30,437 You have a hot plasma electrons are, passing by protons, all ionized. 46 00:03:30,438 --> 00:03:35,240 They're decelerated by, electron, electric interaction, they emit photons then. 47 00:03:35,240 --> 00:03:40,594 So it's not like a Planck black body, but it's, thermal emission nevertheless. 48 00:03:40,594 --> 00:03:46,038 And, that would, imply, on the face value of it, that it's, the universe is somehow 49 00:03:46,038 --> 00:03:49,639 filled with hot plasma and this is where it comes from. 50 00:03:51,140 --> 00:03:54,360 But that doesn't work, because we simply haven't seen it. 51 00:03:54,360 --> 00:03:59,057 Instead of that, we see a lot of active galactic nuclei and x-rays that are 52 00:03:59,057 --> 00:04:03,974 identified optical radio sources. But the problem is that their spectra 53 00:04:03,974 --> 00:04:08,366 don't look like this. So how can you have collective emission, 54 00:04:08,366 --> 00:04:14,078 with a spectrum that looks like thermal emission from plasma, composed of a lot of 55 00:04:14,078 --> 00:04:18,584 non-thermal sources? It turns out to be a combination of their 56 00:04:18,584 --> 00:04:24,109 evolution, in time and red shift, and slightly different ways in which x-rays 57 00:04:24,109 --> 00:04:30,044 are being released. Some of it comes directly from [unknown] 58 00:04:30,044 --> 00:04:35,450 electrons. Some of it comes from reflected, and 59 00:04:35,450 --> 00:04:42,897 continuing emission from, dust surrounding the active nucleus. 60 00:04:42,898 --> 00:04:47,838 The way to find this out was to obtain really deep images in x-rays of selected 61 00:04:47,838 --> 00:04:52,359 patches in the sky. And then try to identify the sources. 62 00:04:52,360 --> 00:04:58,200 This was done in Hubble deep field, and other deep fields that are done. 63 00:04:58,200 --> 00:05:04,448 Chandra telescope had its own deep fields, one which are, Hubble, the other one in 64 00:05:04,448 --> 00:05:08,030 the south, and so on. Typical exposures for these were in 65 00:05:08,030 --> 00:05:11,572 millions of seconds. So a lot of invested time in these 66 00:05:11,572 --> 00:05:14,694 observations. And as you can see, there are plenty of 67 00:05:14,694 --> 00:05:19,010 x-ray sources. They look sharp in the middle, and they 68 00:05:19,010 --> 00:05:24,151 look big on the outskirts, and that's purely the effects of the x-ray optics. 69 00:05:24,151 --> 00:05:29,506 It's not so easy to make x-ray mirrors, and so the image quality deteriorates in 70 00:05:29,506 --> 00:05:32,956 radius. They're not really bigger they're just, 71 00:05:32,956 --> 00:05:39,220 images are fatter on the detector itself. And this is precise enough, that we can 72 00:05:39,220 --> 00:05:42,407 actually seek optical or radio counterparts. 73 00:05:42,408 --> 00:05:48,114 We can also count the sources, and see if the total emission, from all of the 74 00:05:48,114 --> 00:05:53,818 sources, counted to the faintest level, adds up to the overall integrated 75 00:05:53,818 --> 00:05:57,530 background. And the answer is, it pretty much does. 76 00:05:57,530 --> 00:06:02,480 And so, in different energy bands, you get counts to a different depth, but they 77 00:06:02,480 --> 00:06:07,430 follow more or less the same kind of curve, which is some sensible evolutionary 78 00:06:07,430 --> 00:06:11,371 models. And, if you extrapolate to fainter source 79 00:06:11,371 --> 00:06:18,160 counts, because they're reasonably well behaved, just like in the optical they're 80 00:06:18,160 --> 00:06:24,577 reasonably well behaved counts of both optimum play and galaxies, you can account 81 00:06:24,577 --> 00:06:30,157 for, reasonably for 90% of the total observed x-ray background, and the 82 00:06:30,157 --> 00:06:35,490 remaining 10%, could be just uncertainties of extrapolation. 83 00:06:35,490 --> 00:06:41,414 So what are the x-ray sources? Here's a collection of, whole bunch of, 84 00:06:41,414 --> 00:06:45,119 Chandra sources, identified in the optical. 85 00:06:45,119 --> 00:06:49,894 And, a lot of them look point-like. These are active galactic nuclei, 86 00:06:49,894 --> 00:06:54,038 sometimes you can see the galaxy, those are all to near by ones. 87 00:06:54,039 --> 00:07:02,124 But, by and large, those all seem to be associated with no thermal activity in 88 00:07:02,124 --> 00:07:08,278 quasar-like objects. Interestingly enough, some of these do not 89 00:07:08,278 --> 00:07:14,075 show any signs of non-thermal AGN in the middle, in optical. 90 00:07:14,075 --> 00:07:19,120 So the visible ray, UV ray, for red light is completely hidden, but x-rays do 91 00:07:19,120 --> 00:07:23,310 penetrate the dust, and we see them. It turns out that there is not a good 92 00:07:23,310 --> 00:07:25,772 correlation between x-ray and optical fluxes. 93 00:07:25,772 --> 00:07:30,964 Something can be a very luminous x-ray source and very faint in the optical or 94 00:07:30,964 --> 00:07:35,882 vice versa. And, they come in different nature if we 95 00:07:35,882 --> 00:07:41,059 plot here x-ray fluxes, essentially. Optical flux, you see there is no 96 00:07:41,059 --> 00:07:45,335 correlation whatsoever. And sources of different kinds appear in 97 00:07:45,335 --> 00:07:49,171 different way. There are some galaxies which are powered 98 00:07:49,171 --> 00:07:53,323 by star formation. And there, those tend to be lower 99 00:07:53,323 --> 00:07:59,108 luminosity x-ray sources, whereas active nuclei or quasars tend to be higher 100 00:07:59,108 --> 00:08:03,492 luminosity x-ray sources. Redshifts were measured, for a whole lot 101 00:08:03,492 --> 00:08:07,350 of them, they turn out not to be particularly high ratchet objects. 102 00:08:07,350 --> 00:08:11,034 Most of them are to ratchet two or so, typical for quasars. 103 00:08:11,035 --> 00:08:20,572 And, here are the, central Hubble diagrams of sorts, for the x-ray sources. 104 00:08:20,572 --> 00:08:24,793 Although it's not, apparent flux, it's, absolute luminosity. 105 00:08:24,793 --> 00:08:28,425 The reason you don't see faint sources of high red shift is that, they're just too 106 00:08:28,425 --> 00:08:32,403 faint to detect. But, you can see they do extend to very 107 00:08:32,403 --> 00:08:36,790 high luminosity's. And, essentially all of the ones at higher 108 00:08:36,790 --> 00:08:41,283 edges, which are solid squares, are active galactic nuclei. 109 00:08:41,284 --> 00:08:46,090 Well, what about gamma rays? The story there is very similar. 110 00:08:46,091 --> 00:08:51,896 Being active galactic nuclei, those red-shifts are, ought to be the principal 111 00:08:51,896 --> 00:08:57,314 extra-galactic gamma ray sources, aside from gamma ray bursts, which, are 112 00:08:57,314 --> 00:09:01,597 spectacular, but do not contribute much energy overall. 113 00:09:01,598 --> 00:09:05,835 This is a sky map from Fermi satellite after first year of operation, and, can, 114 00:09:05,835 --> 00:09:10,060 plotted in galactic coordinates, and see Milky Way plane has a lot of gamma ray 115 00:09:10,060 --> 00:09:16,745 emission from variety of sources. But, outside the plane, there are point 116 00:09:16,745 --> 00:09:24,095 sources and essentially all of them are associated with active galactic nuclei and 117 00:09:24,095 --> 00:09:29,709 specifically with blazars. After few years of work with Fermi, they 118 00:09:29,709 --> 00:09:35,382 actually could add up light from non-blazars and turned out that, they do 119 00:09:35,382 --> 00:09:39,629 not, add up to the full observed gamma ray background. 120 00:09:39,630 --> 00:09:46,842 So those are, the obvious sources, but there could be extra kinds of sources that 121 00:09:46,842 --> 00:09:52,085 might be contributing. Well, some of the caplogs of these beamed 122 00:09:52,085 --> 00:09:58,120 AGN are probably incomplete on a factor 2, probably, because when measurements were 123 00:09:58,120 --> 00:10:02,616 done, sources in low states, were never made it into a caplog. 124 00:10:02,616 --> 00:10:08,083 But even so, there isn't enough in the known active galactic nucleus population. 125 00:10:09,160 --> 00:10:14,736 Additional sources could include some star formation where gamma rays come from 126 00:10:14,736 --> 00:10:20,066 shocks and super nova remnants, or even shocks and, and gas, and it was also 127 00:10:20,066 --> 00:10:25,396 proposed that some component of dark matter may be decaying and, and emitting 128 00:10:25,396 --> 00:10:30,160 gamma rays, although so far no evidence was seen for that whatsoever. 129 00:10:30,160 --> 00:10:36,208 So there could be also fainter populations of being active nuclei that somehow didn't 130 00:10:36,208 --> 00:10:40,932 catch our attention earlier, perhaps because they are too faint. 131 00:10:40,932 --> 00:10:46,956 So this is still and area of ongoing work, but, still being active nuclei, are 132 00:10:46,956 --> 00:10:53,120 probably the main contributors. Something else interesting, is that beam 133 00:10:53,120 --> 00:10:58,750 nuclei blazars can produce photons of tera electron volt energies. 134 00:10:58,750 --> 00:11:03,909 Typical gamma ray observations would be of giga electron volt energies, or mega 135 00:11:03,909 --> 00:11:07,876 electron volt energies. But, these things go up to tera electron 136 00:11:07,876 --> 00:11:10,575 volts. They can be actually observed from the 137 00:11:10,575 --> 00:11:13,550 ground. Earth's atmosphere is opaque to the 138 00:11:13,550 --> 00:11:19,139 photons themselves, but these high-energy photons can knock out particles from, 139 00:11:19,139 --> 00:11:24,242 atoms and molecules in the upper atmosphere, and gain some kinetic energy, 140 00:11:24,242 --> 00:11:29,169 and then you have charged particles moving at a very high speed. 141 00:11:29,170 --> 00:11:34,905 Which can emit, so-called [unknown] radiation, and those can be seen with 142 00:11:34,905 --> 00:11:39,730 large telescopes on the ground. So here are the light curves of several 143 00:11:39,730 --> 00:11:45,810 blazars in tera-electronvolts, and they've been also detected in the optical in terms 144 00:11:45,810 --> 00:11:50,853 of their Lorentz factors, or axis of 50, and, that was interesting itself. 145 00:11:50,854 --> 00:11:56,298 This, gives us some confidence that probably the highest energy cosmic rays 146 00:11:56,298 --> 00:12:01,816 also come from these beam active nuclei. Remember, those are accelerators in the 147 00:12:01,816 --> 00:12:05,516 sky. And, interestingly enough, the highest 148 00:12:05,516 --> 00:12:12,149 energy cosmic rays are 100 million times more energetic than particles in Large 149 00:12:12,149 --> 00:12:16,532 Hadron Collider, in Geneva. So there are many uses for these cosmic 150 00:12:16,532 --> 00:12:20,175 accelerators. First of all, they probe the demographics 151 00:12:20,175 --> 00:12:25,400 unification of active nuclei. They, they're obvious contributor to the 152 00:12:25,400 --> 00:12:29,802 cosmic gamma ray background. There is lot of physics of relativistic 153 00:12:29,802 --> 00:12:35,725 jets that can be learned from them. And there may be even the ultimate sources 154 00:12:35,725 --> 00:12:40,057 of high energy particles. So there could be the future of particle 155 00:12:40,057 --> 00:12:44,611 physics because we'll never have an accelerator that is hundred million times 156 00:12:44,611 --> 00:12:49,950 more powerful than large hadron collider. They do other interesting things. 157 00:12:49,950 --> 00:12:55,630 For example, they're very compact radio sources, and they are the only important 158 00:12:55,630 --> 00:13:01,070 radio source population, foreground population, to the measurements of cosmic 159 00:13:01,070 --> 00:13:06,350 microwave background at high angular resolution, so they have to be accounted 160 00:13:06,350 --> 00:13:11,050 for precisely before, better cosmological conclusions are drawn. 161 00:13:11,050 --> 00:13:14,940 And, they can also serve as a probe of star formation, even though they 162 00:13:14,940 --> 00:13:17,880 themselves have nothing to do with star formation. 163 00:13:17,880 --> 00:13:22,716 And the reason for that is that, intergalactic starlight came from all 164 00:13:22,716 --> 00:13:27,396 different galaxies, as well as the microwave background, are opaque to 165 00:13:27,396 --> 00:13:31,019 high-energy photons, and, this sounds a little weird. 166 00:13:32,080 --> 00:13:37,200 That gas of photons is opaque to other photons, but in the center of energy, of 167 00:13:37,200 --> 00:13:42,000 the two photons, one of which can be infrared and the other is really high 168 00:13:42,000 --> 00:13:46,960 energy gamma ray, they're both high energy, and they can exceed [unknown] 169 00:13:46,960 --> 00:13:50,470 production energy for positrons, and electrons. 170 00:13:50,470 --> 00:13:55,390 So they can, so the two photons interacting can turn electron-positron 171 00:13:55,390 --> 00:14:01,212 pairs, which later, an islet somewhere, but essentially, intergalactic star light 172 00:14:01,212 --> 00:14:04,577 forms a fog for high-energy gamma ray sources. 173 00:14:04,578 --> 00:14:09,883 And so, if you can measure the cutoff in their energy, as a function of red-shift, 174 00:14:09,883 --> 00:14:14,860 you can show energy density of intergalactic star light, without recourse 175 00:14:14,860 --> 00:14:18,065 to any red-shift survey, or any deep counts. 176 00:14:18,066 --> 00:14:23,592 Which is a nice independent way, of constraining the history of cosmic star 177 00:14:23,592 --> 00:14:26,180 formation. And here is another plot. 178 00:14:26,180 --> 00:14:31,167 It's likely to express in a slightly different way of all the different cosmic 179 00:14:31,168 --> 00:14:34,952 diffused backgrounds. They're diffused when you don't have a 180 00:14:34,952 --> 00:14:39,440 good resolution, but all of them except cosmic microwave background, break into 181 00:14:39,440 --> 00:14:43,257 individual sources, once good enough observations are obtained. 182 00:14:43,257 --> 00:14:49,337 And, roughly speaking, in radio, it's mostly from active galactic nuclei, but 183 00:14:49,337 --> 00:14:54,122 there's some star formation. Microwave background is cosmological 184 00:14:54,122 --> 00:14:59,006 origin, sub millimeter, far infrared, visible, and near ultraviolet all come 185 00:14:59,006 --> 00:15:02,102 from star formation. Obscured or not obscured. 186 00:15:02,102 --> 00:15:06,512 X-rays mostly come from active galactic nuclei. 187 00:15:06,513 --> 00:15:11,575 Some small component comes from star forming regions, and gamma rays as far, as 188 00:15:11,575 --> 00:15:16,900 we can tell, mostly, maybe entirely, come from active nuclei, even though we haven't 189 00:15:16,900 --> 00:15:23,252 accounted for all of them yet. Next we will turn to the study of the 190 00:15:23,252 --> 00:15:28,202 evolution of active galactic nuclei.