1 00:00:00,012 --> 00:00:06,407 Galaxies don't live in a perfect vacuum. They're embedded in the intergalactic 2 00:00:06,407 --> 00:00:12,487 medium, which is gas, which, some which was never in galaxies, some which was 3 00:00:12,487 --> 00:00:17,838 expelled from the galaxies. And galaxies are a sort of ecological 4 00:00:17,838 --> 00:00:21,606 equilibrium. So briefly, intergalactic medium is 5 00:00:21,606 --> 00:00:26,542 barriers between galaxies. That gas tends to be very highly ionized, 6 00:00:26,542 --> 00:00:30,176 mostly by radiation from our two galactic nuclei. 7 00:00:30,176 --> 00:00:35,475 And like any other test particles it follows the evolution of the gravitational 8 00:00:35,475 --> 00:00:41,061 potential Orbital large scale structure forming voids and filaments and sheets. 9 00:00:41,061 --> 00:00:46,103 In other words, just like those structures we've seen in numerical simulations. 10 00:00:46,103 --> 00:00:49,853 And because of that it's also sometimes called a cosmic web. 11 00:00:49,853 --> 00:00:54,643 So, unlike the gas in galaxies, which is falling into deep potential wells. 12 00:00:54,643 --> 00:00:57,602 Probably has dissipated quite a bit of energy. 13 00:00:57,602 --> 00:01:01,655 This gas is still in the linear region, and because of that it offers a 14 00:01:01,655 --> 00:01:06,341 possibility of being used to trace the large-scale structure evolution in the 15 00:01:06,341 --> 00:01:09,174 linear region that is not probed by galaxies. 16 00:01:09,175 --> 00:01:14,635 When the gas falls into the galaxy it replenishes the fuel for star formation. 17 00:01:14,635 --> 00:01:20,092 You may recall that the amount of neutral hydrogen that is in spirals now would last 18 00:01:20,092 --> 00:01:23,974 them only about a billion years so it had to be replenished. 19 00:01:23,974 --> 00:01:28,972 Likewise as we've seen from galactic winds, some gas gets to be expelled back 20 00:01:28,972 --> 00:01:33,787 into the intergalactic medium, and this is where the metals come from. 21 00:01:33,787 --> 00:01:39,731 So in some sense, the chemical evolution of inter-galactic medium, which can be 22 00:01:39,731 --> 00:01:45,174 probed in very effective ways, traces that one/g of galaxies themselves. 23 00:01:45,174 --> 00:01:51,028 This is now being simulated numerically at the, in great detail, using[UNKNOWN] 24 00:01:51,028 --> 00:01:55,708 simulations with gas affiliated with large-scale structure. 25 00:01:55,709 --> 00:02:01,192 So you can shoot lines of sight through these theoretical representations of the 26 00:02:01,192 --> 00:02:06,392 cosmic web and compute what would the absorption spectrum look like if you had 27 00:02:06,392 --> 00:02:11,521 light shining from the other end. So this actually provides a complimentary 28 00:02:11,521 --> 00:02:16,958 way to study galaxies and their evolution. Typically, we would look for galaxies in 29 00:02:16,958 --> 00:02:19,790 emission. Visible light maybe re-radiated from 30 00:02:19,790 --> 00:02:22,502 infrared from the dust or something like that. 31 00:02:22,502 --> 00:02:25,095 Here we don't look for galaxies in emission. 32 00:02:25,095 --> 00:02:28,951 It doesn't matter how bright they are or if you can see them at all. 33 00:02:28,951 --> 00:02:34,366 We're looking for galaxies in absorption. There is a cloud of gas, which will absorb 34 00:02:34,366 --> 00:02:40,042 certain wavelengths, and from that we can learn about physics of the gas, maybe 35 00:02:40,042 --> 00:02:44,762 fixed and evolutionary state. So it's a very complementary way of 36 00:02:44,762 --> 00:02:50,358 studying galaxies at high redshifts. Traditionally, quasars provided those 37 00:02:50,358 --> 00:02:54,443 sources of light. That shine through the intergalactic gas. 38 00:02:54,443 --> 00:02:59,620 They're very bright, seen far away, and their spectra tend to be fairly simple. 39 00:02:59,620 --> 00:03:04,481 They have broad emission lines but usually it's a fairly smooth continuum. 40 00:03:04,481 --> 00:03:10,108 So any absorption lines due to the clouds will be easily distinguished against such 41 00:03:10,108 --> 00:03:14,010 a continuum source. More recently gamma ray bursts, which have 42 00:03:14,010 --> 00:03:17,702 bright optical afterglows provided another way of doing this. 43 00:03:17,702 --> 00:03:22,237 And they probed somewhat different regime. But maybe we'll get to that later. 44 00:03:22,237 --> 00:03:27,004 I mentioned repeatedly during this class how selection affects are something one 45 00:03:27,004 --> 00:03:31,282 has to be aware of because we're looking at very faint objects far away, there is 46 00:03:31,282 --> 00:03:34,796 always flux limit, there is surface brightness selection. 47 00:03:34,796 --> 00:03:39,724 There's all kinds of selection effects. When you look for sources in emission. 48 00:03:39,724 --> 00:03:42,879 But when looking in absorption, none of that matters. 49 00:03:42,879 --> 00:03:47,439 It's a little bit like[INAUDIBLE] effect where you're looking for clusters of 50 00:03:47,439 --> 00:03:51,401 galaxies using their shadow, if you will, in microwave background. 51 00:03:51,401 --> 00:03:56,031 Well here we're looking some sense the shadow produced by the gas associated with 52 00:03:56,031 --> 00:04:00,720 galaxies or illogical/g structure. There are a variety of different kinds of 53 00:04:00,720 --> 00:04:05,832 absorption systems, usually called QSO absorption lines because the QSOs, quasars 54 00:04:05,832 --> 00:04:09,703 were used to find them. Hydrogen being the most common element, 55 00:04:09,703 --> 00:04:13,782 accounts for most of it. And there, they're divided according to 56 00:04:13,782 --> 00:04:18,874 their column density, by the amount of hydrogen That you look through along that 57 00:04:18,874 --> 00:04:22,436 line of sight. The thinnest are so called Lyman alpha 58 00:04:22,436 --> 00:04:26,374 forest clouds. They're small subgalactic size clouds, and 59 00:04:26,374 --> 00:04:30,657 they're heavily ionized. But they're easily seen in absorption 60 00:04:30,657 --> 00:04:34,385 lyman alpha line. As you increase the column density you 61 00:04:34,385 --> 00:04:39,678 find so called lyman limit systems and then damp lyman alpha observers, and Talk 62 00:04:39,678 --> 00:04:43,733 about that in more detail. Usually there is some metallic line 63 00:04:43,733 --> 00:04:47,958 absorption associated with high column density hydrogen clouds. 64 00:04:47,958 --> 00:04:52,539 This is probably because they are associated with galaxies in someway or 65 00:04:52,539 --> 00:04:55,873 another. There also helium equivalents of all this 66 00:04:55,873 --> 00:05:00,965 seen further in Ultraviolet but since hydrogen is more common it's easier to 67 00:05:00,965 --> 00:05:04,323 obseve usually people just worry about hydrogen. 68 00:05:04,324 --> 00:05:07,761 And finally they're metallic line absorbers. 69 00:05:07,761 --> 00:05:13,656 This is the gas that was processed inside galaxies, expelled in supernovae winds. 70 00:05:13,656 --> 00:05:19,357 And it can be studied in absorption as parts of maybe extended galaxial halos, or 71 00:05:19,357 --> 00:05:24,472 even clouds between. So this is what the, spectrum of the quasa 72 00:05:24,472 --> 00:05:28,356 looks like being ultraviolet rest in ultraviolet. 73 00:05:28,356 --> 00:05:34,147 The Quasa has sort of power like, power like spectrum with very broad emission 74 00:05:34,147 --> 00:05:38,550 lines superimposed on. But now on top of that there are all these 75 00:05:38,550 --> 00:05:42,305 absorption lines. And you can see the bluer/g of the center 76 00:05:42,305 --> 00:05:45,686 of the Lyman alpha line. There is a whole lot of them. 77 00:05:45,686 --> 00:05:49,495 This is not noise. These are actually individual absorption 78 00:05:49,495 --> 00:05:52,866 lines piled close together. The reason why they're. 79 00:05:52,866 --> 00:05:58,314 Occurring there is that Lyman Alpha is a resonant line, a very strong absorbing 80 00:05:58,314 --> 00:06:04,082 line, and the stuff that's between us and the quasar is, of course, redshifted less. 81 00:06:04,082 --> 00:06:09,017 So all those would be on the blue side of the quasar's Lyman Alpha line. 82 00:06:09,018 --> 00:06:14,591 To the red of the Lyman Alpha line you see Some absorbers, not as many, they are 83 00:06:14,591 --> 00:06:20,286 always due to some kind of metallic ionic species, often as carbon or nitrogen or 84 00:06:20,286 --> 00:06:25,864 magnesium or iron, things like that. Because the Lyman Alpha Forest clouds are 85 00:06:25,864 --> 00:06:31,508 so numerous and crowded together, they're, they're often called the Lyman Alpha 86 00:06:31,508 --> 00:06:34,690 Forest. And here is what they might look like in 87 00:06:34,690 --> 00:06:39,904 typical spectrum a quasar, not such a high signal to noise as the one I'm showing 88 00:06:39,904 --> 00:06:42,387 you. The Lyman Alpha Forest lines are 89 00:06:42,387 --> 00:06:46,775 unsaturated sharp lines. But if you have enough column density of 90 00:06:46,775 --> 00:06:52,295 hydrogen, you'll start making very broad saturated line and those are Lyman limit 91 00:06:52,295 --> 00:06:55,872 systems. They're called so because in the limit of 92 00:06:55,872 --> 00:07:02,247 Lyman series,[UNKNOWN], and 916 angstroms, they're very effectively absorbing most of 93 00:07:02,247 --> 00:07:05,051 the light. So, you see the limit to it. 94 00:07:05,051 --> 00:07:10,497 And then there are damped Lyman alpha absorbers, which are really saturated 95 00:07:10,497 --> 00:07:15,111 lines of high column inside, and they of course will also have. 96 00:07:15,111 --> 00:07:19,226 Strong limit systems. So this is just a zoom in on[UNKNOWN] 97 00:07:19,226 --> 00:07:24,544 spectrum I've shown you before. To show you in little more detail, what's 98 00:07:24,544 --> 00:07:27,997 going on. Since we know the redshift of Lyman alpha 99 00:07:27,997 --> 00:07:33,821 very precisely, then we know the redshift of these absorbers with great precision. 100 00:07:33,821 --> 00:07:39,078 And we can fit theoretical profiles through those lines to estimate their 101 00:07:39,078 --> 00:07:43,848 internal broadening or to the Doppler effect and things like that. 102 00:07:43,848 --> 00:07:49,314 This is a table of some of the most commonly observed transitions in observing 103 00:07:49,314 --> 00:07:52,662 clouds. Not for you to remember, but it's a good 104 00:07:52,662 --> 00:07:57,601 Able to have some where should you ever need it to look up such things. 105 00:07:57,601 --> 00:08:02,807 And can see they're all in ultraviolet because that's where, where observing 106 00:08:02,807 --> 00:08:07,226 quasars which at now at high redshift. So how do we quantify that? 107 00:08:07,226 --> 00:08:10,801 This was a very general discussion of absorption in. 108 00:08:10,801 --> 00:08:15,831 Transpiring plasmas, if you will. And there is some atomic chorionic species 109 00:08:15,831 --> 00:08:21,252 that absorbs at a particular wavelength. The strength of the absorption, which can 110 00:08:21,252 --> 00:08:26,565 be measured as the equivalent width, which is defined as the width of a rectangular 111 00:08:26,565 --> 00:08:31,513 line that would cover the same area. [unknown] amount of absorption. 112 00:08:31,513 --> 00:08:37,503 What] we want with is obviously going to be proportional in some way to the numbers 113 00:08:37,503 --> 00:08:42,636 of absorbers along the line of sight. Since we're now looking at things 114 00:08:42,636 --> 00:08:45,968 projected on the sky, it is the column density. 115 00:08:45,968 --> 00:08:52,194 It is a number of hydrogen atoms. Projected in the sky per, say, centimeter 116 00:08:52,194 --> 00:08:55,935 squared. And in case of hydrogen clouds, the 117 00:08:55,935 --> 00:09:02,635 relevant regime is in 10 to the 12 or so, which is the weakest clouds we can detect, 118 00:09:02,635 --> 00:09:07,166 to about 10 to the 21. Hydrogen atoms per square centimeter 119 00:09:07,166 --> 00:09:12,578 projected line, along the line of sight. Incidentally, if you were to take the 120 00:09:12,578 --> 00:09:18,290 Milky Way disk, and squish it, into a plane, the surface density would be about 121 00:09:18,290 --> 00:09:21,501 that much. 10 to the twentieth, or 10 to the 21, 122 00:09:21,501 --> 00:09:26,266 atoms per square centimeter. The, the physics of this is well 123 00:09:26,266 --> 00:09:30,025 understood. It is very well established atomic 124 00:09:30,025 --> 00:09:34,039 physics. And at first as we increase column density 125 00:09:34,039 --> 00:09:39,767 you have proportional growth. But then there is a little wiggle as you 126 00:09:39,767 --> 00:09:43,992 start saturating. And the line can't be any deeper than 127 00:09:43,992 --> 00:09:48,731 complete absorption in, in the middle, so zero transmitive flux. 128 00:09:48,731 --> 00:09:53,026 Then it can only go wider, and so then starts going up again. 129 00:09:53,026 --> 00:09:58,897 The exact position of this regal, depends on the doppler broadening its present in 130 00:09:58,897 --> 00:10:02,532 the line. And so by studying the equivalence with, 131 00:10:02,532 --> 00:10:08,227 by calibrating this, we can then measure, observe, properties, and the find out 132 00:10:08,227 --> 00:10:13,197 both, what the column density is, and also the Doppler broadening is. 133 00:10:13,197 --> 00:10:18,535 Another thing that we can do, is we can count how many clouds that we see, for 134 00:10:18,535 --> 00:10:23,742 unit redshift, and then. Therefore per unit area projected on sky, 135 00:10:23,742 --> 00:10:29,722 and from that, we can infer what the relative sizes of these absorbers are, and 136 00:10:29,722 --> 00:10:36,070 find out as far a hydrogen concern, the higher culumn densities are corresponding 137 00:10:36,070 --> 00:10:40,563 to smaller sizes, their rare. And that's easily understood. 138 00:10:40,563 --> 00:10:46,050 Because the higher column density presumably correspond to inner parts of 139 00:10:46,050 --> 00:10:49,446 the galaxies. Whereas, the outskirts will be much 140 00:10:49,446 --> 00:10:54,225 thinner, and obviously bigger. Likewise, different ionic speeches show 141 00:10:54,225 --> 00:10:57,550 differences in absorption cross section, I guess. 142 00:10:57,550 --> 00:11:02,929 And that's partly due to their, spatial extent and distribution. 143 00:11:02,929 --> 00:11:10,185 Next we will talk in more detail about hydrogen absorber of different kinds.