1 00:00:00,012 --> 00:00:06,849 Let's now talk about the most common absorbers which are those due to hydrogen. 2 00:00:06,849 --> 00:00:13,447 We can divide them according to the projected column density, as I mentioned 3 00:00:13,447 --> 00:00:17,224 earlier. So, Lyman alpha Forest, are clouds which 4 00:00:17,224 --> 00:00:22,536 have unsaturated lines, and they correspond to column densities ranging up 5 00:00:22,536 --> 00:00:26,453 to ten to the 16 per square centimeter, or thereabouts. 6 00:00:26,453 --> 00:00:32,171 They have very little, if any metals, and their, their sizes are bigger than those 7 00:00:32,171 --> 00:00:36,115 of galaxies, they're not yet collapsed into galaxies. 8 00:00:36,116 --> 00:00:44,417 Next are the Lymon limit systems. Those have column densities ranging from 9 00:00:44,417 --> 00:00:51,313 10 to the 16 through maybe 10 to the 18 or so square centimeters. 10 00:00:51,314 --> 00:00:58,512 They're called so because there is enough absorption going on that you can see the 11 00:00:58,512 --> 00:01:05,116 break of the alignment series at rest frame by length of the 912 angstroms. 12 00:01:05,116 --> 00:01:10,148 The Lyman alpha lines themselves are already saturated. 13 00:01:10,149 --> 00:01:15,405 And so it's the amplitude of the break that's actually interesting quantity here. 14 00:01:15,405 --> 00:01:20,299 Going to the higher densities, we see so-called damped Lyman alpha systems. 15 00:01:20,299 --> 00:01:24,906 They're completely saturated. They're called damped, because what you 16 00:01:24,906 --> 00:01:29,323 see is the absorption by so-called damping rings of absorption line. 17 00:01:29,323 --> 00:01:32,268 And if you take in any stellar atmospheres, or. 18 00:01:32,268 --> 00:01:37,313 Maybe atomic physics that deals with this, then you know what it means. 19 00:01:37,313 --> 00:01:43,059 We believe that these high-cone density systems are almost certainly parts of this 20 00:01:43,059 --> 00:01:48,553 galaxy with high redshift, or at least their progenitors.The things that we can 21 00:01:48,553 --> 00:01:53,420 measure, therefore, are the equivalent widths and the broadening. 22 00:01:53,420 --> 00:01:57,336 And there are many Codes that can be used for this. 23 00:01:57,336 --> 00:02:00,911 Now this is a well understood and well developed art. 24 00:02:00,911 --> 00:02:05,661 And so you're in highly complex spectrum, like the one I show you here. 25 00:02:05,661 --> 00:02:11,499 Can be fitted very well with a collection of modeled lines, that go well through the 26 00:02:11,499 --> 00:02:15,082 data points. And this is what a damped Lyman alpha 27 00:02:15,082 --> 00:02:18,934 system looks like. It looks like there this hole in the 28 00:02:18,934 --> 00:02:24,058 spectrum with kind of curved wings to it and the, the amount of absorption 29 00:02:24,058 --> 00:02:27,392 determines how wide that system is going to be. 30 00:02:27,392 --> 00:02:32,003 So how many of which they there? It turns out that the probability 31 00:02:32,003 --> 00:02:35,558 distribution of column densities is a parallel. 32 00:02:35,558 --> 00:02:39,156 As shown here. And it's a power wall with a slope of 33 00:02:39,156 --> 00:02:44,868 minus 1.7, which is interestingly close to the slope of the two point correlation 34 00:02:44,868 --> 00:02:48,378 function and that's not entirely an accident. 35 00:02:48,378 --> 00:02:53,384 But it goes from the lowest column densities that we can see to the highest 36 00:02:53,384 --> 00:02:56,220 ones. [inaudible] So now let's take a look at 37 00:02:56,220 --> 00:03:00,893 the evolution of these absorbers. They're comoving number density, their 38 00:03:00,893 --> 00:03:06,003 comoving cubic megaparsec changes in time. It's lower here and it's much higher at 39 00:03:06,003 --> 00:03:09,486 high redshifts and here is a good illustration of this. 40 00:03:09,486 --> 00:03:14,517 There are two quasars, very different redshifts, one low, one high, and they've 41 00:03:14,517 --> 00:03:18,132 been plotted in the rest frame wavelength, so see that. 42 00:03:18,132 --> 00:03:23,328 For the lower redshift one there hardly any Lyman alpha clouds to be seen. 43 00:03:23,328 --> 00:03:26,094 At high red shift there is ubiquity of them. 44 00:03:26,094 --> 00:03:31,122 And so that's probably because these clouds eventually get absorbed into 45 00:03:31,122 --> 00:03:34,434 galaxies. Using simple Freedman models you can 46 00:03:34,434 --> 00:03:38,113 derive formulae that will correspond to number. 47 00:03:38,114 --> 00:03:43,485 Per unit ratchet interval, as a function of ratchet, as a function of cosmological 48 00:03:43,485 --> 00:03:48,105 parameters, and here, I'm giving you a formula for models with now, with 49 00:03:48,105 --> 00:03:53,043 non-cosmological constant, although the, those can be computed as well. 50 00:03:53,043 --> 00:03:59,626 And it turns out that also high ratchets. The number increases as parallel of the 51 00:03:59,626 --> 00:04:08,099 stretch factor 1 plus z to the 1.8 power. But, does this not apply to all redshifts. 52 00:04:08,099 --> 00:04:14,804 And one of the interesting results from Hubble space telescope was. 53 00:04:14,805 --> 00:04:20,340 When we can finally observe quasar spectra outside of Earth's atmosphere so that 54 00:04:20,340 --> 00:04:25,199 ultraviolet can be observed. Note that the atmospheric transmission 55 00:04:25,199 --> 00:04:30,496 window starts around 3,000 angstroms. [unknown] of that it's all absorbed by 56 00:04:30,496 --> 00:04:35,193 the, by the oxygen or ozone, but outside the atmosphere we don't have that 57 00:04:35,193 --> 00:04:38,882 limitation. So we could now look at Lymon alpha forest 58 00:04:38,882 --> 00:04:42,372 at lower redshifts then we could do from the ground. 59 00:04:42,372 --> 00:04:46,457 Say from the ground it can do from about redshift 1.6, 1.8. 60 00:04:46,457 --> 00:04:49,670 Up. And, from space, you can look at arbitrary 61 00:04:49,670 --> 00:04:52,809 low red shifts. And they turn out that there is a 62 00:04:52,809 --> 00:04:57,077 substantial change in the slope. There is a change in population. 63 00:04:57,077 --> 00:05:02,327 This may or may not having something to do with the peak of the cosmic star formation 64 00:05:02,327 --> 00:05:05,633 history that happens at around the same red shift. 65 00:05:05,633 --> 00:05:09,344 You may think that. Clouds have been absorbed into galaxies up 66 00:05:09,344 --> 00:05:13,833 until then and then things kind of slow down so the column density doesn't change 67 00:05:13,833 --> 00:05:16,748 very much. And, indeed, as we go to higher redshifts 68 00:05:16,748 --> 00:05:21,038 you get thicker and thicker forests but then what happens is that lines start to 69 00:05:21,038 --> 00:05:24,726 overlapping. The trees are lined up so dense that you 70 00:05:24,726 --> 00:05:30,816 don't see gaps between the trees and this is illustrated here as a set of windows in 71 00:05:30,816 --> 00:05:36,094 redshift space and lines of sight towards some high redshift quasars. 72 00:05:36,095 --> 00:05:40,774 At around redshift of four you see a very thick Lymon alpha forest and then it 73 00:05:40,774 --> 00:05:45,954 superficially looks at, like it's thinning out, but actually it's just overlaps of 74 00:05:45,954 --> 00:05:49,477 the lines. And what looks like little emission spikes 75 00:05:49,477 --> 00:05:54,578 are just gaps between the lines. As you go to ever higher redshifts, it all 76 00:05:54,578 --> 00:05:57,741 fills up. And so the inter-galactic medium is 77 00:05:57,741 --> 00:06:00,784 effectively opaque to the Lyman alpha line. 78 00:06:00,784 --> 00:06:06,472 This leads into the so-called reionization that we will talk about in more detail. 79 00:06:06,472 --> 00:06:10,171 It is quantified by so-called Gunn-Peterson effect. 80 00:06:10,172 --> 00:06:13,792 Named after the two astronomers who came up with it. 81 00:06:13,792 --> 00:06:19,538 And they noticed that if you increase the net column density of neutral hydrogen, at 82 00:06:19,538 --> 00:06:25,196 some point it is so saturated that you're just going to completely lose all the flux 83 00:06:25,196 --> 00:06:29,239 below of to the center of[UNKNOWN]. It doesn't take a lot. 84 00:06:29,239 --> 00:06:35,011 It takes maybe one part in 10,000 of the neutral hydrogen, in the IGM at that 85 00:06:35,011 --> 00:06:39,887 redshift, to do this. So, recall the history of the universe, in 86 00:06:39,887 --> 00:06:45,802 some sense, is that there was first ionized plasma, it recombines, and this is 87 00:06:45,802 --> 00:06:49,448 when cosmic microwave background is relased. 88 00:06:49,449 --> 00:06:55,216 Age little shy of 400 thousand years. Then there are no sources of light in the 89 00:06:55,216 --> 00:07:01,180 universe, and the universe is filled with neutral hydrogen, still some, and neutral 90 00:07:01,180 --> 00:07:06,347 helium, as well as dark matter, and so on. And that neutral hydrogen would 91 00:07:06,347 --> 00:07:11,018 effectively absorb all photons blueward at 1216 axtroms in space. 92 00:07:11,019 --> 00:07:13,997 Radio waves of course can still go through. 93 00:07:13,997 --> 00:07:16,958 Then sources of light may start turning on. 94 00:07:16,958 --> 00:07:22,852 They reionize the intergalactic medium. And thus they make it, transparent again, 95 00:07:22,852 --> 00:07:27,892 to ultraviolet light, except where there are little clouds of hydrogen. 96 00:07:27,892 --> 00:07:30,987 And those are the Lyman alpha forest clouds. 97 00:07:30,988 --> 00:07:36,847 So transition from purely opaque more or less continues not very ionized 98 00:07:36,847 --> 00:07:42,832 intergalactic medium with very high redshifts to the one that's ionized by 99 00:07:42,832 --> 00:07:47,880 stars and quasars. Is called the reionization era and until 100 00:07:47,880 --> 00:07:54,379 recently this was one of the major goals of cosmology to find where it happens and 101 00:07:54,379 --> 00:07:58,530 now we know. The first examples of this we're seeing 102 00:07:58,530 --> 00:08:04,961 circa 2000, 2001 the[UNKNOWN] quaser is discovered by Sloan digital sky survey. 103 00:08:04,961 --> 00:08:10,752 And now there is a number of those several tens and around redshift of six those 104 00:08:10,752 --> 00:08:15,665 effects start to come in. You see sudden drop, essentially complete 105 00:08:15,665 --> 00:08:20,977 absorption[UNKNOWN] the Lyman alpha line until you get to sufficiently low 106 00:08:20,977 --> 00:08:26,643 redshifts so then things can start coming up again and then you run into the limit. 107 00:08:26,643 --> 00:08:31,853 This is the end of[UNKNOWN] era. The beginning starts maybe oh, redshift 20 108 00:08:31,853 --> 00:08:35,807 or. 30, that's still subject of some research, 109 00:08:35,807 --> 00:08:42,734 but we've seen this signature at least. And we'll talk more about this in the next 110 00:08:42,734 --> 00:08:47,991 chapter when we talk about galaxy formation and reionization. 111 00:08:47,991 --> 00:08:53,437 So here is a plot of the transmitted flux at Lyman alpha wavelength as a function of 112 00:08:53,437 --> 00:08:56,979 red shift from a large number of different quasars. 113 00:08:56,979 --> 00:09:01,393 And as you can see, as you go to every higher red shifts, more and more is 114 00:09:01,393 --> 00:09:06,561 absorbed, less and less gets through, and then there is essentially waterfalls, 115 00:09:06,561 --> 00:09:11,578 steep cut at around red shift of six. Which is essentially the effected that I 116 00:09:11,578 --> 00:09:20,216 just described to you. Next we will talk about connection between 117 00:09:20,216 --> 00:09:26,820 galaxies and intergalactic absorbers.