1 00:00:00,012 --> 00:00:05,241 So galaxies and intergalactic medium have a sort of ecological interplay. 2 00:00:05,241 --> 00:00:09,701 The gas comes into galaxies, gets expelled from the galaxies. 3 00:00:09,701 --> 00:00:13,816 This is where metallic line absorbents have to come from. 4 00:00:13,816 --> 00:00:18,946 And I already mentioned that very high-column density ones probably are 5 00:00:18,946 --> 00:00:23,138 associated with galactic disks or progenitors thereof. 6 00:00:23,139 --> 00:00:28,123 As the line of sight for some high redshift quasar goes through space, it can 7 00:00:28,123 --> 00:00:32,926 encounter more than one of these. And here's a good example of a quasar, and 8 00:00:32,926 --> 00:00:37,546 along the line of sight to this quasar there is a whole number of separate 9 00:00:37,546 --> 00:00:41,964 metallic line absorbers. Each of which we believe now is affiliated 10 00:00:41,964 --> 00:00:45,444 with some sort of halo of some small galaxy or another. 11 00:00:45,445 --> 00:00:50,492 And the way to find out is to first take deep images of fields around those 12 00:00:50,492 --> 00:00:56,288 background quasars, identify galaxies around them, get their redshifts, and see 13 00:00:56,288 --> 00:01:01,664 if any of those actually correspond in redshift to one of the absorption line 14 00:01:01,664 --> 00:01:05,147 systems. This was now done for a large number of 15 00:01:05,147 --> 00:01:08,894 these, and in fact. Essentially every time there is an 16 00:01:08,894 --> 00:01:12,891 association of a galaxy with a given metallic line absorber. 17 00:01:12,891 --> 00:01:17,874 Even so, some of those absorbers can be a very large apparent radius from the 18 00:01:17,874 --> 00:01:20,777 galaxy. And we, we know that this is the case 19 00:01:20,777 --> 00:01:24,402 because we can absorb. Multi-baryonic species. 20 00:01:24,402 --> 00:01:27,968 Not just one absorption line, but whole number of them. 21 00:01:27,968 --> 00:01:30,795 Which then correspond to the same redshift. 22 00:01:30,795 --> 00:01:34,357 And they're matched against the galaxy's own redshift. 23 00:01:34,357 --> 00:01:38,938 So here is an example of whole number of those in one of the deep fields. 24 00:01:38,938 --> 00:01:44,342 And those galaxies tend to have pretty much properties of field galaxies at same 25 00:01:44,342 --> 00:01:49,351 redshifts, which is reassuring. Now what about damp Lyman alpha systems? 26 00:01:49,351 --> 00:01:53,947 Which we always thought were counterparts of high redshift disks. 27 00:01:53,948 --> 00:01:59,745 For some number of them we can also detect Lyman alpha in emission that's offset from 28 00:01:59,745 --> 00:02:04,629 the absorption line of sight. And so here is an example of a quasar with 29 00:02:04,629 --> 00:02:09,642 a damped Lyman-alpha object present. Then in a plot on the right you can see a 30 00:02:09,642 --> 00:02:14,611 spectrum from blue to red and the orthogonal direction is the longest slit. 31 00:02:14,611 --> 00:02:19,699 There's a gap that corresponds to the damped absorber and just offset from it is 32 00:02:19,699 --> 00:02:23,719 a blob that corresponds to emission from the same absorber. 33 00:02:23,720 --> 00:02:29,992 From the strengths of those emission lines, from their separation, from, other 34 00:02:29,992 --> 00:02:36,328 properties, it was concluded that, indeed, they're perfectly consistent with being 35 00:02:36,328 --> 00:02:41,624 disks or youngest, or progenitors of young disks, at high redshifts. 36 00:02:41,625 --> 00:02:47,432 So if metallic line absorbers are associated with galaxies, they should also 37 00:02:47,432 --> 00:02:50,702 cluster like galaxies, and in fact they do. 38 00:02:50,702 --> 00:02:56,658 Here is a comparison of such a coloration function for the metallic line absorbers, 39 00:02:56,658 --> 00:03:02,523 adjusting radio coordinate that gets the only one we can measure and also for Lyman 40 00:03:02,523 --> 00:03:06,235 alpha clouds. Lyman alpha clouds are also correlated but 41 00:03:06,235 --> 00:03:09,803 much, much less. So, metallic absorbers do correspond to 42 00:03:09,803 --> 00:03:15,132 high redshift galaxies whereas Lyman alpha clouds are still material that's in linear 43 00:03:15,132 --> 00:03:19,943 regime that belongs to large scale structure, not to galaxies themselves. 44 00:03:19,943 --> 00:03:26,162 And then we can compare how the absorbers of different types involving red-shift. 45 00:03:26,162 --> 00:03:32,018 As we look at hydrogen absorbers, their numbers increase with redshift, as we 46 00:03:32,018 --> 00:03:36,621 already noted before. For the metallic line absorbers, we see 47 00:03:36,621 --> 00:03:41,781 first an, well, looking from high redshifts, towards us, meaning in the 48 00:03:41,781 --> 00:03:45,463 direction of time. We see an increase as you build up 49 00:03:45,463 --> 00:03:51,243 galaxies and star formation, presumably, and then things roll over and flatten out. 50 00:03:51,243 --> 00:03:55,746 So this is exactly what we would expect from this picture, in which. 51 00:03:55,746 --> 00:03:59,326 Galaxies are built up slowly, then they eject gas out. 52 00:03:59,326 --> 00:04:04,356 And so the metal rich intergalactic medium is also being accumulated in time. 53 00:04:04,356 --> 00:04:09,444 So, I mentioned that damp Lyman alpha absorbers are likely to be progenitors of 54 00:04:09,444 --> 00:04:13,159 present day disks. And one way to look at this is to look at 55 00:04:13,159 --> 00:04:17,672 their chemical abundances. The column density is so high that in 56 00:04:17,672 --> 00:04:23,067 addition to hydrogen we can measure various metallic species and from that we 57 00:04:23,067 --> 00:04:26,608 can infer metallicity just as we would for stars. 58 00:04:26,608 --> 00:04:31,762 Here the histograms comparing metallicity distributions in Damped Ly-alpha 59 00:04:31,762 --> 00:04:36,295 absorbers, the solid blue. With metallicities of stars in the thick 60 00:04:36,295 --> 00:04:39,511 disks of the Milky Way on the left, the red one. 61 00:04:39,512 --> 00:04:44,437 And the thin disk of the Milky Way, the green one, on the right. 62 00:04:44,437 --> 00:04:50,175 As you can see the, they overlap. But Lymanova absorbers are much closer in 63 00:04:50,175 --> 00:04:56,545 their metallicity to the thick disk stars, the old disk population, than to the thin 64 00:04:56,545 --> 00:05:00,775 disk stars, which are just relatively recently made. 65 00:05:00,775 --> 00:05:05,022 Chemically evolved systems. And, just as we looked at the history of 66 00:05:05,022 --> 00:05:10,197 chemical enrichment in galaxies we can do the same thing now for absorption clouds 67 00:05:10,197 --> 00:05:14,811 and here is the plot of the metal abundance of damped Lyman alpha systems. 68 00:05:14,811 --> 00:05:19,371 It's a function of redshift. The individual red dots are individual 69 00:05:19,371 --> 00:05:24,018 objects and then they're being, those are the big air process. 70 00:05:24,018 --> 00:05:29,303 So you can see there's a gradual increase. You start with the broad distribution 71 00:05:29,303 --> 00:05:34,462 tending to belong with the list of high redshift and you gradually climb towards 72 00:05:34,462 --> 00:05:40,121 almost solar abundance low redshifts. Now one has to beware that different types 73 00:05:40,121 --> 00:05:43,646 of absorbers may be evolving in a different fashion. 74 00:05:43,646 --> 00:05:48,722 In fact, they correspond to different parts of galaxies so damped Lyman alpha 75 00:05:48,722 --> 00:05:54,026 absorbers are kiloparsec scales, disc scales whereas Lyman alpha forest clouds 76 00:05:54,026 --> 00:05:59,231 may be hundreds of kiloparsec scales. On the other hand you can look at things 77 00:05:59,231 --> 00:06:04,901 like quasars whose metallicity you can also measure from their spectra and they 78 00:06:04,901 --> 00:06:10,085 can be up to 10 times solar and they probably correspond to regions in the very 79 00:06:10,085 --> 00:06:13,770 cores of their host galaxies or near the black hole. 80 00:06:13,770 --> 00:06:19,562 So the chemical evolution will proceed a different pace in different environments. 81 00:06:19,562 --> 00:06:24,631 It will grow faster in the dense environments, we get more of recycling of 82 00:06:24,631 --> 00:06:29,375 stars and so on but we can outgrow that over both range of scale. 83 00:06:29,375 --> 00:06:36,363 So we can add up all this hydrogen that is to be used for star formation in galaxies 84 00:06:36,363 --> 00:06:42,646 and if we have proper census of absorbers per unit redshift and so on we can 85 00:06:42,646 --> 00:06:48,764 integrate that as briefly shown here. So from that we can then deduce. 86 00:06:48,765 --> 00:06:54,922 The evolution of the, say, omega in hydrogen gas, that belongs to these clouds 87 00:06:54,922 --> 00:06:58,419 as a function of redshift. And so here it is. 88 00:06:58,419 --> 00:07:03,069 Compared to the measurement of the local stellar dentsity. 89 00:07:03,069 --> 00:07:09,507 That's the red point on the left, upper left, as well, some of the theoretical 90 00:07:09,507 --> 00:07:12,362 models. And you can see that there is 91 00:07:12,362 --> 00:07:18,526 approximately one part in thousand of omega in hydrogen gas that belongs to this 92 00:07:18,526 --> 00:07:23,723 class. You may remember that omega variance, all 93 00:07:23,723 --> 00:07:29,845 variance, is approximately 0.45. Which is, so 45 times this. 94 00:07:29,845 --> 00:07:36,715 But the omega of stars that we see in galaxies is only something like, 5 times 95 00:07:36,715 --> 00:07:41,564 10 to the minus 3. So this is, a fraction, but sizable 96 00:07:41,564 --> 00:07:45,676 fraction, of all material that we see in stars. 97 00:07:45,677 --> 00:07:49,913 So to recap, the intergalactic medium is forming cosmic web. 98 00:07:49,913 --> 00:07:55,839 It's associated with large scale structure that's still collapsing linear regime, but 99 00:07:55,839 --> 00:08:00,959 metal rich parts in the highest density hydrogen clouds are associated with 100 00:08:00,959 --> 00:08:04,569 galaxies. Pieces that have already fallen together 101 00:08:04,569 --> 00:08:08,672 and made stars. The intergalactic medium is almost 102 00:08:08,672 --> 00:08:15,112 entirely ionized from here out to redshift about six, at which point you start to get 103 00:08:15,112 --> 00:08:21,184 more neutral hydrogen because stars and quasars simply didn't have time yet to 104 00:08:21,184 --> 00:08:26,797 re-ionize it fully before that. The metals themselves come from galactic 105 00:08:26,797 --> 00:08:32,917 winds, products of stellar evolution that have been expelled from galaxies and mixed 106 00:08:32,917 --> 00:08:39,114 in with the rest of intergalactic medium. And these studies using absorption 107 00:08:39,114 --> 00:08:46,324 systems, using quasars or gamma ray bursts as background lights A compliment other 108 00:08:46,324 --> 00:08:54,176 ways of studying galaxy evolution. In the next chapter, we will finally look 109 00:08:54,176 --> 00:09:01,961 at galaxy formation itself and cosmic reionization in more detail.