1 00:00:03,240 --> 00:00:08,085 So, we saw how the primordial star formation, the very first stars of 2 00:00:08,085 --> 00:00:14,437 Population II Stars, reionized the inverse as the first protogalactic fragments form. 3 00:00:14,438 --> 00:00:17,002 Let's take a look at this in a little more detail. 4 00:00:17,002 --> 00:00:23,220 Here, again, is the same schematic outline of how things happen that you start with 5 00:00:23,220 --> 00:00:29,467 my career background, really, is an age of 300 or 400,000 years after the Big Bang. 6 00:00:29,468 --> 00:00:34,512 Then, dark halos keep forming through what's called Dark Ages, when the universe 7 00:00:34,512 --> 00:00:39,614 was about a few hundred million years old. Maybe about 500 million years old, the 8 00:00:39,614 --> 00:00:44,810 very first stars formed. Then, through the next few hundred million 9 00:00:44,810 --> 00:00:52,160 years until about age of about one billion years, the reionization commences and it's 10 00:00:52,160 --> 00:00:57,854 complete by about the redshift of six. We can simulate the very first star 11 00:00:57,854 --> 00:01:03,014 formation and this is from one of such simulations by Tom Abel and his 12 00:01:03,014 --> 00:01:07,693 collaborators. And some of these first stars can ionize 13 00:01:07,693 --> 00:01:13,587 the gas around them, as well as contribute metals through their explosions. 14 00:01:13,588 --> 00:01:17,257 The way we can actually detect reionization here is through the 15 00:01:17,257 --> 00:01:22,190 Gunn-Peterson Effect that we mentioned earlier when we spoke about intergalactic 16 00:01:22,191 --> 00:01:27,398 absorption. This was developed already in the 1960s, 17 00:01:27,398 --> 00:01:32,482 it was recognized when quasars were discovered that they can pro, provide 18 00:01:32,482 --> 00:01:36,967 excellent probe of intergalactic medium with high redshifts. 19 00:01:36,968 --> 00:01:41,798 And there is even small amount of neutral hydrogen, it will be very effective in 20 00:01:41,798 --> 00:01:45,260 absorbing radiation blueward of the Lyman-alpha line. 21 00:01:45,260 --> 00:01:53,170 It will manifest itself as a sudden drop influx blueward of the Lyman-alpha line 22 00:01:53,170 --> 00:01:59,290 and that was finally seen circa 2000. Here is how it works. 23 00:01:59,290 --> 00:02:06,016 If you're looking at the quasar, it's just inside the reionization era zone. 24 00:02:06,017 --> 00:02:11,849 Then, the line of sight will go through a lot of these bubbles and the gaps between 25 00:02:11,849 --> 00:02:18,287 them will be where neutral hydrogen is and they'll manifest themselves as continuum 26 00:02:18,287 --> 00:02:22,812 between absorption lines. As you get closer and closer to the 27 00:02:22,812 --> 00:02:28,775 reionization itself, the overlaps of the bubbles will be less and less and there 28 00:02:28,775 --> 00:02:34,738 will be more neutral hydrogen that will just absorb large chunks of spectrum 29 00:02:34,738 --> 00:02:40,790 blueward of the Lyman-alpha of the, of the quasar itself and the spectrum may look 30 00:02:40,790 --> 00:02:45,608 like something like this. And indeed, this was observed. 31 00:02:45,608 --> 00:02:51,251 In essentially all quasars at about redshift of 6 and above, the same 32 00:02:51,251 --> 00:02:55,545 phenomenon was seen. There was a very sudden drop of flux at 33 00:02:55,545 --> 00:03:00,680 Lyman-alpha line, and then at lower redshifts, the Lyman-alpha forest would 34 00:03:00,680 --> 00:03:05,736 continue, but then you would run into the Lyman limit that corresponds to the 35 00:03:05,736 --> 00:03:09,467 redshift of the quasar, and then flux will be gone again. 36 00:03:09,468 --> 00:03:15,312 So, this was the most distant quasar for a while, and, shows beautiful example of the 37 00:03:15,312 --> 00:03:19,664 Gunn-Peterson effect. So, the face value of it, we have found 38 00:03:19,664 --> 00:03:28,038 reionization at about redshift of 6. If you look at transmission of Lyman-alpha 39 00:03:28,038 --> 00:03:33,934 flux against redshift, the higher redshift you go, the less flux it goes through 40 00:03:33,934 --> 00:03:39,110 because of Lyman-alpha forest. But then, by about red shift of 6, there 41 00:03:39,110 --> 00:03:44,720 is a, just, dramatic increase in absorption, which was seen and manifested 42 00:03:44,720 --> 00:03:50,415 itself as Gunn-Peterson effect, and this is why people think that that's pretty 43 00:03:50,415 --> 00:03:56,124 much when reionization era ended. You can convert the observed absorption 44 00:03:56,124 --> 00:04:02,858 into the opacity of neutral hydrogen or fraction of the neutral hydrogen, and the 45 00:04:02,858 --> 00:04:09,592 sudden drop in transmitted flux maps into sudden increase in the fraction of neutral 46 00:04:09,592 --> 00:04:14,641 hydrogen. But it's enough that just only a percent 47 00:04:14,641 --> 00:04:20,287 or so of hydrogen is there to achieve the same effect. 48 00:04:20,288 --> 00:04:24,264 There is also variaion in different lines of sight. 49 00:04:24,264 --> 00:04:29,080 This shows the Lyman-alpha forest in the approach to reionization along four 50 00:04:29,080 --> 00:04:33,239 different lines of sight to four different high redshift quasars. 51 00:04:33,239 --> 00:04:38,078 We think we understand this because of the biasing, that there'll be substantial 52 00:04:38,078 --> 00:04:42,693 cosmic variance, there'll be different large scale structures, there'll be 53 00:04:42,693 --> 00:04:47,620 structures forming at the highest peaks. That means that there will be actually a 54 00:04:47,620 --> 00:04:53,495 certain lumpiness in the reionization. It will not proceed everywhere at exactly 55 00:04:53,495 --> 00:04:56,921 same pace. A new effect that was seen with WMAP 56 00:04:56,921 --> 00:05:03,151 satellite that measured fluctuations in cosmic microwave background was a sudden 57 00:05:03,151 --> 00:05:09,203 increase in electron opacity that corresponds to reionization, but this one 58 00:05:09,203 --> 00:05:15,522 indicated that it happened around redshift of 10 or 20, which would be substantially 59 00:05:15,522 --> 00:05:21,380 higher than what quasar data indicated. Now, the more modern analysis of the 60 00:05:21,380 --> 00:05:27,788 microbackground data gives us optical depth due to free electrons, which come 61 00:05:27,788 --> 00:05:33,840 from ionization of hydrogen and have a peak of reionization around redshift of 10 62 00:05:33,840 --> 00:05:36,411 or so. It's consistent with an instant 63 00:05:36,411 --> 00:05:41,521 reionization but that's physically not very likely and most likely, we're dealing 64 00:05:41,521 --> 00:05:46,558 with an extended period from the formation of very first stars until the time where 65 00:05:46,558 --> 00:05:52,075 all the ionization bubbles overlaped. And that may have taken form redshift that 66 00:05:52,075 --> 00:05:56,150 are 20 or slightly higher, all the way down to redshift of 6. 67 00:05:56,150 --> 00:06:02,000 In terms of actual age, that means from few hundred million years after the Big 68 00:06:02,000 --> 00:06:06,470 Bang to close to about one billion years after the Big Bang. 69 00:06:06,470 --> 00:06:12,083 And even though the quasar data by themselves cannot really guarantee that 70 00:06:12,083 --> 00:06:18,402 there was a substantial amount of neutral hydrogen, remember, only a percent or even 71 00:06:18,402 --> 00:06:22,830 a fraction of a percent is enough to cause the same effect. 72 00:06:22,830 --> 00:06:28,193 We have other ways of looking at it. One is counting how many Lyman-alpha 73 00:06:28,193 --> 00:06:32,683 galaxies are there in any given redshift interval. 74 00:06:32,683 --> 00:06:37,301 And we expect that as we get into the region of neutral hydrogen, sufficiently 75 00:06:37,301 --> 00:06:41,703 high redshifts, suddenly there'll be a drop in the numbers of Lyman-alpha 76 00:06:41,703 --> 00:06:45,237 emitters, because light will be absorbed by the hydrogen. 77 00:06:45,238 --> 00:06:49,501 And that's exactly what we've seen. There seems to be a, a rapid decline in 78 00:06:49,501 --> 00:06:54,372 the number density of Lyman-alpha galaxies past redshift of 6. 79 00:06:54,372 --> 00:07:01,688 Also, in the most distant gamma burst and quasars, we can fit the shape of the 80 00:07:01,688 --> 00:07:10,040 absorption the cause is, is actually the, the damp wing of Lyman-alpha line and its 81 00:07:10,040 --> 00:07:14,934 shape depends on the amount of neutral hydrogen. 82 00:07:14,934 --> 00:07:21,240 So, that indicates that there was about 10% of neutral hydrogen by mass or by 83 00:07:21,240 --> 00:07:27,862 volume at around redshift of 6. So, it's, the universe was not fully 84 00:07:27,862 --> 00:07:35,467 neutral but it was pretty much at the end of that reionization phase then. 85 00:07:35,468 --> 00:07:40,586 So, combining the data from Lyman-alpha galaxies, from quasars, and from microid 86 00:07:40,586 --> 00:07:45,618 background yields a fairly consistent picture that we can see how star formation 87 00:07:45,618 --> 00:07:50,664 density in the universe is dropping. This galaxy, as we go to high redshifts, 88 00:07:50,664 --> 00:07:56,076 these galaxies are just forming back then that resulted in change of ionized or 89 00:07:56,076 --> 00:08:00,528 neutral hydrogen fraction. It also, as galaxies were being built up, 90 00:08:00,528 --> 00:08:05,424 you can infer how much mass was generated in stars nd you can also make predictions 91 00:08:05,424 --> 00:08:10,572 of what should have been seen in microid background measurements and in, in fact 92 00:08:10,572 --> 00:08:15,342 that matches the observations. So, we now seem to have a reasonably 93 00:08:15,342 --> 00:08:21,326 consistent story about formation of first stars and galaxies in that era between 94 00:08:21,326 --> 00:08:27,046 redshifts 20, 30 or so, when first stars form until about redshift of 6 when the 95 00:08:27,046 --> 00:08:31,562 reorganization is complete. Going even deeper would require a 96 00:08:31,562 --> 00:08:36,416 different kind of measurement. Neutral hydrogen, even before any stars 97 00:08:36,416 --> 00:08:40,555 are formed, will still be detectable to 21 centimeter line. 98 00:08:40,555 --> 00:08:46,570 That was used so effectively to map kinematics of gas and spiral galaxies. 99 00:08:46,570 --> 00:08:52,520 And there are theoretical studies of this so that even before first stars are formed 100 00:08:52,520 --> 00:08:57,875 that you can principal map assembly of baryons in potential wells of future 101 00:08:57,875 --> 00:09:00,681 galaxies. And there are radio telescopes and 102 00:09:00,681 --> 00:09:04,290 experiments being developed to try to detect precisely this. 103 00:09:04,290 --> 00:09:10,591 So far it's too early to actually expect any detections, but within next several 104 00:09:10,591 --> 00:09:16,891 years or 10 years everybody thinks that we will actually be able to observe these 105 00:09:16,891 --> 00:09:22,831 protogalaxies before they even make stars using 21 centimeter line of neutral 106 00:09:22,831 --> 00:09:28,712 hydrogen. And next, we will finally turn to active 107 00:09:28,712 --> 00:09:37,353 galactic nuclei and their role in cosmology and galaxy formation.