1 00:00:03,220 --> 00:00:09,049 Let us now turn to the Reionization Era which is currently one of the frontiers of 2 00:00:09,049 --> 00:00:13,037 observational cosmology and theoretical on as well. 3 00:00:13,038 --> 00:00:16,262 A brief history of the universe is as follows. 4 00:00:16,262 --> 00:00:21,085 The, there is a combination, one universe was about 380,000 years old and 5 00:00:21,085 --> 00:00:25,721 microbackground is released. At that point, there were no more sources 6 00:00:25,721 --> 00:00:29,848 of light in the universe. But the dark matter fluctuations keep 7 00:00:29,848 --> 00:00:35,536 collapsing, and eventually, gas that would fall into those potential wells would get 8 00:00:35,536 --> 00:00:40,908 sufficiently dense to make stars,, igniting star formation for proto-galactic 9 00:00:40,908 --> 00:00:44,622 segments. Their UV radiation would then start 10 00:00:44,622 --> 00:00:50,920 reionizing neutral hydrogen in the universe around them and eventually, those 11 00:00:50,920 --> 00:00:56,654 bubbles of ionized gas would start to overlap at which point the universe 12 00:00:56,654 --> 00:01:00,593 becomes transparent again to the UV radiation. 13 00:01:00,593 --> 00:01:03,829 It's, of course, always transparent to the radio waves and infrared. 14 00:01:05,190 --> 00:01:09,734 But, at wavelengths shorter than Lyman-alpha line of neutro hydrogen it was 15 00:01:09,734 --> 00:01:14,188 completely opaque. That is what we call the reionization era, 16 00:01:14,188 --> 00:01:18,997 and at that point, we can that galaxy evolution proper begins. 17 00:01:18,998 --> 00:01:24,690 The universe was reionized by the first stars in flowing to massive black holes or 18 00:01:24,690 --> 00:01:29,068 any black holes. Probably also played a role, but we think 19 00:01:29,068 --> 00:01:33,067 that most of it was done by the first generations of stars. 20 00:01:33,068 --> 00:01:38,800 It turns out that those first stars that formed just from hydrogen and helium, 21 00:01:38,800 --> 00:01:44,696 without dust, as we see in Milky Way today or heavier elements like carbon, carbon 22 00:01:44,696 --> 00:01:48,080 monoxide molecules and so on were different. 23 00:01:48,080 --> 00:01:53,450 In the process of star formation, the protostar has to collapse. 24 00:01:53,450 --> 00:01:58,715 That means it has to lose energy or cool just as we're talking about collapse of 25 00:01:58,715 --> 00:02:04,198 galaxies requiring extra cooling. And, in star formation around us molecular 26 00:02:04,198 --> 00:02:10,804 lines play substantial role in doing that. But, for the first stars only molecular 27 00:02:10,804 --> 00:02:15,991 hydrogen was available, so they couldn't cool quite as efficiently. 28 00:02:15,991 --> 00:02:21,297 And, that leads to formation of very massive generation of first stars. 29 00:02:21,298 --> 00:02:25,909 Multiple theoretical studies have indicated the same thing, that the very 30 00:02:25,909 --> 00:02:30,829 first stars probably had masses of the order of 100s or 1000s of solar masses. 31 00:02:30,830 --> 00:02:37,318 Much more so than stars that are formed today, because the very massive stars are 32 00:02:37,318 --> 00:02:42,840 also very hot and very luminous. They provided excellent sources of UV 33 00:02:42,840 --> 00:02:48,580 radiation to reionize the universe. And indeed, if you look theoretical 34 00:02:48,580 --> 00:02:55,490 Hertzsprung-Russell diagram, HR diagram the plot of luminosity versus temperature. 35 00:02:55,490 --> 00:03:01,958 And if you compare where those stars without heavy elements, just hydrogen and 36 00:03:01,958 --> 00:03:06,217 helium are. They kind of parallel the main sequence of 37 00:03:06,217 --> 00:03:10,427 the young stars today, except they're much hotter. 38 00:03:10,428 --> 00:03:17,073 And the reason they're hotter is that there were slightly smaller and also there 39 00:03:17,073 --> 00:03:21,468 were no metals to absorb UV radiation in the spectra. 40 00:03:21,468 --> 00:03:28,639 Which means they're even more efficient as sources of ionizing radiation [unknown] 41 00:03:28,639 --> 00:03:32,258 galaxies. As those first stars evolved, they would 42 00:03:32,258 --> 00:03:34,814 explode as Population [inaudible] Supernovae. 43 00:03:34,815 --> 00:03:39,125 You may remember, a Population I of stars with a solar discs. 44 00:03:39,126 --> 00:03:42,558 Population II where all stars are galactic halo and bulge. 45 00:03:42,558 --> 00:03:45,966 And by the extension, Population [inaudible] where the very first 46 00:03:45,966 --> 00:03:49,474 generation of stars, made up of hydrogen and helium alone. 47 00:03:49,474 --> 00:03:54,510 Well, they do cook up some elements, some heavy elements and disperse them in some 48 00:03:54,510 --> 00:03:59,394 supernova explosions, that creates the, some small abundance, from which the 49 00:03:59,394 --> 00:04:05,708 Population II stars are formed. People have looked very hard to discover 50 00:04:05,708 --> 00:04:08,823 stars with no metals, but there aren't any left. 51 00:04:08,823 --> 00:04:13,177 The oldest stars, the most metal porous stars are maybe 100,000th of a solar 52 00:04:13,177 --> 00:04:17,800 abundance, but they still have something, and so, that's what we think the first 53 00:04:17,800 --> 00:04:23,197 Population II stars are from. One interesting thing is that many of 54 00:04:23,197 --> 00:04:28,664 these massive stars probably exploded and created black holes, and in that process, 55 00:04:28,664 --> 00:04:33,271 also created gamma-ray bursts. What's shown here is the mass of the 56 00:04:33,271 --> 00:04:38,796 remaining remnant Newton star or black hole versus the initial mass, and for a 57 00:04:38,796 --> 00:04:44,152 large range of masses black holes formed, and gamma-ray burst ensues. 58 00:04:44,152 --> 00:04:49,572 Except for a small, [inaudible] mass, where star is completely destructed and 59 00:04:49,572 --> 00:04:53,881 there is no remnant. So we can expect to see gamma-ray bursts 60 00:04:53,881 --> 00:04:59,650 from the very first generation of super massive stars in the reionization era. 61 00:04:59,650 --> 00:05:05,067 Indeed, here was the very high redshift gamma-ray burst discovered in 2005. 62 00:05:05,067 --> 00:05:10,287 And, just as we can look for quasars or galaxies using the drop tech, drop 63 00:05:10,287 --> 00:05:15,768 technique, here, we can also see it as a strong dropping continuum like that 64 00:05:15,768 --> 00:05:19,853 Lyman-alpha line. And that is characteristic of the high 65 00:05:19,853 --> 00:05:25,165 redshift objects and real spectrum obtained as shown here and that confirms 66 00:05:25,165 --> 00:05:30,294 the photometric determination. Interestingly enough, in this particular 67 00:05:30,294 --> 00:05:33,260 object, the host galaxy of it shows some metals. 68 00:05:33,260 --> 00:05:36,000 So this was not from a really young galaxy. 69 00:05:36,000 --> 00:05:42,721 This galaxy already evolved some. And here is one other redshift 8.2. 70 00:05:42,722 --> 00:05:47,012 The redshift measurement is not 100% certain, because the spectrum was very 71 00:05:47,012 --> 00:05:51,445 poor, but photometric dating indicate that was indeed the correct redshift. 72 00:05:51,445 --> 00:05:56,485 And, this is probably the most distant object to which somewhat reliable redshift 73 00:05:56,485 --> 00:06:00,776 estimate was made. What's shown here is the histogram of 74 00:06:00,776 --> 00:06:05,594 gamma-ray bursts redshift that pretty much parallels the history of star formation in 75 00:06:05,594 --> 00:06:09,791 the universe. And this one is obviously out there beyond 76 00:06:09,792 --> 00:06:13,637 redshifts where other galaxies have been seen. 77 00:06:13,638 --> 00:06:18,732 We think that these first stars started forming maybe around edshifts 20 or 30. 78 00:06:18,732 --> 00:06:23,918 The first proto-galactic fragments, they probably reionized the universe by about 79 00:06:23,918 --> 00:06:26,980 redshift 10 or 15. And that's indicated by micro background 80 00:06:26,980 --> 00:06:29,361 measurements, as we'll see in the next module. 81 00:06:29,362 --> 00:06:36,782 But then, conceivably there was a low in, in that primordial star formation, and in 82 00:06:36,782 --> 00:06:42,311 the first supernovae from Population [inaudible] stars exploded, then they 83 00:06:42,311 --> 00:06:50,130 started to make young Population II stars. And they may have reionized the universe 84 00:06:50,130 --> 00:06:54,937 in a second wave of primordial star formation. 85 00:06:54,938 --> 00:06:59,507 And by about redshift of six, the whole thing is pretty much finished. 86 00:06:59,508 --> 00:07:04,700 So one possible scenario is that after the recombination in the Dark Ages when there 87 00:07:04,700 --> 00:07:09,302 are no stars and all sources of light. You have the very first generation of 88 00:07:09,302 --> 00:07:13,982 supermassive stars reionizing the universe, creating the metals, creating 89 00:07:13,982 --> 00:07:18,260 now Population II stars. The low mass Population II stars are still 90 00:07:18,260 --> 00:07:20,784 with us. They're in galactic halo and globular 91 00:07:20,784 --> 00:07:23,345 clusters. But the very massive ones have, of course, 92 00:07:23,345 --> 00:07:28,056 exploded early on. And then, they can do the second wave of 93 00:07:28,056 --> 00:07:32,357 reionization, which may or may not be really distinct. 94 00:07:32,358 --> 00:07:38,530 We can model this process using hydro simulations that include dark matter, gas 95 00:07:38,530 --> 00:07:42,061 as well as radiation and supernova explosions. 96 00:07:42,061 --> 00:07:47,740 And, here are snapshots from one of those. I can see that the ionization fronts 97 00:07:47,740 --> 00:07:53,482 propagate through the primordial intergalactic medium, the original cosmic 98 00:07:53,482 --> 00:07:59,050 web, in a way that's not simple and spherical, because some hydrogen shields 99 00:07:59,050 --> 00:08:03,305 the material behind it. But eventually, it all does get ionized. 100 00:08:03,305 --> 00:08:13,253 In the next module, we will talk about this in a little more detail.