1 00:00:00,012 --> 00:00:05,087 Now let's see what the observations say about galaxy evolution. 2 00:00:05,087 --> 00:00:10,673 The simplest thing to do is take images of different wavelengths that is 3 00:00:10,673 --> 00:00:17,462 observationally much cheaper than taking spectra, which require longer integration 4 00:00:17,462 --> 00:00:20,948 times. And this can be used to do galaxy counts 5 00:00:20,948 --> 00:00:26,334 as a function of their brightness, color etc., but that can only go so far. 6 00:00:26,334 --> 00:00:31,762 And to really understand what's going on, redshifts really are necessary. 7 00:00:31,762 --> 00:00:38,032 So it was only in the recent years that we have obtained sufficiently large samples 8 00:00:38,032 --> 00:00:41,120 of. Galaxies in deep fields to really reach a 9 00:00:41,120 --> 00:00:44,857 good observational understanding of galaxy evolution. 10 00:00:44,857 --> 00:00:49,782 There is one important dichotomy. You can think of star formation as coming 11 00:00:49,782 --> 00:00:55,008 two flavors: the simple, unobscured star formation where you see the light from 12 00:00:55,008 --> 00:01:00,624 stellar photospheres as they are, or light absorbed and re-radiated by interstellar 13 00:01:00,624 --> 00:01:05,835 dust, which now moves all of the energy into infrared or submillimeter regime. 14 00:01:05,835 --> 00:01:10,477 So there are two regimes in which we can observe galaxy evolution and star 15 00:01:10,477 --> 00:01:15,417 formation in them, and each of them has its own tools and selection effects and 16 00:01:15,417 --> 00:01:19,079 limitations. You may recall, when we first mentioned 17 00:01:19,079 --> 00:01:22,394 Source counts as a potential cosmological test. 18 00:01:22,394 --> 00:01:26,051 That the evolutionary effects really mess thing up. 19 00:01:26,051 --> 00:01:31,433 And they always work in the sense of making counts higher than they would be in 20 00:01:31,433 --> 00:01:36,370 the absence of the evolution. Even the galaxies that are evolving in 21 00:01:36,370 --> 00:01:41,326 brightness say due to the fading of stellar populations that were more 22 00:01:41,326 --> 00:01:46,929 luminous in the past and therefore they would be at the higher magnitude level. 23 00:01:46,929 --> 00:01:52,120 But there would be many more of them thus, they'd be moving to the left, and 24 00:01:52,120 --> 00:01:57,507 producing a less declining curve. Likewise, if galaxies are assembled from 25 00:01:57,507 --> 00:02:02,853 smaller pieces, there are more smaller pieces in the past than exactly the same 26 00:02:02,853 --> 00:02:07,834 observational effect appears. To disentangle these we need redshift 27 00:02:07,834 --> 00:02:11,400 theories. Still here at the deep galaxy counts from 28 00:02:11,400 --> 00:02:16,484 the Hubble deep fields and these days we do this down to about twenty-ninth 29 00:02:16,484 --> 00:02:22,306 magnitude or thereabouts which is really spectacular and by extrapolation over the 30 00:02:22,306 --> 00:02:27,472 entire sky, there may be a couple of hundred billion galaxies within the 31 00:02:27,472 --> 00:02:31,622 observable universe. You may also recall that evolution is 32 00:02:31,622 --> 00:02:36,614 expected to appear stronger in bluer wavelengths, and less so in redder, and 33 00:02:36,614 --> 00:02:40,564 that's indeed exactly what's seen here. Now, this is now. 34 00:02:40,564 --> 00:02:45,848 Infrared galaxy counts and they show roughly minor discrepancy compared to the 35 00:02:45,848 --> 00:02:51,196 much stronger ones that are observable in the, in the bluer parts of the spectrum. 36 00:02:51,196 --> 00:02:55,388 But those galaxies evolve as their stellar populations evolve. 37 00:02:55,388 --> 00:02:59,775 Their colors evolve too, generally going from bluer to redder. 38 00:02:59,775 --> 00:03:03,209 However that's also complicated by the redshift. 39 00:03:03,209 --> 00:03:08,995 The whole spectral entry distribution is moving from bluer filters to the redder 40 00:03:08,995 --> 00:03:12,406 filters. So, any given time you can look at color, 41 00:03:12,406 --> 00:03:16,094 color space and see what galaxies will do in there. 42 00:03:16,095 --> 00:03:21,668 Now, you can Make use of their complex trajectories and use those to evaluate 43 00:03:21,668 --> 00:03:26,997 redshifts, from colors alone. Those are so-called photometric redshifts. 44 00:03:26,997 --> 00:03:31,448 You can think of those as a really low-resolution spectroscopy. 45 00:03:31,448 --> 00:03:37,234 And this seem to work remarkably well, typically in at least four or five filters 46 00:03:37,234 --> 00:03:42,652 and here are examples of some of the measurements, those are dots with their 47 00:03:42,652 --> 00:03:46,773 bars with models of stellar populations drawn to them. 48 00:03:46,773 --> 00:03:52,383 This actually looks too good, and there could be many different models that can 49 00:03:52,383 --> 00:03:57,866 fit the same set of measurements. But that can be also about a statistic. 50 00:03:57,866 --> 00:04:02,600 So here's a typical plot. Usually there is a really excellent 51 00:04:02,600 --> 00:04:08,138 agreement between spectroscopic redshifts. Done as a control and predicted 52 00:04:08,138 --> 00:04:12,366 photometric redshifts. The state of the art is that maybe down to 53 00:04:12,366 --> 00:04:15,946 a few percent. However there always out-layers, galaxies 54 00:04:15,946 --> 00:04:20,209 for which gross error is made. And that's usually due to something like 55 00:04:20,209 --> 00:04:24,847 presence of an active nucleus, or some other peculiar happening like that. 56 00:04:24,848 --> 00:04:31,753 A particular form of photometric redshifts relies on the presence of deep jumps in 57 00:04:31,753 --> 00:04:36,401 the spectrum of the galaxy. There are a couple of those. 58 00:04:36,401 --> 00:04:43,479 There is the limit of the Balmar series of hydrogen which then results in a step of, 59 00:04:43,479 --> 00:04:49,003 of what is magnitude around 3,600 angstroms in the red stream. 60 00:04:49,003 --> 00:04:54,421 So you can use that by measuring flux in filters bluer than the gem, in redwood of 61 00:04:54,421 --> 00:05:00,161 the gem, an even stronger effect occurs at the limit of the alignment series, and 62 00:05:00,161 --> 00:05:04,668 those are extremely useful to find galaxies of very high riches. 63 00:05:04,668 --> 00:05:09,794 Moreover, for the reasons we'll be discussing later, intergalactic medium 64 00:05:09,794 --> 00:05:12,946 absorbs light. Blueward of Lyman alpha line. 65 00:05:12,946 --> 00:05:17,770 So it's really the variant of the Lyman alpha line that serves as an interesting 66 00:05:17,770 --> 00:05:21,229 conjunct point. This has been used to great effect, in 67 00:05:21,229 --> 00:05:26,696 particular by Steidel and collaborators, who discover large numbers of galaxies of 68 00:05:26,696 --> 00:05:30,841 high redshifts and then study their evolution and properties. 69 00:05:30,841 --> 00:05:36,216 But again colors and magnitudes have their limitations and redshifts are needed. 70 00:05:36,216 --> 00:05:41,640 So that the advent of eight, ten meter class telescopes like BLT in Chile, or 71 00:05:41,640 --> 00:05:47,144 [unknown] telescopes in Hawaii, in Subaru and so on, we can begin possible to 72 00:05:47,144 --> 00:05:52,933 actually do this in an effective way. Also, with the development of multifibre 73 00:05:52,933 --> 00:05:58,713 spectrographs, which you may recall also revolutionized redshift series of low 74 00:05:58,713 --> 00:06:02,794 redshift, and nowadays. Thousands if hundreds of thousands of 75 00:06:02,794 --> 00:06:07,295 faint galaxy redshifts have been obtained. A good winning strategy is to obtain 76 00:06:07,295 --> 00:06:11,650 really deep images from space where you don't have to worry about the effect of 77 00:06:11,650 --> 00:06:15,046 Earth's atmosphere. Images taken from Hubble can go much 78 00:06:15,046 --> 00:06:19,091 deeper than those taken from the ground, with a better resolution. 79 00:06:19,091 --> 00:06:24,113 And so there is a set of selected fields in the sky, where very deep observations 80 00:06:24,113 --> 00:06:27,712 have been obtained. Hubble deep field was the first one, 81 00:06:27,712 --> 00:06:32,576 followed by the ultra deep field, and Chandra deep field, and extremely deep 82 00:06:32,576 --> 00:06:35,717 field. So these are the deepest windows in the 83 00:06:35,717 --> 00:06:40,389 universe we obtained so far. Once you have these images in a number of 84 00:06:40,389 --> 00:06:45,919 filters you can deploy large telescopes to measure[UNKNOWN] of as many galaxies as 85 00:06:45,919 --> 00:06:49,607 you possible can. And that's still very much an ongoing 86 00:06:49,607 --> 00:06:55,059 enterprise in observational cosmology. Here is the first one of those, the Hubble 87 00:06:55,059 --> 00:06:59,878 Deep Field with it's characteristic B2 bomber shape and the histogram of 88 00:06:59,878 --> 00:07:03,055 redshifts obtained with the Keck telescope. 89 00:07:03,055 --> 00:07:06,761 So even those where the deepest observation is. 90 00:07:06,761 --> 00:07:12,997 Until then, the bulk of these galaxies is actually at the very high redshifts, about 91 00:07:12,997 --> 00:07:17,099 redshift 1/2. They go beyond redhift of 1, but not by a 92 00:07:17,099 --> 00:07:20,654 lot. In subsequent work, pushing ever deeper, 93 00:07:20,654 --> 00:07:26,234 galaxies were found at redshifts of 5 or even almost 6, but still the bulk of 94 00:07:26,234 --> 00:07:29,251 these. Even at the limit of the present day 95 00:07:29,251 --> 00:07:34,516 observations with eight and ten meter class telescope is of the order of unity 96 00:07:34,516 --> 00:07:37,519 or less. So we do not actually probe evolution of 97 00:07:37,519 --> 00:07:40,723 galaxies very deep through direct measurements. 98 00:07:40,723 --> 00:07:45,346 Small numbers we do see, but then one has to beware of selection effects. 99 00:07:45,346 --> 00:07:49,447 The complete understanding is really a thread use less than one. 100 00:07:49,447 --> 00:07:56,278 This was done now by numerous groups, both in north and south, and tens of thousands 101 00:07:56,278 --> 00:08:00,057 of feign galaxy redshifts would be obtained. 102 00:08:00,058 --> 00:08:05,272 Results are usually in a really good, mutual agreement. 103 00:08:05,272 --> 00:08:11,830 This one is from an SL Beam survey in Circle Good Field, which is where Hubble 104 00:08:11,830 --> 00:08:17,233 Deep Field was, plus other observations surrounding it. 105 00:08:17,233 --> 00:08:20,344 And so here are a couple interesting diagrams. 106 00:08:20,344 --> 00:08:23,332 On the left, you see the redshift histogram. 107 00:08:23,332 --> 00:08:27,328 And it's very spiky. It's not spiky because it's noisy, it's 108 00:08:27,328 --> 00:08:32,384 spiky because of the large scale structure that the line of sight intersects 109 00:08:32,384 --> 00:08:37,440 filaments or even clusters and voids, and that's what produces the observed 110 00:08:37,440 --> 00:08:41,456 distribution. On the right you see absolute magnitudes 111 00:08:41,456 --> 00:08:45,431 in the rest frame of galaxies plotted versus redshift. 112 00:08:45,431 --> 00:08:50,641 And you see there is a sharp cutoff that corresponds to magnitude limit. 113 00:08:50,641 --> 00:08:56,087 People who do these surveys decide that they can only go down to same magnitude 114 00:08:56,087 --> 00:09:01,687 level, say 24 magnitude, and that maps into different absolute magnitudes with 115 00:09:01,687 --> 00:09:07,407 different redshifts, so this is a built in but well understood selection of facts. 116 00:09:07,407 --> 00:09:12,326 Nevertheless, one has to be aware of it. Naiively, if you looked at this, you would 117 00:09:12,326 --> 00:09:16,143 conclude that galaxies of higher redshifts are more luminous. 118 00:09:16,143 --> 00:09:20,973 No, you simply only see the luminous ones with high redshifts, you are not seeing 119 00:09:20,973 --> 00:09:25,186 the fainter ones. So this was done for deep fields and the 120 00:09:25,186 --> 00:09:29,740 result is as follows. Individual galaxies cannot be really 121 00:09:29,740 --> 00:09:33,736 compared. You need to look at the whole population. 122 00:09:33,736 --> 00:09:40,081 And the simplest description of the entire population is the luminosity function. 123 00:09:40,081 --> 00:09:45,702 Distributional galaxy luminosities. So if you look at that in different 124 00:09:45,702 --> 00:09:51,912 redshift shells, you find out that the observed galaxy luminosity function is 125 00:09:51,912 --> 00:09:57,696 very similar to the one we seen in yours. It revolves very slowly but you begin to 126 00:09:57,696 --> 00:10:02,961 see couple of interesting effects, by about refuge of half or so, the faint end 127 00:10:02,961 --> 00:10:07,023 steepens. We see more evolution in intrinsically 128 00:10:07,023 --> 00:10:11,920 fainter galaxies. And second thing is that as you push deep 129 00:10:11,920 --> 00:10:18,520 enough, you start to see brightening at the bright end, which is what you expect 130 00:10:18,520 --> 00:10:24,626 from fading of stellar populations. The steepening was a little bit of a 131 00:10:24,626 --> 00:10:28,362 surprise. It was thought before that galaxies 132 00:10:28,362 --> 00:10:34,632 responsible for the excess counts in the skies, in the imaging are evolving 133 00:10:34,632 --> 00:10:40,131 galaxies with larger redshifts. The bulk of them turn out to be dwarf 134 00:10:40,131 --> 00:10:45,959 galaxies at modest redshifts. And The evolution of galaxies depends very 135 00:10:45,959 --> 00:10:51,378 much on their intrinsic luminosity. This is known as the downsizing. 136 00:10:51,378 --> 00:10:57,684 In face value, it's exactly opposite what you expect from a hierarchical structure 137 00:10:57,684 --> 00:11:03,984 formation where you slowly build up large galaxies, you expect the larger galaxies 138 00:11:03,984 --> 00:11:08,631 to be evolving fastest, but it's the, exactly the opposite. 139 00:11:08,631 --> 00:11:14,864 Moreover, the evolution of the lumosity function depends on the galaxy morphology. 140 00:11:14,865 --> 00:11:20,812 And if you can split them in morphological types, say early and late type spirals and 141 00:11:20,812 --> 00:11:26,608 ellipticals, you find out that the later type galaxies, those further to the right 142 00:11:26,608 --> 00:11:32,619 in Hubble sequence star forming disks and irregulars have the strongest evolution. 143 00:11:32,619 --> 00:11:38,103 Galaxies done earlier part to Hubble sequence pretty much done their evolving 144 00:11:38,103 --> 00:11:42,956 by about a redshift of one. They do evolve since then, but both of the 145 00:11:42,956 --> 00:11:48,852 change appears in the faint population and also the late Hubble Types, it is only 146 00:11:48,852 --> 00:11:54,572 when we reach redshifts of the order of two and beyond that we start to see clear 147 00:11:54,572 --> 00:12:00,236 effects of strong evolutional stellar populations at the very bright end. 148 00:12:00,236 --> 00:12:05,912 So the generic conclusion these days is that most of the Hubble sequence was 149 00:12:05,912 --> 00:12:09,668 pretty much in place by about ratchet point one. 150 00:12:09,669 --> 00:12:15,176 And things have been evolving a relatively modest space since then. 151 00:12:15,176 --> 00:12:19,591 We'll see some other approaches to this a little later. 152 00:12:19,591 --> 00:12:26,029 Next we will talk more about some of the observed results as well as the evolution 153 00:12:26,029 --> 00:12:31,725 in clusters as opposed to field, which is what we just talked about now.