1 00:00:00,012 --> 00:00:06,123 So we have started talking about the results on galaxy evolution studies in 2 00:00:06,123 --> 00:00:10,411 general field. First we found out how the luminosity 3 00:00:10,411 --> 00:00:16,625 function seems to be evolving slightly. But now let's see some of the other 4 00:00:16,625 --> 00:00:22,320 results that have been obtained. A simple thing to do is to look at colors 5 00:00:22,320 --> 00:00:27,730 of galaxies as a function of redshift. And if we then predict what galaxies of 6 00:00:27,730 --> 00:00:32,793 different Hubble types would look like if there was no evolution, just a k 7 00:00:32,793 --> 00:00:38,769 correction, which you may recall from way back in Hubble diagrams, what measurements 8 00:00:38,769 --> 00:00:42,208 would look like given the redshifted spectrum. 9 00:00:42,208 --> 00:00:47,858 This is what it's like, the points are the actual measurements and the three lines 10 00:00:47,858 --> 00:00:52,921 labeled with different Hubble types correspond to the colors that those 11 00:00:52,921 --> 00:00:57,155 particular Hubble types would have with a given redshift. 12 00:00:57,155 --> 00:01:01,270 And, as you can see, the observations pretty much follow the band. 13 00:01:01,270 --> 00:01:06,883 So there isn't very much, in terms of color evolution for the Hubble type 14 00:01:06,883 --> 00:01:11,119 sequence galaxies all the way after redshift affinity. 15 00:01:11,119 --> 00:01:18,154 Hubble space telescope has also afforded us another possible measurement, which is 16 00:01:18,154 --> 00:01:23,520 to look at galaxy sizes. Say, looking at their half light radii or 17 00:01:23,520 --> 00:01:27,529 some other form of objectively defined radius. 18 00:01:27,529 --> 00:01:33,084 And so this is the result of what. The radii, average radio galaxies do as a 19 00:01:33,084 --> 00:01:36,595 function of redshift, namely galaxies grow in time. 20 00:01:36,595 --> 00:01:41,499 If the radii were fixed in proper coordinates, if there was no evolution at 21 00:01:41,499 --> 00:01:44,789 all, then solid line shows what f would look like. 22 00:01:44,789 --> 00:01:49,709 But instead of that the points show that there is a substantial growth of the 23 00:01:49,709 --> 00:01:53,142 galaxy. This is consistent with our understanding 24 00:01:53,142 --> 00:01:56,486 of how this galaxies seem to form from the inside out. 25 00:01:56,486 --> 00:02:00,101 As you'll recall the bulge is all the spark in the middle. 26 00:02:00,101 --> 00:02:04,111 Then you have stellar disk. Then hydrogen extending beyond it. 27 00:02:04,111 --> 00:02:09,422 That hydrogen gas has to turn into stars. And here we'll probably see the collective 28 00:02:09,422 --> 00:02:13,207 effect of that. Modern spectra of high redshift galaxies 29 00:02:13,207 --> 00:02:18,287 can be used to measure not just the redshifts, but also velocity broadening. 30 00:02:18,287 --> 00:02:23,710 In other words, get a velocity dispersion. With radii measured from Hubble space 31 00:02:23,710 --> 00:02:28,766 telescope, we can infer dynamical masses, or from their, from fitting of the 32 00:02:28,766 --> 00:02:33,150 spectral energy distribution we can infer their stellar masses. 33 00:02:33,150 --> 00:02:37,292 So that is shown here. The sizes of symbols correspond to the 34 00:02:37,292 --> 00:02:41,259 magnitudes. We can see that the most massive galaxies 35 00:02:41,259 --> 00:02:44,697 seem to be already in place at redshift one or two. 36 00:02:44,697 --> 00:02:48,544 This is sort of the upper envelope of this distribution. 37 00:02:48,544 --> 00:02:51,986 Whereas the lower masses keep seem to be evolving. 38 00:02:51,986 --> 00:02:56,978 This is known as galaxy downsizing. Naively you would expect a hierarchical 39 00:02:56,978 --> 00:03:00,580 formation scenario. That you make small ones first, and then 40 00:03:00,580 --> 00:03:04,632 you gradually build the big ones. But that seems to be the opposite to 41 00:03:04,632 --> 00:03:07,977 what's observed. The solution to this is probably due to 42 00:03:07,977 --> 00:03:10,771 biasing effect we talked about late, earlier. 43 00:03:10,771 --> 00:03:13,686 We cannot wait long enough to see galaxies merge. 44 00:03:13,686 --> 00:03:17,428 But we can. Assume that some number of close projected 45 00:03:17,428 --> 00:03:22,215 pairs will eventually merge. So by doing statistics of close pairs of 46 00:03:22,215 --> 00:03:27,791 galaxies, allowing for projection effects and things like that, we can infer the 47 00:03:27,791 --> 00:03:31,056 likely merger rate as a function of redshift. 48 00:03:31,056 --> 00:03:34,565 And here it is. It's a power of 1 plus z, and it's more or 49 00:03:34,565 --> 00:03:39,724 less exactly what's expected from the modern models of hierarchical structure 50 00:03:39,724 --> 00:03:43,192 formation. You will recall that when we talked about 51 00:03:43,192 --> 00:03:48,428 scaling relations, like the fundamental plane, I mentioned that they can be used 52 00:03:48,428 --> 00:03:53,462 as a sharp probes of galaxy evolution. Luminosity function is a very broad 53 00:03:53,462 --> 00:03:59,232 distribution, but these correlations are by construction the sharpest we can have. 54 00:03:59,232 --> 00:04:04,568 And so if we can follow, say the evolution of their intercept or maybe slope as a 55 00:04:04,568 --> 00:04:09,488 function of redshift, we can gain new understanding in the evolution of 56 00:04:09,488 --> 00:04:14,384 galaxies, in this case ellipticals. What's shown here in the open points 57 00:04:14,384 --> 00:04:19,844 through which lines fit Is the essentially zero red shift from the mental plane edge 58 00:04:19,844 --> 00:04:22,585 on. The solid dots are ellipticals in one of 59 00:04:22,585 --> 00:04:27,307 the most distant cluster now known. And there are too few points, but you can 60 00:04:27,307 --> 00:04:32,191 see that they're consistent with being on a shifted version of the fundamental 61 00:04:32,191 --> 00:04:35,587 plane. Numerous studies have been now done, both 62 00:04:35,587 --> 00:04:40,782 for field and cluster ellipticals. And it was always seen that ellipticals at 63 00:04:40,782 --> 00:04:46,062 higher edge show a shift in the intercept to the fundamental plane, the surface 64 00:04:46,062 --> 00:04:51,582 brightness, that would correspond to the fading of stellar population in time, as 65 00:04:51,582 --> 00:04:55,332 expected from evolutionary stellar populations. 66 00:04:55,332 --> 00:05:00,651 And we can even say something about the star formation histories. 67 00:05:00,651 --> 00:05:06,904 We can fit different evolution models. And find out which ones seem to describe 68 00:05:06,904 --> 00:05:11,280 the data the best. And the answer is that models in which 69 00:05:11,280 --> 00:05:16,432 elliptical galaxies form. Relatively early on and don't evolve very 70 00:05:16,432 --> 00:05:21,478 much since then, just passively fade away, seem to fit the data fairly well. 71 00:05:21,478 --> 00:05:26,528 That is shown in the plot on the left. The plot on the right shows fading, or if 72 00:05:26,528 --> 00:05:31,520 you're look[UNKNOWN], brightning of surface brightness which remember is 73 00:05:31,520 --> 00:05:35,459 related to luminosity density of sterllar populations. 74 00:05:35,460 --> 00:05:40,882 As a function of red-shit and you can see that it is systematically increasing in 75 00:05:40,882 --> 00:05:44,341 red-shift for both field and cluster ellipticals. 76 00:05:44,341 --> 00:05:49,627 So these studies are consistent with what we've seen earlier that as far Hubble 77 00:05:49,627 --> 00:05:54,907 sequence galaxies are concerned there seem to be pretty much in place buy about 78 00:05:54,907 --> 00:05:59,151 red-shit of unity. The ellipticals are evolving in a way we 79 00:05:59,151 --> 00:06:04,871 will expect from early burst of star formation and relatively modest star 80 00:06:04,871 --> 00:06:10,461 formation history after that. And we also begin to see changing the tilt 81 00:06:10,461 --> 00:06:15,097 of fundamental plane. Which really means that galaxies of 82 00:06:15,097 --> 00:06:19,356 different masses evolve at slightly different pace. 83 00:06:19,356 --> 00:06:25,757 And it goes in the sense that those at the lowest mass end evolve fastest, which is 84 00:06:25,757 --> 00:06:29,876 a, again another example of the galaxy downsizing. 85 00:06:29,876 --> 00:06:33,661 That's a first from the mental plane's concern. 86 00:06:33,661 --> 00:06:39,225 What about Tully-Fisher? Well, that has been done as well, but it 87 00:06:39,225 --> 00:06:45,525 turns out to be much more difficult and complex to do it for spiral of high red 88 00:06:45,525 --> 00:06:49,082 shifts. And these results are still not a hundred 89 00:06:49,082 --> 00:06:53,616 percent clear but at least broadly consistent with this picture. 90 00:06:53,616 --> 00:06:57,868 So far we've looked at the evolution of galaxies in the field. 91 00:06:57,868 --> 00:07:02,066 What about clusters? This is a dense environment, you expect 92 00:07:02,066 --> 00:07:07,412 galaxy interactions to play some role. The first hint that something interesting 93 00:07:07,412 --> 00:07:12,312 is going on in clusters, the so-called Butcher-Oemler effect established very 94 00:07:12,312 --> 00:07:15,511 early on. These astronomers found out that clusters 95 00:07:15,511 --> 00:07:20,551 at larger etchers seems to contain larger proportion of bluer galaxies, for whatever 96 00:07:20,551 --> 00:07:23,161 reason. Generically you expect the galaxy 97 00:07:23,161 --> 00:07:27,921 evolution leads from bluer galaxies to other galaxies so at least qualitatively 98 00:07:27,921 --> 00:07:32,082 this seem about right. Remember that in clusters, some merges 99 00:07:32,082 --> 00:07:36,882 will occur, but vast majority of interactions will not lead to merging, 100 00:07:36,882 --> 00:07:42,242 however it will disrupt galaxies, may remove vially some of their gas or starts, 101 00:07:42,242 --> 00:07:46,867 dark matter, and that's the process called the galaxy harassment. 102 00:07:46,867 --> 00:07:52,377 There'll be a lot of cumulative small encounters that would maybe remove gas 103 00:07:52,377 --> 00:07:57,669 from galaxies little by little. This would tend to transform Lay type, 104 00:07:57,669 --> 00:08:02,733 spirals and dwarf irregulars, into S0's and ellipticals in time. 105 00:08:02,733 --> 00:08:08,845 With the Hubble space telescope, it became possible to look directly at a morphology 106 00:08:08,845 --> 00:08:14,521 of galaxies in distant clusters, and here is One of them were different symbols 107 00:08:14,521 --> 00:08:18,862 correspond to galaxies of different morphological types. 108 00:08:18,862 --> 00:08:24,466 With that and spectroscopic measurements we can try to disentangle what's going on. 109 00:08:24,466 --> 00:08:29,380 An interesting finding was made. There is a novel type of galaxies found in 110 00:08:29,380 --> 00:08:34,438 these evolving cluster populations. So called post-starburst galaxies. 111 00:08:34,439 --> 00:08:40,186 Galaxies that are bluer than they should be at the zero redshift there was nothing 112 00:08:40,186 --> 00:08:45,415 happening and have spectral energy distributions consistent with having 113 00:08:45,415 --> 00:08:50,585 undergone a burst of star formation maybe up to a billion years earlier. 114 00:08:50,586 --> 00:08:57,157 This could have been caused by some interactions, and these galaxies will then 115 00:08:57,157 --> 00:09:03,038 presumably fade into Hubble types. And so the summary of these results is 116 00:09:03,038 --> 00:09:09,182 that, as far as we can tell, there is a conversion of light type spirals into 0s 117 00:09:09,182 --> 00:09:15,078 in ellipticals in the clusters. This is related to density of clusters as 118 00:09:15,078 --> 00:09:18,971 well. The more[INAUDIBLE] relation and that can 119 00:09:18,971 --> 00:09:25,043 account for the observed[INAUDIBLE] effect and presence of things like E plus A or 120 00:09:25,043 --> 00:09:29,987 plus starburst galaxies. So one possible scenario is that spirals 121 00:09:29,987 --> 00:09:33,897 from field are falling into cluster potential well. 122 00:09:33,897 --> 00:09:39,182 As they do that, they encounter dense intracluster gas, the extra gas. 123 00:09:39,183 --> 00:09:44,560 Some of their own gas gets stripped away that quenches the star formation in them. 124 00:09:44,560 --> 00:09:50,096 They may undergo a burst of star formation triggered by some of the interactions but 125 00:09:50,096 --> 00:09:55,338 eventaully that fades too and in the end you may have an all disk like S0 galaxy. 126 00:09:55,338 --> 00:10:01,827 So this is a plausible scenario. This is not necessarily the only way to 127 00:10:01,827 --> 00:10:06,033 make zeros. But it can account for the observed 128 00:10:06,033 --> 00:10:11,209 effects in clusters. Next we will talk about the obscured 129 00:10:11,209 --> 00:10:15,935 component of star formation in history in galaxies.