1 00:00:00,012 --> 00:00:05,365 So, I didn't get a chance to give, a lot of detail about elliptic galaxies or the 2 00:00:05,365 --> 00:00:10,213 patterns of, mass versus rotation and spiral galaxies, we'll get to some of 3 00:00:10,213 --> 00:00:13,520 that maybe later. But, I want to tell you what we know 4 00:00:13,520 --> 00:00:18,515 about galactic evolution, because it'll give us a framework for thinking about 5 00:00:18,515 --> 00:00:23,131 this in the same way that the history of the solar system gave us a framework 6 00:00:23,131 --> 00:00:27,729 thinking of the solar system. galaxies, it turns out, evolve in a very 7 00:00:27,729 --> 00:00:32,388 different way than stars, stars essentially unless they're in a very 8 00:00:32,388 --> 00:00:35,444 close binary, evolve pretty much independently. 9 00:00:35,444 --> 00:00:38,854 A star, evolves, on it's own evolutionary track. 10 00:00:38,854 --> 00:00:43,765 Galaxies are far more social creatures remember that most galaxies are found in 11 00:00:43,765 --> 00:00:47,860 clusters, many stars are found in clusters, but in a cluster of galaxies 12 00:00:47,860 --> 00:00:51,620 the fraction of the volume taken up by the galaxies is much larger. 13 00:00:51,620 --> 00:00:55,913 Remember that if you count the halos Andromeda and the Milky Way essentially 14 00:00:55,913 --> 00:00:59,359 are touching. Even if you don't count the galaxies 15 00:00:59,359 --> 00:01:04,313 compute the ratio of size to distance between say, Andromeda and the Milky Way 16 00:01:04,313 --> 00:01:09,386 and size to distance between say the sun and Proxima Centauri, and you will get 17 00:01:09,386 --> 00:01:14,428 the idea that while interstellar space is vast and empty, intergalactic space is 18 00:01:14,428 --> 00:01:18,052 far less vast especially in the context of a cluster. 19 00:01:18,052 --> 00:01:24,782 And so interactions between galaxies, therefore are going to be far more 20 00:01:24,782 --> 00:01:30,408 important in the evolution of a single galaxy than is the case in stars. 21 00:01:30,408 --> 00:01:36,020 Now, I talked about spiral galaxies we find as part of our hints to 22 00:01:36,020 --> 00:01:42,195 understanding how galaxies evolve That elliptical galaxies are far more common 23 00:01:42,195 --> 00:01:47,673 in the interior center of the core of the densest clusters in the areas where they 24 00:01:47,673 --> 00:01:51,927 are densest. And this is the region where you expect 25 00:01:51,927 --> 00:01:56,392 galactic interactions to occur more than anywhere else, because the galaxies are 26 00:01:56,392 --> 00:02:00,657 packed tight near the center. And galactic interactions, it turns out, 27 00:02:00,657 --> 00:02:04,607 can destabilize the disc structure. This is sort of a reasonable thing. 28 00:02:04,607 --> 00:02:09,482 imagine remember that the disc maintains itself because of conservation of angular 29 00:02:09,482 --> 00:02:12,697 momentum. So you have a bunch of stars orbiting in 30 00:02:12,697 --> 00:02:17,263 one particular direction, but then they collide with another bunch of stars with 31 00:02:17,263 --> 00:02:19,732 a different direction of angular momentum. 32 00:02:19,732 --> 00:02:24,658 Those collisions will lead to a combinium momentum, but the flatten disk structure 33 00:02:24,658 --> 00:02:28,517 will probably not survive. And the result of collisions is likely to 34 00:02:28,517 --> 00:02:33,672 be since you have collisions with I mean collisions of stars are gravitational 35 00:02:33,672 --> 00:02:36,717 collisions. They're scattering events, but the result 36 00:02:36,717 --> 00:02:40,982 will be that stars will go off in with varying orbital parameters and the 37 00:02:40,982 --> 00:02:45,392 net result is sort of this elliptical galaxy where stars orbit with all kinds 38 00:02:45,392 --> 00:02:49,030 of inclinations. And so if you believe that collisions and 39 00:02:49,030 --> 00:02:53,844 mergers happen in a cluster you would expect ellipticals to survive in clusters 40 00:02:53,844 --> 00:02:58,741 and that is indeed what, in the centers of clusters, that is indeed what we find. 41 00:02:58,741 --> 00:03:03,865 There's another difference between galaxy clusters and star clusters or galaxies as 42 00:03:03,865 --> 00:03:08,955 large clusters of stars, which is that in between stars, the interstellar medium is 43 00:03:08,955 --> 00:03:12,945 there but the interstellar medium, remember gas and dust per whole 44 00:03:12,945 --> 00:03:17,302 interstellar medium in the Milky Way comprises the tiny fraction of the mass 45 00:03:17,302 --> 00:03:21,797 of the Milky Way unless you count the extended halo in which case may be it 46 00:03:21,797 --> 00:03:25,713 counts for about half the mass of the Milky Way excluding dark matter. 47 00:03:25,713 --> 00:03:28,848 In the case of galaxies we find this is very different. 48 00:03:28,848 --> 00:03:33,583 The typical galaxy cluster, galactic cluster is full of hot gas whereby hot, 49 00:03:33,583 --> 00:03:38,974 again I mean million kelvin gasses and the mass of this hot gas usually exceeds 50 00:03:38,974 --> 00:03:41,985 the sum of the masses of all of the galaxies. 51 00:03:41,985 --> 00:03:47,223 So, a majority of the, again excluding dark matter, a majority of the mass of a 52 00:03:47,223 --> 00:03:52,513 galactic cluster is actually spread out in between the galaxies in the form of 53 00:03:52,513 --> 00:03:59,205 intergalactic medium and again we think of interactions as the reason for this in 54 00:03:59,205 --> 00:04:04,636 inter, intergalactic interactions, tidal forces can strip a small fraction of a 55 00:04:04,636 --> 00:04:09,402 galaxy's gas away and ejected out into the interstellar medium. 56 00:04:09,402 --> 00:04:13,723 And then, if we have recurring interactions, then over time some 57 00:04:13,723 --> 00:04:18,712 significant quantity of gas will be transferred to the intsteller medium. 58 00:04:18,712 --> 00:04:24,033 The net result is the observation that in galactic clusters, most of the mass is in 59 00:04:24,033 --> 00:04:26,994 between the galaxies and not in the galaxies. 60 00:04:26,994 --> 00:04:32,240 So what do interactions between galaxies look like? Well, galaxies colliding is 61 00:04:32,240 --> 00:04:37,644 certainly not like cars colliding because galaxies are sort of for all that they 62 00:04:37,644 --> 00:04:43,735 are massive they are extended objects. And the main interaction between two 63 00:04:43,735 --> 00:04:48,222 galaxies is gravitational. So, the typical interaction at slow 64 00:04:48,222 --> 00:04:53,851 relative velocities is a very stately thing, it's a gravitational interaction. 65 00:04:53,851 --> 00:05:00,047 and a typical case to study is sort of the motion of a dwarf galaxy through the 66 00:05:00,047 --> 00:05:04,762 plain of the Milky Way or a globular cluster on it's orbit through the plain 67 00:05:04,762 --> 00:05:09,362 of the Milky Way or of a globular cluster from one galaxy through the plain of 68 00:05:09,362 --> 00:05:14,597 another galaxy, and, it's a little bit tricky to make the calculation, but you 69 00:05:14,597 --> 00:05:20,557 can sort of imagine but if you have a bunch of stars scattered around, and 70 00:05:20,557 --> 00:05:26,782 along comes a globular cluster, pushing through moving this way, then in its 71 00:05:26,782 --> 00:05:33,697 wake, what is going to happen is that the stars are going to gravitationally 72 00:05:33,697 --> 00:05:57,532 attracted to this visitor and so there will be a motion of stars towards and 73 00:05:57,532 --> 00:05:57,532 after it takes a while for these stars to move and after the cluster of pass, has 74 00:05:57,532 --> 00:05:57,532 pass, there will be a wake. A region of increased mass density from 75 00:05:57,532 --> 00:05:57,953 where it is attracted star. 76 00:05:57,953 --> 00:05:59,241 So, an object moving through a field of stars always is dragging behind it 77 00:05:59,241 --> 00:06:02,345 increased mass density which is decelerating it. 78 00:06:02,345 --> 00:06:06,655 And it in turn is of course accelerating these stars so momentum is being 79 00:06:06,655 --> 00:06:11,468 transferred from the moving object to the galaxy relative to collection of stars 80 00:06:11,468 --> 00:06:15,869 relative to which it is moving. This is called dynamical friction and 81 00:06:15,869 --> 00:06:20,831 because it's second order of gravitational effect, it's not surprising 82 00:06:20,831 --> 00:06:25,929 that it increases like GM^2. Why GM^2 because the first GM is the 83 00:06:25,929 --> 00:06:30,519 magnitude of the wake, that the star creates and the second GM is the 84 00:06:30,519 --> 00:06:34,175 magnitude of the force that that wake exercises on the star. 85 00:06:34,175 --> 00:06:39,179 So, 2 GM row is the density of the medium of course the denser the medium exerts 86 00:06:39,179 --> 00:06:45,310 more friction and then this 1 / v^2 can be argued either from dimensional grounds 87 00:06:45,310 --> 00:06:50,496 v being the speed with which this thing is moving relative to the stars around it 88 00:06:50,496 --> 00:06:55,393 or because the, the basic idea is, the faster you're moving through the less 89 00:06:55,393 --> 00:06:59,362 time there is for this wake to establish itself and exert a force. 90 00:06:59,362 --> 00:07:04,492 The net result is that a visitor will be slowed down so a globular cluster, as it 91 00:07:04,492 --> 00:07:09,547 passes through the disk, through the disk population, will be slowed down and 92 00:07:09,547 --> 00:07:14,984 slowly spiral into a smaller and smaller orbit and eventually its interaction with 93 00:07:14,984 --> 00:07:19,764 the discs will become so frequent. That tidal and forces will then break it 94 00:07:19,764 --> 00:07:22,912 up and the globular cluster will not live forever. 95 00:07:22,912 --> 00:07:27,797 It will eventually merge with a disc. And, this will happen faster for massive 96 00:07:27,797 --> 00:07:31,246 clusters. so if you started globular clusters with 97 00:07:31,246 --> 00:07:35,637 all kinds of masses, the smallest one would survive the longest. 98 00:07:35,637 --> 00:07:41,012 Another, sort of, observation to file away this is what a slow collison between 99 00:07:41,012 --> 00:07:44,952 galaxies look like. there are faster collisions where the 100 00:07:44,952 --> 00:07:49,617 relative speeds are bigger and, essentially, the 2 galaxies pass through 101 00:07:49,617 --> 00:07:52,852 each other without time for wakes to get organized. 102 00:07:52,852 --> 00:07:56,573 Without the stars having time to react, v is much larger. 103 00:07:56,573 --> 00:08:01,015 The stars still rearrange each other but they rearrange with a delay, this 104 00:08:01,015 --> 00:08:05,489 liberates gravitational energy that was converted while the galaxies were 105 00:08:05,489 --> 00:08:09,815 accelerating towards each other. Some of that is converted into kinetic 106 00:08:09,815 --> 00:08:15,536 energies of the stars, the result can be the explosion. 107 00:08:15,536 --> 00:08:23,395 A large amount, of kinetic energy of the stars in one galaxy and in dramatic 108 00:08:23,395 --> 00:08:28,992 cases, like the the, something cartwheel nebula. 109 00:08:28,992 --> 00:08:35,762 [LAUGH] Like the cartwheel galaxy, this has caused the emission of a lot of gas 110 00:08:35,762 --> 00:08:42,582 in a circular ring around the galaxy, and the stars that we see here, these blue 111 00:08:42,582 --> 00:08:48,205 stars are of course nothing to do With a. They were not ejected from the galaxy. 112 00:08:48,205 --> 00:08:51,937 The gas was ejected. The gas is plowing into the intergalactic 113 00:08:51,937 --> 00:08:54,934 medium. That's creating a high density shock wave 114 00:08:54,934 --> 00:08:59,945 and that's generating star creation so this beautifully decorative ring around 115 00:08:59,945 --> 00:09:04,899 the galaxy is marking the place where the shock wave is progressing through the 116 00:09:04,899 --> 00:09:09,885 intergalactic medium and this may well have resulted From a collision with one 117 00:09:09,885 --> 00:09:12,583 of these two objects here over on the right. 118 00:09:12,583 --> 00:09:17,911 another case is this elliptical galaxy, which has what is called a polar ring, 119 00:09:17,911 --> 00:09:20,627 likely the result similarly of a collision. 120 00:09:20,627 --> 00:09:25,397 Perhaps the collider was destroyed and merged with the elliptical but the 121 00:09:25,397 --> 00:09:30,442 resulting energy ejected this ring. Now, these rings are very useful because 122 00:09:30,442 --> 00:09:34,202 they do not orbit in the plane of a galaxy, they are influenced by the 123 00:09:34,202 --> 00:09:38,622 gravity of whatever is around there. This gives us an opportunity to measure 124 00:09:38,622 --> 00:09:42,377 by studying their orbit or parameters, the dark matter distribution 125 00:09:42,377 --> 00:09:46,327 perpendicular to the plane. And somebody might have argued based on 126 00:09:46,327 --> 00:09:50,984 the orbit of the sun and the stars and the clouds and the Milky Way that dark 127 00:09:50,984 --> 00:09:54,100 matter could be confined to the disk of the Milky Way. 128 00:09:54,100 --> 00:09:59,311 the reason we are absolutely certain it's a spherical distribution is the, that, 129 00:09:59,311 --> 00:10:04,394 the orbits of these objects, which orbit way outside the plane of the galaxy agree 130 00:10:04,394 --> 00:10:08,175 with the predictions of the dark matter, cold dark matter model. 131 00:10:08,175 --> 00:10:13,258 when the collisions are more delicate or in any collision between galaxies, 132 00:10:13,258 --> 00:10:17,788 remember galaxies are in free fall. The gravitational attraction or the 133 00:10:17,788 --> 00:10:22,966 gravitational interaction of one with the other, is a tidal interaction and this 134 00:10:22,966 --> 00:10:27,910 can lead to these beautiful objects. On the left, of course M51 the whirlpool 135 00:10:27,910 --> 00:10:33,806 galaxy interacting with not quite a collision, but obviously an interaction 136 00:10:33,806 --> 00:10:36,850 with the smaller galaxy in the upper right, 137 00:10:36,850 --> 00:10:41,852 We see that one of the spiral arms has grown and is extending towrads this 138 00:10:41,852 --> 00:10:47,957 galaxy and slightly outside the frame the other spiral arm extends very far to the 139 00:10:47,957 --> 00:10:52,467 left. These are both tidal effects gas or 140 00:10:52,467 --> 00:10:58,282 material and stars are being attracted on the one hand and attracted away from the 141 00:10:58,282 --> 00:11:04,409 galaxy on the other hand in the same way that we get high tide on two sides of the 142 00:11:04,409 --> 00:11:07,490 Earth. These are the tidal effects of the 143 00:11:07,490 --> 00:11:11,520 passing galaxy. And an even more brilliant example of 144 00:11:11,520 --> 00:11:17,103 these sort of tidal tails, are these antennae galaxies, as they are called 145 00:11:17,103 --> 00:11:20,654 here. And then a pair of galaxies in collision. 146 00:11:20,654 --> 00:11:29,632 And we see the effects of tidal forces in these large tendrils of gas that have 147 00:11:29,632 --> 00:11:38,922 been emitted by both of the galaxies. in another recent observation of the same 148 00:11:38,922 --> 00:11:44,610 effect, NGC68672 has recently been, been measured to be the largest spiral galaxy 149 00:11:44,610 --> 00:11:48,067 known. the scale here is 100,000 light years so 150 00:11:48,067 --> 00:11:52,629 this thing is about 2 or 3 times the size of the Milky Way, but that's kind of 151 00:11:52,629 --> 00:11:55,693 cheating. One of the reasons these spiral arms are 152 00:11:55,693 --> 00:12:01,219 so extended is because it seems that this has undergone a collision, perhaps over 153 00:12:01,219 --> 00:12:06,553 here, in the upper left, and so it has got these two tightly extended spiral 154 00:12:06,553 --> 00:12:11,146 arms coming out on two sides. And the most exciting thing here is that 155 00:12:11,146 --> 00:12:16,739 there is some structure in this upper left its tidal tail and it seems as 156 00:12:16,739 --> 00:12:21,776 though some of the tail has become sufficiently cold and dense to collapse 157 00:12:21,776 --> 00:12:27,394 and there's the region that is in circle there is a region where astronomers 158 00:12:27,394 --> 00:12:32,512 suspect that a actual globular cluster or a dwarf galaxy might be forming. 159 00:12:32,512 --> 00:12:37,499 So this is the weird case where rather than destroying galaxies and merging 160 00:12:37,499 --> 00:12:42,413 them, here a collision is actually creating a new galaxy, a new wrinkle on 161 00:12:42,413 --> 00:12:47,399 our theories of galaxy interactions, something modelers will indubitably 162 00:12:47,399 --> 00:12:51,622 incorporate as time goes on, if indeed this thing is verified. 163 00:12:51,622 --> 00:12:55,002 Now an interesting question you might ask, is, well. 164 00:12:55,002 --> 00:12:59,912 So, if what you are telling me is that very often, two galaxies, will in fact, 165 00:12:59,912 --> 00:13:04,697 merge, because they will be slowed down, ripped apart by each other's tidal 166 00:13:04,697 --> 00:13:07,892 effects, and in fact, end up as one merged object. 167 00:13:07,892 --> 00:13:10,682 Well that's very nice for the stars, they can orbit whatever they want and the gas. 168 00:13:10,682 --> 00:13:15,242 Each of these galaxies would have had a super massive black hole in its center. 169 00:13:15,242 --> 00:13:18,461 What do those do? Well that's an interesting question. 170 00:13:18,461 --> 00:13:22,751 And what you would guess, of course, is that the two black holes, being so 171 00:13:22,751 --> 00:13:27,005 massive, would very rapidly find themselves in the core, of the combined 172 00:13:27,005 --> 00:13:29,495 object. So you could find yourself, in the 173 00:13:29,495 --> 00:13:36,909 interesting situation, like NGC6240 which is a galaxy whose shape clearly suggest, 174 00:13:36,909 --> 00:13:44,012 a past in which some interaction happened and a x-ray image of the center, shows 2, 175 00:13:44,012 --> 00:13:49,756 x-ray sources, two black holes. This has a binary super massive black 176 00:13:49,756 --> 00:13:55,232 hole and they are seperated by, this doesn't say, but what it meant to say, is 177 00:13:55,232 --> 00:14:00,117 that these things are separated by an order of 3000 light years or a kiloparsec 178 00:14:00,117 --> 00:14:05,096 an eighth of the distance from the sun to the center of the galaxy is the distance 179 00:14:05,096 --> 00:14:07,454 between these two super massive black holes. 180 00:14:07,454 --> 00:14:12,954 Those are some very interesting highly relativistic, orbital dynamics, and over 181 00:14:12,954 --> 00:14:18,303 time of course friction with the dynamical friction, with the medium 182 00:14:18,303 --> 00:14:23,088 as well as gravitational radiation will cause these things to eventually over 183 00:14:23,088 --> 00:14:28,042 hundreds of millions of years or billions of years to move closer to each other, 184 00:14:28,042 --> 00:14:32,187 eventually merge with each other, and there you will get a blaze of 185 00:14:32,187 --> 00:14:37,008 gravitational radiation, that perhaps someone will be able to detect at that, 186 00:14:37,008 --> 00:14:40,842 to detect at that point. So, based on all these observations of 187 00:14:40,842 --> 00:14:46,141 galactic interactions we can try to understand, a model of galaxy formation. 188 00:14:46,141 --> 00:14:50,595 And the original model of galaxy formation was basically the solar nebula 189 00:14:50,595 --> 00:14:53,550 at large. So, remember, the solar nebula formed 190 00:14:53,550 --> 00:14:56,982 from a cloud that collapsed, a proto star, that story. 191 00:14:56,982 --> 00:15:02,502 Imagine the same thing on a galactic scale so this is the this is the cartoon 192 00:15:02,502 --> 00:15:05,872 version of the ELS model of galaxy formation. 193 00:15:05,872 --> 00:15:11,657 You start with a protogalactic cloud. The protogalactic cloud collapses in the 194 00:15:11,657 --> 00:15:17,106 center where the density gets highest. stars starts forming long before the 195 00:15:17,106 --> 00:15:19,933 densities of the outskirts are high enough. 196 00:15:19,933 --> 00:15:24,969 Eventually of course angular momentum will flatten the cloud just as it did the 197 00:15:24,969 --> 00:15:30,172 protosteller disc but, in the center perhaps enough stars formed in clusters 198 00:15:30,172 --> 00:15:34,636 managed to collapse so there was fragmentation and smaller sub clouds 199 00:15:34,636 --> 00:15:39,647 became critical and collapsed before the flattening so then you'd find the old 200 00:15:39,647 --> 00:15:44,163 globular cluster stars orbiting in orbits that are not necessarily in the plane of 201 00:15:44,163 --> 00:15:47,911 the disk, and then eventually the disk flattens out and you get the issue of 202 00:15:47,911 --> 00:15:50,313 the, the, the shape of the galaxy as we know it. 203 00:15:50,313 --> 00:15:53,791 This is a very reasonable model, and it probably is part of the truth. 204 00:15:53,791 --> 00:15:57,132 There are some issues with it. One is, it does, it explains why you 205 00:15:57,132 --> 00:16:01,121 find, halo stars and halo clusters orbiting in inclined orbits, but not why 206 00:16:01,121 --> 00:16:05,900 so many of them are in retrograde orbits. Orbits considering that as the cloud 207 00:16:05,900 --> 00:16:11,028 collapsed angle of momentum reservation would have sped up whatever rotation it 208 00:16:11,028 --> 00:16:13,834 has. It also turns out that this collapse 209 00:16:13,834 --> 00:16:19,162 occurs very rapidly and it does not have, it does not leave for a cloud the size of 210 00:16:19,162 --> 00:16:23,777 The Milky Way this is the Kelvin Helmoltz gravitational collapse time scale. 211 00:16:23,777 --> 00:16:28,385 It's a rapid timescale and even in the case of a cloud the size of the Milky Way 212 00:16:28,385 --> 00:16:32,010 you don't have 2 billion years in which to create halo stars. 213 00:16:32,010 --> 00:16:36,692 Remember the ages of globular clusters were between 11 and 13 billion years. 214 00:16:36,692 --> 00:16:41,104 This is 1 of the key pieces of evidence that this model is no sufficient. 215 00:16:41,104 --> 00:16:45,731 In addition there's this correlation between distance, orbital radius and 216 00:16:45,731 --> 00:16:50,086 modelocity in globular clusters that says that the distant clusters are 217 00:16:50,086 --> 00:16:53,217 younger. This does not match, in this theory you'd 218 00:16:53,217 --> 00:16:56,802 expect the clusters to be formed in the center to be the. 219 00:16:56,802 --> 00:17:02,960 The oldest ones and, and also the fact that the disk components, the thin and 220 00:17:02,960 --> 00:17:09,482 thick disk differ in age, in this case, in this model everything happens fast and 221 00:17:09,482 --> 00:17:15,383 expect stellar creation to start in the disk as soon as densities got high 222 00:17:15,383 --> 00:17:18,579 enough. And there is not, again a billion, 223 00:17:18,579 --> 00:17:24,351 billions of years of separation that can explain all of the phenomena that we see. 224 00:17:24,351 --> 00:17:29,624 So this is not a sufficient model. Another sort of opposing model is what's 225 00:17:29,624 --> 00:17:35,317 called the heirarchical merger model, in this case a galaxy forms more the way we 226 00:17:35,317 --> 00:17:39,598 said planets formed. Form, so, there is a collapse and what 227 00:17:39,598 --> 00:17:44,402 collapses are basically globular clusters or dwarf galaxies, protogalactic 228 00:17:44,402 --> 00:17:49,193 fragments and they can a mass from a million solar masses to maybe bigger than 229 00:17:49,193 --> 00:17:52,272 a galaxy. But in the usual way that fragmentation 230 00:17:52,272 --> 00:17:56,047 works we know that the most common will be the smallest ones. 231 00:17:56,047 --> 00:17:59,502 So you'll have a lot of, globular cluster size. 232 00:17:59,502 --> 00:18:04,606 Protogalactic fragments, with a mass of about a million solar masses. 233 00:18:04,606 --> 00:18:10,190 And then, as these orbit each other, and interact with each other, dynamical 234 00:18:10,190 --> 00:18:15,425 friction slows them down, and, eventually, they are brought into this, 235 00:18:15,425 --> 00:18:20,959 big spherical, mass distribution. And, there, density becomes high enough 236 00:18:20,959 --> 00:18:26,494 that they start forming stars and globular clusters in the center, and 237 00:18:26,494 --> 00:18:33,183 because these things have formed stars individually then in the center of each 238 00:18:33,183 --> 00:18:39,424 cloud you start star formation then this explains why you get different chemical 239 00:18:39,424 --> 00:18:44,382 histories because different clusters independently initiated different 240 00:18:44,382 --> 00:18:49,052 fragments independently initiated star formation and then these things are now 241 00:18:49,052 --> 00:18:53,662 beginning star formation at the same time that they're interacting with each other. 242 00:18:53,662 --> 00:18:56,552 Collisions and tidal forces will break some of them. 243 00:18:56,552 --> 00:19:00,972 Which ones will break most likely? Well the ones with, which experience the most 244 00:19:00,972 --> 00:19:04,543 dynamical friction namely the mass, most massive one. 245 00:19:04,543 --> 00:19:09,472 So the more massive ones are more likely to be slowed down and disrupted because 246 00:19:09,472 --> 00:19:15,172 of dynamical friction those that have been disrupted will produce the halo 247 00:19:15,172 --> 00:19:20,201 field stars and, some of the others the stars and the gas will have been blown 248 00:19:20,201 --> 00:19:24,553 away, but the globular clusters as more massive, will have survived. 249 00:19:24,553 --> 00:19:29,522 So we'll have bare globular clusters and we'll still have some dwarf galaxies. 250 00:19:29,522 --> 00:19:34,009 Now, in this model about 90% of all the globular clusters formed, especially the 251 00:19:34,009 --> 00:19:37,079 massive ones, as I've said, are going to be destroyed. 252 00:19:37,079 --> 00:19:41,677 The globular clusters we see in the halo today will be the 10% that survived and 253 00:19:41,677 --> 00:19:46,139 with an emphasis on the less massive ones which is why we typically find globular 254 00:19:46,139 --> 00:19:48,951 clusters with masses up to a million solar masses. 255 00:19:48,951 --> 00:19:54,180 And then very early in the dense center as this these things are starting to 256 00:19:54,180 --> 00:19:57,235 merge. You form a bulge, so in this case star 257 00:19:57,235 --> 00:20:02,141 formation proceeds from the inside out, in this hierarchical merger model and so 258 00:20:02,141 --> 00:20:07,030 you'd expect very old stars in the bulge. Explaining why there's new stars in the 259 00:20:07,030 --> 00:20:11,382 bulge, is a detail of the model I won't really have time to get into. 260 00:20:11,382 --> 00:20:15,258 Of course in principal both models probably have a role to play. 261 00:20:15,258 --> 00:20:20,180 We do not have I should say a satisfactory theory of galaxy formation, 262 00:20:20,180 --> 00:20:24,968 this is an area of active research. But I think the, the consensus is that, 263 00:20:24,968 --> 00:20:29,623 the Hierarchical Merger Model in a slightly more elaborate version than what 264 00:20:29,623 --> 00:20:33,645 I've described is the more likely, it's more likely to be 265 00:20:33,645 --> 00:20:38,881 closer to what it is that really goes on. Can we see the evidence of this? So for 266 00:20:38,881 --> 00:20:43,622 stars, we used clusters to test our theories of stellar evolution. 267 00:20:43,622 --> 00:20:49,055 How do you test your theories of galactic evolution? we do not have a long main 268 00:20:49,055 --> 00:20:54,803 sequence understanding so we don't, can't time galaxies in the way we timed stars, 269 00:20:54,803 --> 00:20:59,612 but we can look for very young galaxies.Where would you find very young 270 00:20:59,612 --> 00:21:05,180 galaxies? You'd find very young galaxies by looking very, very far so this is the 271 00:21:05,180 --> 00:21:08,660 very famous Hubble extreme dark field. 272 00:21:08,660 --> 00:21:14,957 basically the Hubble Telescope was aimed at a region, the constellation Formax. 273 00:21:14,957 --> 00:21:20,144 This shows just a region 3 by 3 arc minutes in the sky which as far as anyone 274 00:21:20,144 --> 00:21:25,088 knew when they aimed at it, was completely empty of anything. 275 00:21:25,088 --> 00:21:29,514 There was no known objects in that region and then they took a very long exposure 276 00:21:29,514 --> 00:21:32,753 and in this field they found about 10,000 formerly unknown galaxies. 277 00:21:32,753 --> 00:21:37,753 They looked at a region where there was nothing in order to estimate, in order to 278 00:21:37,753 --> 00:21:40,773 be able to focus on very dim and therefore very distant objects. 279 00:21:40,773 --> 00:21:44,002 These galaxies are billions of light years away. 280 00:21:44,002 --> 00:21:48,818 Aha, billions of light years away tells us that we are seeing them billions of 281 00:21:48,818 --> 00:21:53,935 years ago, we'll get into that soon and this means that the galaxies we see here 282 00:21:53,935 --> 00:21:58,613 are likely to be younger than the ones we see a million light years away like 283 00:21:58,613 --> 00:22:01,912 Andromeda or the Milky way which we see right now. 284 00:22:01,912 --> 00:22:09,616 And so, in this Hubble Deep Field indeed, we see that the galaxies seem to be bluer 285 00:22:09,616 --> 00:22:16,523 than the galaxies we see around us, and their shapes are far more irregular. 286 00:22:16,523 --> 00:22:21,552 It is a possibility that's under study that what we're seeing here is, in some 287 00:22:21,552 --> 00:22:27,605 of these cases the proto-galactic fragment that will eventually form the 288 00:22:27,605 --> 00:22:32,290 galaxies that we see today. As I said, galaxy formation is still a 289 00:22:32,290 --> 00:22:37,497 very active field of study and the understanding we have of the dynamics of 290 00:22:37,497 --> 00:22:42,516 the qualitative level we can handle, I think is all we're going to be able to 291 00:22:42,516 --> 00:22:43,406 achieve today.