1 00:00:00,012 --> 00:00:04,154 So far, we have reviewed the morphology of galaxies. 2 00:00:04,154 --> 00:00:08,979 But now let's take a look at whether that morphology actually correlates 3 00:00:08,979 --> 00:00:14,492 meaningfully with some outer properties of galaxies or their formation history. 4 00:00:14,492 --> 00:00:19,353 First, let me critique the traditional approach to galaxy morphology. 5 00:00:19,353 --> 00:00:23,941 It is subjective, it is based on the appearance, visual appearance in 6 00:00:23,941 --> 00:00:29,945 particular wavelength, traditionally those would be blue light sensitive photographic 7 00:00:29,945 --> 00:00:33,163 plates. But now we have the panchromatic view of 8 00:00:33,163 --> 00:00:38,091 the universe, and when you look at galaxies in different wavelength regimes 9 00:00:38,091 --> 00:00:42,491 they look very different. And, classification would presumably be 10 00:00:42,491 --> 00:00:46,712 very different depending on which wavelength regime was chosen. 11 00:00:46,712 --> 00:00:51,781 Now we happened to have chosen visible light and, in vicinity of visible light. 12 00:00:51,781 --> 00:00:56,276 And you, if you look at bluer wavelengths, ultraviolet included, you'll be very 13 00:00:56,276 --> 00:01:00,497 sensitive to the star formation, so galaxies will be clumpier, there'll be 14 00:01:00,497 --> 00:01:03,869 more strong features due to the regions of star formation. 15 00:01:03,869 --> 00:01:08,029 If you look towards redder wavelengths, which are not so sensitive to star 16 00:01:08,029 --> 00:01:12,518 formation, but reflect the bulk of the whole stellar population, galaxies will 17 00:01:12,518 --> 00:01:17,267 look much smoother than that. So there are three major problems with the 18 00:01:17,267 --> 00:01:20,950 traditional morphological classification of galaxies. 19 00:01:20,950 --> 00:01:25,692 First it is subjective, it is based on just the visual appearance, looking at 20 00:01:25,692 --> 00:01:30,454 things and deciding whether some galaxies have more open spiral arms or not. 21 00:01:30,454 --> 00:01:35,260 However, these days we can actually, process digital images of galaxies and 22 00:01:35,260 --> 00:01:40,386 define in objective, consistent fashion some of those structural parameters. 23 00:01:40,386 --> 00:01:44,581 The second problem is that it's superficial, it really is based on just 24 00:01:44,581 --> 00:01:49,125 vis-, visual appearance of particular wavelength region, and not on actual 25 00:01:49,125 --> 00:01:53,385 physical properties that we'd be interested in, such as galaxy mass for 26 00:01:53,385 --> 00:01:56,534 example. A more modern approach is to use 27 00:01:56,534 --> 00:02:02,882 correlations, and clustering of different galaxies and properties in parameter 28 00:02:02,882 --> 00:02:09,165 spaces, and does objectively define galaxy families using those correlations. 29 00:02:09,165 --> 00:02:14,016 Finally it is an incomplete classification, it was based on what was 30 00:02:14,016 --> 00:02:18,950 known, more or less, in Hubble's days. And it completely missed, a major 31 00:02:18,950 --> 00:02:24,164 dichotomy between giant galaxies, by giants I mean those that are usually seen 32 00:02:24,164 --> 00:02:29,694 in Hubble sequence, And dwarfs, which turn out to be largely a completely different 33 00:02:29,694 --> 00:02:32,564 set of beasts, and I'll show you that later. 34 00:02:32,564 --> 00:02:35,750 And they themselves, may split in different categories. 35 00:02:35,750 --> 00:02:40,170 So that was completely missed by the original, classification schemes. 36 00:02:40,170 --> 00:02:45,190 Alright, so what does it mean? Properties of galaxies, including their 37 00:02:45,190 --> 00:02:50,068 appearance, are a product of their formative and evolutionary histories. 38 00:02:50,068 --> 00:02:55,156 And does, if we can interpret morphol, morphological trends, then we can learn 39 00:02:55,156 --> 00:02:58,876 something about galaxy formation and galaxy evolution. 40 00:02:58,876 --> 00:03:03,128 A lot can already be concluded from a very basic fact. 41 00:03:03,128 --> 00:03:08,136 Among the Hubble sequence galaxies at least there seem to be two dominant 42 00:03:08,136 --> 00:03:11,705 components. The elliptical like bulge component and 43 00:03:11,705 --> 00:03:17,087 disk component and the relative probab-, presence of these two determines a lot of 44 00:03:17,087 --> 00:03:21,264 properties of galaxies.. The bold slash elliptical stellar 45 00:03:21,264 --> 00:03:26,412 components are older, and they're schematically supported by random motions 46 00:03:26,412 --> 00:03:29,311 of stars. Stars have to somehow balance the 47 00:03:29,311 --> 00:03:34,683 potential wells, in which they sit, and in elliptical galaxies and bulges, those 48 00:03:34,683 --> 00:03:40,044 motions are largely random, like molecules in a gas, so we call them pressure. 49 00:03:40,044 --> 00:03:43,802 Important. In this galaxy, most of the kinetic energy 50 00:03:43,802 --> 00:03:49,294 is in circular motion around the center. And very productively little kinetic 51 00:03:49,294 --> 00:03:53,326 energy is actually in random motion, though there is some. 52 00:03:53,326 --> 00:03:57,346 And so that's a major distinguishing characteristic. 53 00:03:57,347 --> 00:04:03,094 Note also that we'd be just talking about light and we already know that the 54 00:04:03,094 --> 00:04:06,535 dominant mass component is the dark matter. 55 00:04:06,535 --> 00:04:12,301 We think with some justification that dark matter is probably also pressure 56 00:04:12,301 --> 00:04:16,151 supported, random motions rather than rotation. 57 00:04:16,151 --> 00:04:21,477 And also, discs are certainly distinct kinematical Age type components plus peril 58 00:04:21,477 --> 00:04:26,520 arms which were used for original classification, may be largely ornamental. 59 00:04:26,520 --> 00:04:31,082 They are interesting patterns, but they do not seem to really correlate very much 60 00:04:31,082 --> 00:04:34,449 with anything else. However, having said all that, there's 61 00:04:34,449 --> 00:04:38,652 still very important and significant trends along the Hubble sequence. 62 00:04:38,652 --> 00:04:44,538 As we go from the early types ellipticals and F zeros, towards later types of 63 00:04:44,538 --> 00:04:48,480 spirals, F As, F Bs, F Cs, there are several trends. 64 00:04:48,480 --> 00:04:52,342 The age, average age of the population decreases. 65 00:04:52,342 --> 00:04:57,343 The disks are younger and the later Hubble taps are even younger. 66 00:04:57,344 --> 00:05:02,449 The start formation rate decreases. There is almost none in elliptical and 67 00:05:02,449 --> 00:05:06,451 bulges, and more and more as you go towards the later types. 68 00:05:06,451 --> 00:05:11,817 Because of that, the color also changes, because young stellar populations are 69 00:05:11,817 --> 00:05:16,977 dominated by luminous blue stars. All stellar populations dominated by all 70 00:05:16,977 --> 00:05:21,852 red giants, and so there would be turning form redder colors towards the bluer 71 00:05:21,852 --> 00:05:25,042 colors. The gas content would change, at least the 72 00:05:25,042 --> 00:05:29,747 neutral hydrogen gas will change. There is almost none in ellipticals and 73 00:05:29,747 --> 00:05:34,372 voltages except if it came from disks. And there is more and more hydrogen 74 00:05:34,372 --> 00:05:38,191 relative to the stellar mass as you go towards later types. 75 00:05:38,191 --> 00:05:43,516 Likewise, the outer components of these cold interstellar medium, including dust, 76 00:05:43,516 --> 00:05:47,701 are also increasing in their importance towards the later types. 77 00:05:47,701 --> 00:05:52,657 And finally, an important dynamical characteristic which tells us a lot about 78 00:05:52,657 --> 00:05:57,607 formative processes is that in the early types, most of the kinetic energy is in 79 00:05:57,607 --> 00:06:01,159 random motions. In the later types, most of the kinetic 80 00:06:01,159 --> 00:06:06,634 energy is in ordered, circular motions. So because of these trends, and what they 81 00:06:06,634 --> 00:06:11,938 really mean in direct interpretation, Hubble classification persisted to this 82 00:06:11,938 --> 00:06:14,790 day. It's still useful, even though it's 83 00:06:14,790 --> 00:06:20,218 superficial, incomplete and all that. It's still fairly useful because it does 84 00:06:20,218 --> 00:06:23,694 represent some important properties of galaxies. 85 00:06:23,694 --> 00:06:28,491 Not all, but, but some. However it does not represent others, for 86 00:06:28,491 --> 00:06:33,288 example, galaxy masses. You can't think of any more fundamental 87 00:06:33,288 --> 00:06:38,392 quantity than total mass of a galaxy that does contain at all in Hubble 88 00:06:38,392 --> 00:06:43,049 specifications. In fact here are plots of the mean values 89 00:06:43,049 --> 00:06:50,078 of important characteristics of galaxies like radius, mass, luminosity and mass to 90 00:06:50,078 --> 00:06:54,043 light ratios, as functions of the hubble type. 91 00:06:54,043 --> 00:07:00,583 As you can see these plots are largely flat meaning there is independence of 92 00:07:00,583 --> 00:07:04,835 these quantities on galaxies hubble type except. 93 00:07:04,836 --> 00:07:08,516 For some, maybe a little bit towards the latest Hubble types. 94 00:07:08,516 --> 00:07:13,282 If the Hubble type were representative or indicative of, say, galaxy mass, there 95 00:07:13,282 --> 00:07:17,056 should have been great correlations here and there aren't any. 96 00:07:17,056 --> 00:07:21,616 So this is really probably the most fundamental problem with morphology based 97 00:07:21,616 --> 00:07:26,471 classification as opposed to, say, physical properties based classification. 98 00:07:26,471 --> 00:07:30,431 All of these trends can be interpreted in a simple way. 99 00:07:30,431 --> 00:07:35,076 They are really a sequence of star formation histories of galaxies. 100 00:07:35,076 --> 00:07:40,312 Not just star formation today. There is almost none in ellipticals and a 101 00:07:40,312 --> 00:07:46,560 lot in spirals, and further away, go even more, but integrated over the lifetimes. 102 00:07:46,560 --> 00:07:51,859 Put simply, early types, ellipticals, bulges, form most of their stars early on 103 00:07:51,859 --> 00:07:56,915 and then very little after whereas disks tend to have much more extended star 104 00:07:56,915 --> 00:08:02,366 formation histories and it could be even flat, uniform star formation through the 105 00:08:02,366 --> 00:08:05,842 Hubble time. For some of them the late type discs a 106 00:08:05,842 --> 00:08:09,387 star formation may be even still increasing in time. 107 00:08:09,387 --> 00:08:14,395 Here is an interesting little fact. Typical spirals say like Milky Way form 108 00:08:14,395 --> 00:08:18,152 stars at typical rates of several solar masses per year. 109 00:08:18,152 --> 00:08:22,808 And if you add up all of the hydrogen available for star formation, there is 110 00:08:22,808 --> 00:08:25,559 usually about a billion solar masses or so. 111 00:08:25,559 --> 00:08:30,878 In other words if spirals were to continue like this they'll burn through all of the 112 00:08:30,878 --> 00:08:33,935 available hydrogen in a billion years or less. 113 00:08:33,935 --> 00:08:37,844 So it would seem as if we were really living at special time. 114 00:08:37,845 --> 00:08:40,576 More likely there is a fresh supply of gas. 115 00:08:40,576 --> 00:08:45,682 The fresh supply of gas that comes in from the intergalactic medium and gets still 116 00:08:45,682 --> 00:08:49,837 secreted from galaxies. And here is, schematically, what I meant 117 00:08:49,837 --> 00:08:55,160 by the trend in star formation histories. If you plot, say, star formation rates as 118 00:08:55,160 --> 00:09:00,088 a function of time in ellipticals and bulges, there is a lot of activity early 119 00:09:00,088 --> 00:09:04,081 on, dies off very quickly and there is hardly any after that. 120 00:09:04,081 --> 00:09:08,917 Whereas for these galaxies, there may be little enhancement in the beginning, but 121 00:09:08,917 --> 00:09:12,902 by and large remains flat. Otherwise, I mentioned, it can even be 122 00:09:12,902 --> 00:09:16,219 increasing. So the simple picture can explain all of 123 00:09:16,219 --> 00:09:20,856 the trends that we have seen so far. Now let's recall the concept of stellar 124 00:09:20,856 --> 00:09:25,452 populations as you probably remember from earlier in astronomy study. 125 00:09:25,452 --> 00:09:30,570 Those were introduced by Walter Baade who noticed that there seemed to be two kinds 126 00:09:30,570 --> 00:09:33,620 of stars. There are younger stars that tends to be 127 00:09:33,620 --> 00:09:38,365 constrict and to be in galaxy discs, and older stars that tend to be in bulges or 128 00:09:38,365 --> 00:09:41,789 galactic halos. And he called them population 1 and 129 00:09:41,789 --> 00:09:46,062 population 2. So they differ in their ages and where 130 00:09:46,062 --> 00:09:52,281 they're found, and sometimes in velocity, and also their kinematics. 131 00:09:52,281 --> 00:09:57,987 But as ideas can be extended, you can think of stellar populations as 132 00:09:57,987 --> 00:10:03,892 sub-systems inside galaxies. They're characterized by their location, 133 00:10:03,892 --> 00:10:09,652 by their dense distribution, by kinematics, by the star formation history, 134 00:10:09,652 --> 00:10:15,306 by the resulting metallicity, and so on. And there's probably more than 2. 135 00:10:15,306 --> 00:10:19,637 For example, in the milky way alone, we count at least 4. 136 00:10:19,637 --> 00:10:25,895 There would be old but metal rich bulge. There'll be old but metal poor halo much 137 00:10:25,895 --> 00:10:32,356 more extended, it'll be young stellar disk where stars are now being formed and it'll 138 00:10:32,356 --> 00:10:37,005 be older and somewhat thicker disk composed of older stars. 139 00:10:37,005 --> 00:10:41,421 Again note dark matter is yet whole another issue here. 140 00:10:41,421 --> 00:10:46,734 How can we understand the connection between star formation history and 141 00:10:46,734 --> 00:10:50,686 dynamics? Well, schematically, it works like this. 142 00:10:50,686 --> 00:10:56,677 Stars are essentially mass points, and even in colliding galaxies, galaxy 143 00:10:56,677 --> 00:11:01,055 mergers, they are behaving like dissipation systems. 144 00:11:01,055 --> 00:11:04,517 They just follow what ever potential there is. 145 00:11:04,517 --> 00:11:09,016 And they, in a sense, dynamically remember the dynamics of their birth. 146 00:11:09,016 --> 00:11:13,292 So if you start with a bunch of small galaxies merging together in a random 147 00:11:13,292 --> 00:11:17,842 fashion, then stars that used to belong to them will continue to move in random 148 00:11:17,842 --> 00:11:22,462 directions, and you'll end up with a stellar system that is supported by random 149 00:11:22,462 --> 00:11:25,558 motions which is just like elliptical galaxies. 150 00:11:25,559 --> 00:11:32,550 Now, consider collapse, not of galaxies already made of some stars, but just 151 00:11:32,550 --> 00:11:37,209 hydrogen clouds. They dissipate energy, but they cannot 152 00:11:37,209 --> 00:11:41,475 dissipate angular momentum, therefore, they settle. 153 00:11:41,476 --> 00:11:46,487 Configuration that gives them minimum energy for the given amount of angular 154 00:11:46,487 --> 00:11:49,442 momentum which is a flat, thin rotating disk. 155 00:11:49,442 --> 00:11:54,684 Now they make stars and those stars then remember dynamics of their birth and they 156 00:11:54,684 --> 00:11:59,565 continue moving in circular orbits. There is very little in terms of random 157 00:11:59,565 --> 00:12:03,330 motions. So in this way we can connect the dynamics 158 00:12:03,330 --> 00:12:09,333 of stellar populations or subsystem of these galaxies with the histories of their 159 00:12:09,333 --> 00:12:13,006 star formation. What about their metallicities? 160 00:12:13,006 --> 00:12:18,516 Remember that enrichment of stellar, interstellar medium comes from stars 161 00:12:18,516 --> 00:12:22,325 themselves. They form stars, massive ones explode, 162 00:12:22,325 --> 00:12:28,431 disseminate heavier elements, they cooked up in interstellar medium, new stars are 163 00:12:28,431 --> 00:12:32,860 formed from that, and so on. Now if you have a really massive big 164 00:12:32,860 --> 00:12:38,354 galaxy, supernova ejecta will not be able to escape the potential well, and they 165 00:12:38,354 --> 00:12:43,028 will mix in with the rest of the gas, which serves as a fuel for the next 166 00:12:43,028 --> 00:12:48,316 generation of stars and the metallicity will gradually increase in time. 167 00:12:48,316 --> 00:12:53,347 On the other hand, if you have a very low mass host system, a little dwarf galaxy or 168 00:12:53,347 --> 00:12:58,522 protogalactic fragment, the potential low is not so deep and the kinetic energy of 169 00:12:58,522 --> 00:13:03,751 supernova ejecta is sufficient to expel them out into the intergalactic space. 170 00:13:03,751 --> 00:13:07,801 So that little galaxy does not evolve chemically very much. 171 00:13:07,801 --> 00:13:13,100 It does a little bit but its metallicity remains low and doesn't change very much 172 00:13:13,100 --> 00:13:16,267 in time. So that's the qualitative picture but 173 00:13:16,267 --> 00:13:19,776 let's try to put this in a quantitative footing. 174 00:13:19,776 --> 00:13:24,306 We would like to know about actual distributions and correlations of 175 00:13:24,306 --> 00:13:29,532 meaningful physical properties of galaxies that we can measure in well defined 176 00:13:29,532 --> 00:13:33,657 fashion, and those include first the density distribution. 177 00:13:33,657 --> 00:13:38,027 How are they distributed spatially? What is the density profile? 178 00:13:38,027 --> 00:13:43,010 Their kinematical profile, how is the kinematics changing as a function of 179 00:13:43,010 --> 00:13:46,522 radius? The relative importance of Auld and Yang, 180 00:13:46,522 --> 00:13:52,306 or rotating in random components, and the chemical abundances and how they change. 181 00:13:52,306 --> 00:13:58,144 So by and large, structural properties or density distribution are obtained through 182 00:13:58,144 --> 00:14:03,166 photometry , or surface photometry which is spacial result of pretty much 183 00:14:03,166 --> 00:14:08,350 everything else, kinematics, metallicities, star formation rates, comes 184 00:14:08,350 --> 00:14:13,144 from spectroscopic measurements. And in order to interpret these 185 00:14:13,144 --> 00:14:18,720 measurements, we have to use various population synthesis models and dynamical 186 00:14:18,720 --> 00:14:21,840 models. Dynamical models can be assumed as toy 187 00:14:21,840 --> 00:14:26,124 models with some gravitational potential. See where that goes. 188 00:14:26,124 --> 00:14:30,802 Stellar population synthesis models, meaning what Kind of mix of different 189 00:14:30,802 --> 00:14:35,912 stars would be required to obtain spectrum as observed, and of course because stars 190 00:14:35,912 --> 00:14:40,876 evolve in time, their mixture will evolve and the spectrum will evolve in time and 191 00:14:40,876 --> 00:14:43,833 so we need galaxy evolution models like that. 192 00:14:43,833 --> 00:14:47,644 We have all of those and we'll talk about them a little later. 193 00:14:47,645 --> 00:14:56,659 So next we will turn to the structural properties of spiral galaxies.