1 00:00:00,012 --> 00:00:04,170 Finally, let us now see why spiral galaxies are spiral. 2 00:00:04,170 --> 00:00:09,456 Obviously, the spiral arms are the defining feature, and there are some 3 00:00:09,456 --> 00:00:15,578 important observational facts about. Probably, the most significant of them is 4 00:00:15,578 --> 00:00:21,541 that they're seen only in disks that do contain interstellar material, the gas, 5 00:00:21,541 --> 00:00:24,992 which then causes star formation. There's. 6 00:00:24,992 --> 00:00:30,106 Disks which do not contain much gas, those are the S0 galaxies. 7 00:00:30,106 --> 00:00:36,572 Another important clue is that they're seen in young stars, and since young stars 8 00:00:36,572 --> 00:00:41,866 tend to last less than full rotational period, it suggests that. 9 00:00:41,866 --> 00:00:47,482 Possibly, that spiral arms are transient phenomenon, that they actually last less 10 00:00:47,482 --> 00:00:50,686 than typical rotations of, of, these galaxies. 11 00:00:50,686 --> 00:00:56,052 And, something that's a little tricky is that when you look at the spiral galaxy, 12 00:00:56,052 --> 00:00:59,521 you have the impression of a vortex, and that they. 13 00:00:59,522 --> 00:01:05,448 Rotate in a sense of you would see, in say, water going down the sink. 14 00:01:05,448 --> 00:01:11,352 They knew except that they move at half the angular speed of the disk. 15 00:01:11,352 --> 00:01:18,226 So the disk of the galaxy rotates in the same direction as you'd infer from binding 16 00:01:18,226 --> 00:01:22,993 of the arms and so do the spiral arms but at half the speed. 17 00:01:22,993 --> 00:01:28,184 So a relative of the disk, the spiral arms are actually moving in the other 18 00:01:28,184 --> 00:01:32,011 direction. They move as either scooping up the stars 19 00:01:32,012 --> 00:01:35,284 counter intuitive to what you think. Think. 20 00:01:35,284 --> 00:01:41,017 Now the important feature to remember here is that essentially all galactic disks 21 00:01:41,017 --> 00:01:46,495 have more or less flat rotation curves, meaning the linear velocity is roughly 22 00:01:46,495 --> 00:01:52,130 constant as a function of radius. Which means that the angular velocity goes 23 00:01:52,130 --> 00:01:56,128 as 1 over the radius. And so if you were to start with say 24 00:01:56,128 --> 00:02:02,062 straight, features directly through the galactic center due to the differential 25 00:02:02,062 --> 00:02:07,394 rotation it will naturally produce something that looks like segments of 26 00:02:07,394 --> 00:02:10,711 spiral arms. Which actually presented a dilemma 27 00:02:10,711 --> 00:02:15,547 initially on, because if somehow spiral arms were constant because of the 28 00:02:15,547 --> 00:02:21,163 differential rotation they'll keep winding and winding until they look like a really 29 00:02:21,163 --> 00:02:24,562 tightly wound spiral, and that's not what we see. 30 00:02:24,562 --> 00:02:27,811 An interesting way to approach this is as follows. 31 00:02:27,811 --> 00:02:32,591 Imagine that stars move around center of a galaxy in elliptical orbits because 32 00:02:32,591 --> 00:02:37,776 roughly they do. But its sub, subsequent orbits of larger 33 00:02:37,776 --> 00:02:43,641 radii are shifted a little bit. So it kind of looks like this. 34 00:02:43,641 --> 00:02:50,041 You start with 1, then keep adding, concentric but tilted orbits. 35 00:02:50,041 --> 00:02:55,921 And by the time you're done. You can see that there is something that 36 00:02:55,921 --> 00:03:01,364 looks like two spiral arms. And in fact, this is roughly what happens, 37 00:03:01,364 --> 00:03:07,576 but the only, the question is, why? The answer why, is the so-called density 38 00:03:07,576 --> 00:03:14,444 wave theory, that was developed in 1960s and 70s by Lin, Shu [unknown] and others, 39 00:03:14,444 --> 00:03:21,110 and the upshot is that spiral arms are density waves in deferentially rotating 40 00:03:21,110 --> 00:03:24,702 disks. If you were say to drop a rock in a pond 41 00:03:24,702 --> 00:03:29,221 it will have circular waves going away from the center. 42 00:03:29,221 --> 00:03:35,369 But, galaxies are not stationary points and if you are to make a prediction/g in a 43 00:03:35,369 --> 00:03:41,375 deferentially rotating disk like disk of spirals, what you get is not circular 44 00:03:41,375 --> 00:03:46,172 waves but spiral waves. The reason why they persist at all Is 45 00:03:46,172 --> 00:03:50,790 resonant motions. You can decompose motion of any star 46 00:03:50,790 --> 00:03:57,254 around center of a galaxy to first approximation as circular orbit with some 47 00:03:57,254 --> 00:04:01,270 perturbation. And the perturbation can again be 48 00:04:01,270 --> 00:04:07,510 approximated as, see a star wobbling around at radius, in other words, making 49 00:04:07,510 --> 00:04:14,084 circular orbits, around some hypothetical node on the central part of the orbit. 50 00:04:14,084 --> 00:04:19,343 That resembles epicycles, from Ptolemaic theory of solar system. 51 00:04:19,343 --> 00:04:23,294 And in fact those are called epicycles. So if you. 52 00:04:23,294 --> 00:04:29,332 Have the exact match in the numbers of orbits, around, at center of the epicycle, 53 00:04:29,332 --> 00:04:35,398 and its motion around center of a galaxy. We will have amplification, and that is 54 00:04:35,398 --> 00:04:40,313 exactly what's happening. So if you look at angular frequencies, 55 00:04:40,313 --> 00:04:44,051 omega not to be confused with density parameter. 56 00:04:44,051 --> 00:04:50,224 And then look at the epicyclic frequency. Again angular frequency then divided by 57 00:04:50,224 --> 00:04:55,419 integer number which is usually small one. One or two or something. 58 00:04:55,420 --> 00:05:00,651 Then the first two resonances are called the[UNKNOWN] resonances. 59 00:05:00,651 --> 00:05:06,399 And they actually are the radii in which that Particular rotational frequency 60 00:05:06,399 --> 00:05:12,402 occurs from which or to which spiral arms extend and since usually the 1st harmonic 61 00:05:12,402 --> 00:05:17,796 the first of residence is the strongest one this is why we mostly see two arm 62 00:05:17,796 --> 00:05:22,592 spirals although we do see for example 4 arms spirals and so on. 63 00:05:22,592 --> 00:05:28,707 So another way to phrase this is that spiral's arms are density waves which are 64 00:05:28,707 --> 00:05:34,940 really resonances of perturbations in these differentially rotating disks. 65 00:05:34,940 --> 00:05:40,962 So remember the orbit crowding diagram that is more or less what happens here 66 00:05:40,962 --> 00:05:47,260 where unelliptical orbit is can be really be composed as circle plus an epicycle 67 00:05:47,260 --> 00:05:52,553 that makes exactly one turn. As the big one turns around, and so that, 68 00:05:52,553 --> 00:05:58,261 that causes the elliptical shape. Now the orbit crowding really implies that 69 00:05:58,261 --> 00:06:03,929 there is going to be a density pile up, and that's exactly what spiral arms are. 70 00:06:03,929 --> 00:06:09,220 Now these waves are moving relative to the underlying disk gas and stars. 71 00:06:09,220 --> 00:06:13,539 And so, as the waves hit material, they are compressing it. 72 00:06:13,539 --> 00:06:17,422 Compressed gas. Is liable to make stars, which is why we 73 00:06:17,422 --> 00:06:20,708 see star formation associated with spiral arms. 74 00:06:20,708 --> 00:06:25,424 So this is why star formation can be triggered by spiral density waves. 75 00:06:25,424 --> 00:06:30,230 That's not the only way in which we can trigger star formation disks, but 76 00:06:30,230 --> 00:06:33,736 obviously it works. And there'll be stars made of all 77 00:06:33,736 --> 00:06:37,367 different. Masses, but the most massive stars, as you 78 00:06:37,367 --> 00:06:42,083 probably know, are the more luminous ones, and they live the shortest. 79 00:06:42,083 --> 00:06:47,619 So it's the shortest lived, most luminous stars, which haven't had chance to, drift 80 00:06:47,619 --> 00:06:53,003 away, from the wave, before they explode. That will deleniate, pattern on spiral 81 00:06:53,003 --> 00:06:56,729 density waves. When you look at bluer light ones, which 82 00:06:56,729 --> 00:07:02,101 are more susceptible to the radiation from Yankov/g luminous stars, you see very 83 00:07:02,101 --> 00:07:07,072 prominent spiral density patterns. If you look to the redder light ones. 84 00:07:07,073 --> 00:07:12,290 Say, near infrared, where the dens, where the light is dominated by the older, red 85 00:07:12,290 --> 00:07:15,786 giant stars. The spiral arms are still prominent, the 86 00:07:15,786 --> 00:07:20,574 density wave is still there, but not nearly as much, or not nearly as sharp as 87 00:07:20,574 --> 00:07:23,413 you would see in blue or ultraviolet light. 88 00:07:23,414 --> 00:07:26,533 Flight. So schematically, you'd expect things to 89 00:07:26,533 --> 00:07:29,811 look like this. There is the spiral density wave that 90 00:07:29,811 --> 00:07:34,769 moves relative to the underlying disk, in the opposite way of what you think from 91 00:07:34,769 --> 00:07:39,166 say, water going down the sink. In other words, the arms are scooping up 92 00:07:39,166 --> 00:07:43,980 the materials so the leading edge of the waves is the inner part of the spiral. 93 00:07:43,980 --> 00:07:49,356 This is where density wave compresses the gas, molecular clouds. 94 00:07:49,356 --> 00:07:54,272 Makes dust lanes. And then, inside of them, there'll be star 95 00:07:54,272 --> 00:07:58,252 formation. So you expect to see dark lanes on the 96 00:07:58,252 --> 00:08:03,282 leading edge, which is back-, back side of the spiral arms. 97 00:08:03,283 --> 00:08:09,572 Followed immediately by regions of star formation and then kind of more diffused 98 00:08:09,572 --> 00:08:13,831 stellar population as you go towards the trailing edge. 99 00:08:13,831 --> 00:08:17,786 And now let's look what that looks like in real life. 100 00:08:17,786 --> 00:08:24,182 So this is a picture from Hubble space telescope of, I believe N101 right near my 101 00:08:24,182 --> 00:08:30,436 spiral, and this is exactly what you see. The inner part, which is the leading edge 102 00:08:30,436 --> 00:08:35,706 of the spiral arm, is where you see the dust lines, and then the red, dots and 103 00:08:35,706 --> 00:08:39,577 blobs that you see in the dust lines are immediately. 104 00:08:39,577 --> 00:08:43,466 Past them, or the regions of young star formations. 105 00:08:43,466 --> 00:08:47,103 Those are H alpha, nebuli ionized by young stars. 106 00:08:47,103 --> 00:08:53,093 The young stars burn out through the dust, dissipated, and then you see luminous, 107 00:08:53,093 --> 00:08:57,263 blue, stellar light. And then kind of fades away as you go 108 00:08:57,263 --> 00:09:03,347 towards the trailing edge of the spiral. So the theory predicts exactly this. 109 00:09:03,347 --> 00:09:09,653 So to summarize spiral arms are density waves that occur in deferentially rotating 110 00:09:09,653 --> 00:09:13,894 stellar disks. They will compress gas that will lead to 111 00:09:13,894 --> 00:09:19,261 star formation at edges where gas enters the spiral density wave. 112 00:09:19,261 --> 00:09:23,695 Stars themselves will simply pass through density wave. 113 00:09:23,695 --> 00:09:28,003 Just like water molecules pass nicely through waves in water. 114 00:09:28,003 --> 00:09:33,031 And this dynamical theory is, is very successful in explaining the global 115 00:09:33,031 --> 00:09:36,710 properties of spiral galaxies, but it's not perfect. 116 00:09:36,710 --> 00:09:40,936 First of all it doesn't say why were there waves to begin with. 117 00:09:40,936 --> 00:09:46,846 Some sort of disturbance has to happen and one possibility is that encounters between 118 00:09:46,846 --> 00:09:51,353 galaxies Create such a disturbance. That's entirely possible. 119 00:09:51,353 --> 00:09:56,443 Another part, which is a little more difficult, is that not all spirals are 120 00:09:56,443 --> 00:10:00,797 perfect alarm spirals. They're detached spiral arms, spurs, 121 00:10:00,797 --> 00:10:04,416 things like that. So additional mechanisms might be 122 00:10:04,416 --> 00:10:07,514 responsible for a creation of Such patterns. 123 00:10:07,514 --> 00:10:12,525 Now where the theory predicts exactly what we should see in say, a two arm grand 124 00:10:12,525 --> 00:10:17,302 design spiral, as they're called. Here are other types of these galaxies, 125 00:10:17,302 --> 00:10:22,394 spiral so to speak, in which, the patterns are much more diffuse and they're both 126 00:10:22,394 --> 00:10:27,638 flocculent galaxies that they almost have no spiral arms, but they certainly have 127 00:10:27,638 --> 00:10:31,666 patches of star formation. And even maybe little segments. 128 00:10:31,667 --> 00:10:35,276 They may be caused by different phenomenon. 129 00:10:35,276 --> 00:10:41,837 Namely differential stretching just like we addressed at the beginning of the 130 00:10:41,837 --> 00:10:45,211 lecture. So that's it for these galaxies. 131 00:10:45,211 --> 00:10:49,685 Next, we will start talking about elliptical galaxies.