1 00:00:00,012 --> 00:00:05,726 So we have seen what stars do in spiral galaxies, but what about gas? 2 00:00:05,726 --> 00:00:11,732 This is a composite picture of a lot of neutral hydrogen images of a lot of 3 00:00:11,732 --> 00:00:18,432 different spiral galaxies, and you can see that in many cases you see spiral arms 4 00:00:18,432 --> 00:00:22,712 just fine. Well, there are many components to the gas 5 00:00:22,712 --> 00:00:27,405 in spiral galaxies. There is cool, neutral hydrogen, which we 6 00:00:27,405 --> 00:00:32,381 denote as H-Roman One. Then there is molecular gas, usually, most 7 00:00:32,381 --> 00:00:37,388 of that is actually molecular hydrogen and then carbon monoxide. 8 00:00:37,388 --> 00:00:43,764 But there are hundreds, if not thousands, of others species of molecules out there, 9 00:00:43,764 --> 00:00:48,332 some of them, very complex. And the importance of, of the gas, is 10 00:00:48,332 --> 00:00:52,737 that's the fuel for star formage. You need to have cold gas, cold, 11 00:00:52,737 --> 00:00:58,095 interstellar material clouds to collapse and be able to ignite, nuclear fusion in 12 00:00:58,095 --> 00:01:02,196 the core. Now there is ionized gas as well ionized 13 00:01:02,196 --> 00:01:06,020 by the ultraviolet radiation from young stars. 14 00:01:06,020 --> 00:01:11,792 And that's what makes those pretty pictures of nebulii and star formation 15 00:01:11,792 --> 00:01:15,006 regions. And that, we can observe, observe it 16 00:01:15,006 --> 00:01:20,286 through optical spectroscopy or other forms of spectroscopy, but the neutral 17 00:01:20,286 --> 00:01:25,646 hydrogen is best observed through the spin-flip line which I'll explain in a 18 00:01:25,646 --> 00:01:28,869 moment, at the wavelength of 21 centimeters. 19 00:01:28,869 --> 00:01:32,356 At that wavelength, radio waves go through the dust. 20 00:01:32,356 --> 00:01:37,712 And so we can see all the way through. So just as there are stellar disks, there 21 00:01:37,712 --> 00:01:41,475 are gas disks, and they also can show spiral arms. 22 00:01:41,475 --> 00:01:47,487 But one important thing is that hydrogen disks extend further than the optical 23 00:01:47,487 --> 00:01:53,512 ones, sometimes by a substantial factor. And because this spin flip line is so 24 00:01:53,512 --> 00:01:58,269 sharp and well defined. It can be used as an excellent tracer of 25 00:01:58,269 --> 00:02:04,611 velocities of gas, including rotation of, of galaxies the, ta, the Doppler shift. 26 00:02:04,611 --> 00:02:10,271 This is how it works, in a hydrogen atom, you have a proton and an electron. 27 00:02:10,271 --> 00:02:15,771 And there are 2 possibilities for their spins to be Aligned or anti-aligned, one 28 00:02:15,771 --> 00:02:21,429 has a slightly higher energy level than the other and that transition corresponds 29 00:02:21,429 --> 00:02:24,964 to the photons of the wavelength of 21 centimeters. 30 00:02:24,964 --> 00:02:29,929 So, this is, where it counts. Now, this is not something that's easily 31 00:02:29,929 --> 00:02:35,617 achieved in a lab but can happen easily in densities and temperatures of interstellar 32 00:02:35,617 --> 00:02:38,556 medium. So there's plenty of it. 33 00:02:38,556 --> 00:02:46,200 And this 21 centimeter line has been main tool to probe interstellar material in 34 00:02:46,200 --> 00:02:52,027 these galaxies, near and far. So here is a contour map of neutral 35 00:02:52,027 --> 00:02:59,587 hydrogen superposed on an optical image of this galaxy and it's obvious that hydrogen 36 00:02:59,587 --> 00:03:06,026 goes much further in visible stars. Now by observing Doppler shifts of the 21 37 00:03:06,026 --> 00:03:09,629 centimeter line we can see the velocity field. 38 00:03:09,629 --> 00:03:14,230 And that looks like this. These are the lines of equal radial 39 00:03:14,230 --> 00:03:20,327 velocity and they look like this because on one side The disc is coming towards us 40 00:03:20,327 --> 00:03:25,808 and the other is going away from us. Because the gas goes so much further out 41 00:03:25,808 --> 00:03:31,758 than stars, we can measure rotation curves much further using this method, than say 42 00:03:31,758 --> 00:03:37,075 through optical spectroscopy. So here is a simple comparison of optical 43 00:03:37,075 --> 00:03:42,730 and hydrogen images of famous spiral galaxy M81, in the Virgo cluster, and you 44 00:03:42,730 --> 00:03:48,646 can see that gases nicely concentrated in spiral arms, which is, of course, where 45 00:03:48,646 --> 00:03:52,537 stars are made. Although it tends to be a little more 46 00:03:52,537 --> 00:03:55,256 extended. So that was the atomic gas. 47 00:03:55,256 --> 00:03:59,636 What about molecular gas? Aside from hydrogen molecule, h2. 48 00:03:59,636 --> 00:04:03,061 The next most prominent one is carbon monoxide. 49 00:04:03,061 --> 00:04:08,592 Incidentally, all of the different molecular species tend to be found in same 50 00:04:08,592 --> 00:04:13,412 places, by and large. Now in the case of molecular clouds, which 51 00:04:13,412 --> 00:04:18,868 are colder, they get to be closer to the dust lanes and they're closer to the 52 00:04:18,868 --> 00:04:23,644 regions of star formation. You can see spiral arms outlined even 53 00:04:23,644 --> 00:04:27,333 better. But there are also new features, there are 54 00:04:27,333 --> 00:04:33,030 rings of Molecular gas around galaxies. There are bars and other things. 55 00:04:33,030 --> 00:04:38,368 As you probably learned in astronomy before, the interstellar medium has 56 00:04:38,368 --> 00:04:42,486 multiple phases. I touched upon that a few minutes ago. 57 00:04:42,486 --> 00:04:48,188 The, first there is a cold interstellar medium and that's mostly the neutral 58 00:04:48,188 --> 00:04:54,257 hydrogen gas, temperatures ranging from typically 10's to 100 degrees Kelvin. 59 00:04:54,257 --> 00:04:59,638 So that's all of the cold gas, both atomic and molecular as well as dust. 60 00:04:59,638 --> 00:05:03,298 This is what makes stars in, in this cold galaxy. 61 00:05:03,298 --> 00:05:09,780 Next, there is warm interstellar material. Typically with temperatures of thousands 62 00:05:09,780 --> 00:05:14,910 or tens of thousand of degrees. And, it's warmed up and ionized by the 63 00:05:14,910 --> 00:05:19,722 radiation by young stars. And that's where all these pretty nebula 64 00:05:19,722 --> 00:05:22,907 come from. And finally, there is hot interstellar 65 00:05:22,907 --> 00:05:26,616 medium, which really mostly belongs to the galactic halo. 66 00:05:26,616 --> 00:05:31,797 It's the gas that's expelled from the disc through explosions of supernovi and the 67 00:05:31,797 --> 00:05:36,973 shock waves of supernova remnants are what heats it up to this particular range. 68 00:05:36,974 --> 00:05:42,872 Here, here is the typical spiral galaxy rotation curve, with it's decomposition 69 00:05:42,872 --> 00:05:46,776 into the bulged disc, and dark matter components. 70 00:05:46,776 --> 00:05:52,544 We spoke about those extensively when we talked about dark matter, so, I will not 71 00:05:52,544 --> 00:05:59,978 dwell on this much further. And next, we will look at where does the 72 00:05:59,978 --> 00:06:04,037 spiral structure come from.