1 00:00:00,012 --> 00:00:07,382 We now turn our attention to the subject of galaxy evolution, which is at the core 2 00:00:07,382 --> 00:00:12,836 of most of modern cosmology and extragalactic astronomy. 3 00:00:12,836 --> 00:00:17,626 First of all, galaxies must evolve for two reasons. 4 00:00:17,626 --> 00:00:23,276 They're made out of stellar populations and stars evolve and also galaxies merge 5 00:00:23,276 --> 00:00:28,256 into assembling ever larger objects through hierarchical structural 6 00:00:28,256 --> 00:00:31,917 information. So, therefore, there are two aspects to 7 00:00:31,917 --> 00:00:35,763 galaxy formation. One is the assembly of the mass and the 8 00:00:35,763 --> 00:00:39,403 other one is conversion of gas into stars and energy. 9 00:00:39,404 --> 00:00:45,550 The former is relatively well understood. This is something we can model very well, 10 00:00:45,550 --> 00:00:50,081 in numerical simulations and it's driven by the dark matter. 11 00:00:50,081 --> 00:00:55,967 The later is much more difficult, because it involves messy dissipative processes 12 00:00:55,967 --> 00:01:01,091 including star formation itself, the feedback of stellar populations, 13 00:01:01,091 --> 00:01:07,170 supernovae active nuclei and so on. And there we can model processes up to 14 00:01:07,170 --> 00:01:14,622 some limit, using modern hydrosimulations because the analytic theories of star 15 00:01:14,622 --> 00:01:20,547 formation do not exist. So even though dark matter dominates the 16 00:01:20,547 --> 00:01:27,917 process of galaxy assembly because it's the dominant mass component That's not 17 00:01:27,917 --> 00:01:31,576 what we see. What we see is the light. 18 00:01:31,576 --> 00:01:38,416 And from that we have to infer about the assembly of the mass as well. 19 00:01:38,416 --> 00:01:45,808 First let's take a look at the relevant time scales for galaxy evolution. 20 00:01:45,809 --> 00:01:51,320 One timescale will be typical starburst scale which we now think is in or, of the 21 00:01:51,320 --> 00:01:56,334 order of 10 to 100 million years. That also happens to be the timescale of 22 00:01:56,334 --> 00:02:02,074 the life of the very massive stars which are energetically the dominant ones and 23 00:02:02,074 --> 00:02:07,496 which also provide most of the photons that ionize the interstellar medium. 24 00:02:07,496 --> 00:02:14,119 It also turns out that this is the estimated time scale of active galactic 25 00:02:14,119 --> 00:02:19,081 nucleus episodes. And it's not obvious whether this is 26 00:02:19,081 --> 00:02:24,650 coincidence or not. The internal time scales within galaxies 27 00:02:24,650 --> 00:02:30,600 are commiserate with their freefall time scales, which would be. 28 00:02:30,600 --> 00:02:35,497 Few hundred million years. And the time scales needed for merging an 29 00:02:35,497 --> 00:02:39,476 assembly are typically of the order of a billion years. 30 00:02:39,476 --> 00:02:45,177 And finally, there is Hubble time of the order of 10 billion years, through which 31 00:02:45,177 --> 00:02:49,032 galaxies evolve. There are different observational 32 00:02:49,032 --> 00:02:54,204 approaches to this problem. First of all we can look at our own Milky 33 00:02:54,204 --> 00:02:59,772 Way and try to deduce from it's constituents and properties how it may 34 00:02:59,772 --> 00:03:03,732 have form. Next we can look at nearby galaxies and 35 00:03:03,732 --> 00:03:10,287 study the systematics of their properties as we discussed earlier through Hubble 36 00:03:10,287 --> 00:03:14,603 sequence, through use of scaling relations and so on. 37 00:03:14,604 --> 00:03:20,106 And finally, we can observe it directly by looking deep, since light travels at a 38 00:03:20,106 --> 00:03:24,880 finite speed, the further out we look, the deeper in the past we look. 39 00:03:24,880 --> 00:03:29,410 And so we can see galaxy evolution unfolding on our path light con. 40 00:03:29,410 --> 00:03:35,155 To do this we need a good combination of theoretical tools and observational tools. 41 00:03:35,156 --> 00:03:41,548 On theory side we can use numerical simulations of structure formation to tell 42 00:03:41,548 --> 00:03:45,982 us about mass assembly. We can also predict behavior of the 43 00:03:45,982 --> 00:03:52,088 stellar populations because we know a lot about stellar evolution and so making some 44 00:03:52,088 --> 00:03:57,850 assumptions about Initial mass function of stars and the actual history of star 45 00:03:57,850 --> 00:04:03,698 formation rate in galaxies, we can predict what their spectra would look like as a 46 00:04:03,698 --> 00:04:07,343 composite of these evolving star populations. 47 00:04:07,343 --> 00:04:11,978 And there are also hybrid semi-analytical schemes that combine these. 48 00:04:11,978 --> 00:04:16,439 On the observational side, there again are three approaches. 49 00:04:16,439 --> 00:04:22,225 First, we can take deep images, the deeper the better if you want to look in the past 50 00:04:22,225 --> 00:04:27,387 over a full length of wavelengths. And from those we can infer a lot about 51 00:04:27,387 --> 00:04:32,034 star-forming histories or galaxies that may be merging as well. 52 00:04:32,034 --> 00:04:35,481 More important approach is to spectroscopy. 53 00:04:35,481 --> 00:04:40,887 Because this is what reveals not just retrusive galaxies, but physical 54 00:04:40,887 --> 00:04:46,557 properties that happen in them such as star formation rates, presence of an 55 00:04:46,557 --> 00:04:52,656 active nucleus, if there is one, etc, etc. And finally, we can look at the diffuse 56 00:04:52,656 --> 00:04:57,147 backgrounds, the collective emission of all galaxies ever. 57 00:04:57,147 --> 00:05:02,808 Now that has a drawback of not knowing which source is where until you actually 58 00:05:02,808 --> 00:05:07,812 resolve the background. But, it bypasses selection effect because 59 00:05:07,812 --> 00:05:13,594 you get everything, whether or not individual galaxies are affectable or not. 60 00:05:13,595 --> 00:05:19,504 A very important thing is to beware of, of the selection effects. 61 00:05:19,504 --> 00:05:25,266 And they come in multiple gui, guises. The most obvious one is the flux limit. 62 00:05:25,266 --> 00:05:31,664 Any given astronomical observation runs into the signal to noise problem at some 63 00:05:31,664 --> 00:05:35,079 faint level, and that's how deep you can go. 64 00:05:35,079 --> 00:05:40,453 But more subtly, there is a surface brightness selection limit that, for 65 00:05:40,453 --> 00:05:46,261 extended objects, it's the number of pixels above a certain threshold, that is 66 00:05:46,261 --> 00:05:51,369 not just how much light there is, but how diffuse it's distributed. 67 00:05:51,370 --> 00:05:56,721 The more compact galaxies would be much easier to detect than the diffuse ones. 68 00:05:56,721 --> 00:06:01,687 And so we know that, for sure, the deeper we look, we are getting an ever more 69 00:06:01,687 --> 00:06:07,367 luminous, or higher surface brightness objects and we're missing the intrisically 70 00:06:07,367 --> 00:06:10,663 faint ones or the intrinsically diffuse ones. 71 00:06:10,664 --> 00:06:16,246 If we don't know what we are missing, then it will be very hard to correct. 72 00:06:16,246 --> 00:06:21,577 But we can make reasonable extrapolations from nearer parts of the universe, and see 73 00:06:21,577 --> 00:06:25,197 how that works. Let's refresh our memory of the dynamical 74 00:06:25,197 --> 00:06:29,417 evolution galaxy merging. We've discussed this earlier, and seen 75 00:06:29,417 --> 00:06:33,402 how. Hierarchal assembly of galaxies is a 76 00:06:33,402 --> 00:06:40,222 process the keeps going on from the earliest days of the universe and we see 77 00:06:40,222 --> 00:06:44,281 it even today in major mergers of galaxies. 78 00:06:44,281 --> 00:06:50,902 This obviously arranges the mass but Thanks to the dissipation of all gas to 79 00:06:50,902 --> 00:06:57,622 the centers, it can also trigger a burst of star formation, as well as feeding 80 00:06:57,622 --> 00:07:05,001 active nuclei Thus stellar population evolution of galaxies and their denomical 81 00:07:05,001 --> 00:07:09,028 evolution can be very deeply interconnected. 82 00:07:09,028 --> 00:07:14,754 An important physical process here is dynamical friction, and it works as 83 00:07:14,754 --> 00:07:18,483 follows. If you have a massive body, in this case 84 00:07:18,483 --> 00:07:24,690 say a galaxy, moving through a sea of mass points, stars of another galaxy, say. 85 00:07:24,690 --> 00:07:29,408 It will accelerate them. And as it keeps moving, you'll be 86 00:07:29,408 --> 00:07:35,496 encountering more and more stars to which it will exert its acceleration. 87 00:07:35,496 --> 00:07:42,137 Those stars acquire a certain velocity. And that kinetic energy has to come at the 88 00:07:42,137 --> 00:07:45,065 expense of the perturber. So, the. 89 00:07:45,066 --> 00:07:51,860 Stellar population or population of test particles in which this perturber plunges, 90 00:07:51,860 --> 00:07:56,666 gains kinetic energy usually in the form of random motions. 91 00:07:56,666 --> 00:08:03,252 Whereas the perturber loses it's orbital kinetic energy and this why galaxies that 92 00:08:03,252 --> 00:08:07,303 they're initially in parabolic orbits can merge. 93 00:08:07,304 --> 00:08:13,874 Incidentally, a good local example is the Magellanic Clouds, and they are being torn 94 00:08:13,874 --> 00:08:17,537 apart by tidal interaction with the Milky Way. 95 00:08:17,537 --> 00:08:21,726 In a few billion years the will merge with the Milky Way. 96 00:08:21,726 --> 00:08:27,339 This has already happened to numerous dwarf galaxies in the past, and we see 97 00:08:27,339 --> 00:08:34,387 fossil evidence of that in[UNKNOWN] Halo. A time scale for dynamical friction can be 98 00:08:34,387 --> 00:08:39,568 expressed as follows. You start with the velocity, relative 99 00:08:39,568 --> 00:08:46,638 velocity, of the observer, divided by the rate in which that vel-, velocity is being 100 00:08:46,638 --> 00:08:50,988 diminished. And deceleration due to the energy loss. 101 00:08:50,988 --> 00:08:56,102 After some theoretical computation, you obtain a formula like this. 102 00:08:56,102 --> 00:09:01,642 It has several interesting features. The denser the medium, which has been 103 00:09:01,642 --> 00:09:05,707 perturbed. And the higher the mass of the perturber. 104 00:09:05,707 --> 00:09:11,160 The shorter time scale, which is intuitively clear. 105 00:09:11,160 --> 00:09:19,407 But also, the time scale gets to be longer if the velocity of the observer is high. 106 00:09:19,407 --> 00:09:25,368 And the reason for this is that. Perturbers that zip by very fast simply 107 00:09:25,368 --> 00:09:31,026 don't have enough time to deposit enough kinectic energy in the perturbing galaxy 108 00:09:31,026 --> 00:09:36,766 so it will take many different passes for the dynamical friction to actually do it's 109 00:09:36,766 --> 00:09:39,314 thing. This is why the time goes up. 110 00:09:39,315 --> 00:09:46,232 We've seen numerical simulations of merging galaxies earlier and this is a set 111 00:09:46,232 --> 00:09:52,091 of snapshots from one of those showing what dark matter particles do. 112 00:09:52,091 --> 00:09:59,324 In these simulations, always, eventually, the merger product looks something like an 113 00:09:59,324 --> 00:10:03,954 elliptical galaxy. But if you follow the evolution of the 114 00:10:03,954 --> 00:10:10,302 gas, you find out that the gas is reacting much more to the encounter and it's losing 115 00:10:10,302 --> 00:10:16,282 kinetic energy very quickly and very effectively meaning it has to sink to the 116 00:10:16,282 --> 00:10:21,726 bottom of the potential well. Which also means it will achieve high 117 00:10:21,726 --> 00:10:25,726 densities. This is exactly the kind of conditions 118 00:10:25,726 --> 00:10:31,901 that can lead to burst of star formation or they can also feed a giant black hole 119 00:10:31,901 --> 00:10:36,773 if there is one there. Next we will talk about the models of the 120 00:10:36,773 --> 00:10:39,324 stellar population evolution.