1 00:00:00,012 --> 00:00:07,248 We now turn our attention to the other end of the Hubble sequence, the elliptical 2 00:00:07,248 --> 00:00:12,178 galaxies. In Hubble's definition, ellipticals were 3 00:00:12,178 --> 00:00:19,522 old, boring systems containing just old stars with no star formation, no dust, no 4 00:00:19,522 --> 00:00:22,732 gas. And the first theories of their formation 5 00:00:22,732 --> 00:00:27,542 whether they form through a single monolithic collapse of giant protogalaxy 6 00:00:27,542 --> 00:00:31,811 where all the stars are made. Turns out all of these are incorrect. 7 00:00:31,811 --> 00:00:36,451 The modern view is that ellipticals are actually fairly complex systems. 8 00:00:36,451 --> 00:00:41,625 They do contain lots of gas except most of it is X-ray gas, sometimes they have star 9 00:00:41,625 --> 00:00:45,186 formation. Sometimes they have dust mites, and they 10 00:00:45,186 --> 00:00:51,342 probably form through hierarchical merging of smaller pieces, although, dissimilitude 11 00:00:51,342 --> 00:00:56,968 collapse was probably also involved. They're also not simple systems, they have 12 00:00:56,968 --> 00:01:02,728 subsystems just like spiral galaxies do. Some ellipticals seem to show weak disks, 13 00:01:02,728 --> 00:01:06,212 some have special decoupled cores in the middle. 14 00:01:06,213 --> 00:01:09,879 Most of them have super massive black holes in the centers. 15 00:01:09,879 --> 00:01:14,571 One important distinction between ellipticals and spirals is that, whereas, 16 00:01:14,571 --> 00:01:19,257 spirals have contained most of their kinetic energy in the ordered rotational 17 00:01:19,257 --> 00:01:23,872 motion, in the elliptical galaxies most of the kinetic energy is in the form of 18 00:01:23,872 --> 00:01:28,771 random motions, so we call them pressure supported stars moving like molecules in 19 00:01:28,771 --> 00:01:32,386 gas. And here is a blatant example of dust in a 20 00:01:32,386 --> 00:01:36,768 elliptical galaxies. This is an elliptical galaxy NGC 1316 21 00:01:36,768 --> 00:01:40,583 which is in the center of the nearby Fornax Cluster. 22 00:01:40,583 --> 00:01:46,259 The reason why it has all this dust is that it gobbled up a spiral galaxy which 23 00:01:46,259 --> 00:01:51,978 had plenty of dust in its disks, and so that's still being digested Somehow. 24 00:01:51,978 --> 00:01:58,308 We do see the signature of mergers in ellipticals and a particularly interesting 25 00:01:58,308 --> 00:02:03,426 one is so called, shells. If you turn the contrast up in some of the 26 00:02:03,426 --> 00:02:10,171 elliptical galaxies, you'll see these shell like structures of the radii and now 27 00:02:10,171 --> 00:02:15,642 we think we know where they come from. If you have a largely 2-dimensional 28 00:02:15,642 --> 00:02:21,218 stellar components, such as this galaxy, and then you merge it into an elliptical 29 00:02:21,218 --> 00:02:26,876 galaxy, the stars will still stay on sort of 2-dimensional surface until sometime 30 00:02:26,876 --> 00:02:30,159 later. And so what we see our diluted stretched 31 00:02:30,159 --> 00:02:35,735 curved pieces of former galactic disks that be now wrapped around the elliptical 32 00:02:35,735 --> 00:02:40,270 galaxy as they are being merged in. This is being seen in numerical 33 00:02:40,270 --> 00:02:43,051 simulations and starts on the observations. 34 00:02:43,051 --> 00:02:47,530 The way we quantify structure for elliptical galaxies, is through surface 35 00:02:47,530 --> 00:02:50,977 photometry. We measure brightness as a function of the 36 00:02:50,977 --> 00:02:55,023 radius and the azimuth. And where this is light, we can then try 37 00:02:55,023 --> 00:02:58,945 to convert that into mass through kinematic measurements. 38 00:02:58,945 --> 00:03:03,676 The way we quantify structure of elliptical galaxies is first by measuring 39 00:03:03,676 --> 00:03:08,583 their radial brightness profiles, so called surface brightness profiles. 40 00:03:08,583 --> 00:03:13,377 And there are a number of formulae proposed to account for the shape of these 41 00:03:13,377 --> 00:03:20,296 brightness profiles. The most popular of them is so called the 42 00:03:20,296 --> 00:03:30,566 Vaucouleurs profile, it was invented by Gerard de Vaucouleurs and sub purely 43 00:03:30,566 --> 00:03:34,829 empirical formula. And it says that the log of the surface 44 00:03:34,829 --> 00:03:37,853 brightness which is luminosity per unit area, behaves as the radius to the 1 45 00:03:37,853 --> 00:03:41,021 quarter power, it's proportional to radius to minus 1 quarter power, and it was 46 00:03:41,021 --> 00:03:45,143 something called to the 1 quarteral. There is a parameter because it's an 47 00:03:45,143 --> 00:03:49,611 exponential involved, and that is called the effective radius. 48 00:03:49,611 --> 00:03:56,225 And if you tweak the constant multiplying that parallel that radius can be made so 49 00:03:56,225 --> 00:04:00,731 that it contains exactly one half of all projected light. 50 00:04:00,731 --> 00:04:06,189 So it's sometimes called an effective radius, or a half-light radius. 51 00:04:06,189 --> 00:04:11,535 And typically, for elliptical galaxies, it's value is the order of a few 52 00:04:11,535 --> 00:04:17,409 kiloparsec, not too different from say, typical scaling lengths of spiraling 53 00:04:17,409 --> 00:04:20,585 galaxy disks. So here are some plots of surface 54 00:04:20,585 --> 00:04:23,814 brightness for a thousand elliptical galaxies. 55 00:04:23,814 --> 00:04:29,100 What's show here is a logarithm of the surface brightness and y axis measured in 56 00:04:29,100 --> 00:04:33,687 magnitudes per square, second versus radius to the 1 quarter power. 57 00:04:33,687 --> 00:04:41,200 And the Vaucouleurs profile looks like a straight line in those coordinates. 58 00:04:41,200 --> 00:04:47,014 Indeed for some galaxies it seems to fit remarkable well in at least some part of 59 00:04:47,014 --> 00:04:52,822 the radial range but in the others it does not, and so these deviation are also of 60 00:04:52,822 --> 00:04:56,776 some interest. Other profiles have been suggested, and 61 00:04:56,776 --> 00:05:01,421 the one that's currently most popular is so called, Sersic profile. 62 00:05:01,421 --> 00:05:06,719 Which is a generalization of the Vaucouleurs profile, that log of a surface 63 00:05:06,719 --> 00:05:10,539 brightness goes as the radius to the minus 1 over n. 64 00:05:10,539 --> 00:05:15,788 Where n is some number, and in case of the Vaucouleurs profile, n is equal 4. 65 00:05:15,788 --> 00:05:20,415 So it's proportional to the radius to the minus 1 quarter. 66 00:05:20,415 --> 00:05:25,551 In case of this galaxy n is 1. So then you have just traditional old 67 00:05:25,551 --> 00:05:30,500 declining exponential. This formula turns out to actually to work 68 00:05:30,500 --> 00:05:36,132 remarkably well for elliptical galaxies as well as dark holes which is a very 69 00:05:36,132 --> 00:05:40,634 interesting thing. Its physical origin is not really well 70 00:05:40,634 --> 00:05:44,457 understood, it is also purely an empirical formula. 71 00:05:44,457 --> 00:05:50,007 Hubble himself proposed a different profile, this was essentially a parallel 72 00:05:50,007 --> 00:05:55,635 with surface brightness going as 1 over the radius squared, except in the middle 73 00:05:55,635 --> 00:05:59,802 where it's softened. So there is a parameter called, core 74 00:05:59,802 --> 00:06:03,148 radius. And within that core radius, profile is 75 00:06:03,148 --> 00:06:07,923 more or less flat and then it turns around and goes into the parallel. 76 00:06:07,923 --> 00:06:13,739 Now one profile, problem with Hubble's profile are originally envisioned, is that 77 00:06:13,739 --> 00:06:17,345 it diverges. If you integrate the surface brightness 78 00:06:17,345 --> 00:06:22,727 profile in radius, you'll reach infinite total luminosity at infinite radius and 79 00:06:22,727 --> 00:06:26,977 that clearly can't work. So, Hubble profile has to truncate at some 80 00:06:26,977 --> 00:06:29,914 point. So, it's actually quite remarkable that 81 00:06:29,914 --> 00:06:34,732 the ellipticals can be fit by the same family of profiles that have, say just two 82 00:06:34,732 --> 00:06:39,915 parameters, in case of the Vaucouleurs it's the effective radius that scales the 83 00:06:39,915 --> 00:06:44,733 radial coordinate and effective surface brightness that scales the vertical 84 00:06:44,733 --> 00:06:48,729 coordinate. In case of Sersic profile, there is that 85 00:06:48,729 --> 00:06:52,831 shape parameter, little n for which Vaucouleurs is fixed. 86 00:06:52,831 --> 00:06:58,022 And if it were always fixed then ellipticals would be a homologous family 87 00:06:58,022 --> 00:07:01,326 of objects, one could be scaled into another. 88 00:07:01,326 --> 00:07:05,746 Turns out, that's pretty close but that's not exactly true. 89 00:07:05,746 --> 00:07:10,392 And the deviations of that, from that, are actually quite important. 90 00:07:10,392 --> 00:07:15,557 One deviation that we already talked about before is the diffuse envelopes of cD 91 00:07:15,557 --> 00:07:20,947 galaxies and clusters, but you may recall that those are really stars that belong to 92 00:07:20,947 --> 00:07:24,963 the cluster itself, just cospatial with the galaxy itself. 93 00:07:24,964 --> 00:07:31,438 Nonetheless, if you obtain a surface brightness profile of the many, many 94 00:07:31,438 --> 00:07:38,545 ellipticals and then if you fit Sersic law to all of them and plot the value of the 95 00:07:38,545 --> 00:07:42,731 Sersic parameter. And which in this plot on the left is 96 00:07:42,731 --> 00:07:48,077 confusingly labeled as M by the authors of the paper, you find out that the shape 97 00:07:48,077 --> 00:07:53,423 parameter, Sersic parameter, depends on the size of the galaxy, and by the same 98 00:07:53,423 --> 00:07:58,785 token on the luminosity of the galaxy. And goes in the sense that galaxies with 99 00:07:58,785 --> 00:08:03,015 larger radii or larger luminosities have shallower profiles. 100 00:08:03,015 --> 00:08:08,493 A more direct way to see this is to simply bin together profiles of many ellipticals 101 00:08:08,493 --> 00:08:13,549 average them up, and plot prolific response to certain bin of luminosity and 102 00:08:13,549 --> 00:08:17,134 that's shown in the lower right done by Jim Schombert. 103 00:08:17,134 --> 00:08:21,643 And again you can see that the more luminous ellipticals seem to have 104 00:08:21,643 --> 00:08:25,812 shallower profiles. On the other hand, near the centers of 105 00:08:25,812 --> 00:08:29,051 galaxies, very interesting things happening. 106 00:08:29,051 --> 00:08:34,257 In Hubble's days, seeing an angular resolution simply were not good enough to 107 00:08:34,257 --> 00:08:39,217 actually tell what's happening in the inner arc second or so, and now a days 108 00:08:39,217 --> 00:08:44,417 with Hubble space telescope we can probe the inner portions of the elliptical 109 00:08:44,417 --> 00:08:48,420 galaxies, and interesting things are happening there. 110 00:08:48,420 --> 00:08:53,772 In some cases, there are corse flat density distributions, sort of like Hubble 111 00:08:53,772 --> 00:08:57,264 envisioned. In other cases, their density cusps, 112 00:08:57,264 --> 00:09:01,617 density goes as a parallel all the way in, as far as we can tell. 113 00:09:01,617 --> 00:09:06,542 And those may be related to the presence of super massive black holes. 114 00:09:06,542 --> 00:09:11,668 And carrying on with what happens at larger radii, you see that more luminous 115 00:09:11,668 --> 00:09:17,028 ellipticals tend to be those that have flat cores and small ellipticals tends to 116 00:09:17,028 --> 00:09:20,980 be those with cusps. So, here is just a collection of surface 117 00:09:20,980 --> 00:09:26,234 brightness profiles from Hubble, and they have been divided empirically into those 118 00:09:26,234 --> 00:09:30,896 that show a core, which are shown as solid lines here and those that look like 119 00:09:30,896 --> 00:09:36,078 parallel cusps, not impure parallels. Maybe slightly curved, but nevertheless 120 00:09:36,078 --> 00:09:38,919 the cusps and those are shown as dash lines. 121 00:09:38,920 --> 00:09:45,563 So the shape of the surface brightness profile changes at smaller radii. 122 00:09:45,563 --> 00:09:51,077 And so, people who study this have come up with a broken power law or nuker profile, 123 00:09:51,077 --> 00:09:54,800 that has one parallels synthetically smaller radii. 124 00:09:54,800 --> 00:10:00,062 And a different parallel, synthetically at large radii, and there is some transition 125 00:10:00,062 --> 00:10:03,316 radius between them where one bends into the other. 126 00:10:03,316 --> 00:10:06,801 And so here are the examples what those profiles look like. 127 00:10:06,801 --> 00:10:10,401 On the left, you see what happens as we change the inner slope. 128 00:10:10,401 --> 00:10:15,181 Whereas, the outer ones remain more or less fixed on the right it's the opposite. 129 00:10:15,181 --> 00:10:18,444 We keep the inner slope, but change the outer slope. 130 00:10:18,444 --> 00:10:24,786 Galaxies seem to fit all over this particular family of profiles. 131 00:10:24,786 --> 00:10:31,441 So next, we will talk about two dimensional and three dimensional shapes 132 00:10:31,441 --> 00:10:32,999 of ellipticals.