1 00:00:00,012 --> 00:00:04,021 Let us now turn to the shapes of elliptical galaxies. 2 00:00:04,021 --> 00:00:07,986 They're called elliptical for a really good reason. 3 00:00:07,986 --> 00:00:13,786 Their projected surface brightness profiles look pretty close to ellipses. 4 00:00:13,786 --> 00:00:18,821 And here are 2 intensity maps. Lines of equal surface brightness. 5 00:00:18,821 --> 00:00:23,680 In the image on the left, the little knots are actually stars in the foreground, 6 00:00:23,680 --> 00:00:26,528 because their brightness gets measured too. 7 00:00:26,528 --> 00:00:31,498 And you can obviously see that elliptical galaxies really do look like ellipses, 8 00:00:31,498 --> 00:00:35,406 although, ellipticity may be changing as a function of radius. 9 00:00:35,406 --> 00:00:39,115 Generally, they tend to be a little rounder in the middle. 10 00:00:39,116 --> 00:00:43,218 Originally, people thought that ellipticity is due to rotational 11 00:00:43,218 --> 00:00:46,767 flattening. Elliptical galaxy spins, centrifugal force 12 00:00:46,767 --> 00:00:51,807 stretches it in two directions, orthogonal to the spin axis but turned, that turned 13 00:00:51,807 --> 00:00:55,796 out not to be the case. And more modern studies indicated that in 14 00:00:55,796 --> 00:01:00,828 fact, ellipticals, first of all, don't rotate very much and, second, their shape 15 00:01:00,828 --> 00:01:05,183 is not to do to rotation. It is due to the velocity and isotropy. 16 00:01:05,183 --> 00:01:09,916 As you would recall, the stars in elliptical galaxies move randomly. 17 00:01:09,916 --> 00:01:14,317 Sort of like molecules in gas. That's why they call them pressure 18 00:01:14,317 --> 00:01:17,949 supported. But, it's possible in a dynamical system 19 00:01:17,949 --> 00:01:22,924 like that, that velocity dispersion is different along x, y, and z axis. 20 00:01:22,924 --> 00:01:26,825 A galaxy may be a little hotter in one axis than the other. 21 00:01:26,825 --> 00:01:32,217 And so that means the stars will go further out along that axis so be longer. 22 00:01:32,217 --> 00:01:37,515 And now we think that the shapes of elliptical galaxies are due entirely, to 23 00:01:37,515 --> 00:01:41,408 this anisotropy. The, their temperature if you will is 24 00:01:41,408 --> 00:01:46,989 different, in different directions. We can use statistics of observed shapes 25 00:01:46,989 --> 00:01:50,456 to try to decompose, to project what's going on. 26 00:01:50,456 --> 00:01:56,120 What's shown here in the histogram is the distribution of through ellipticities, 27 00:01:56,120 --> 00:02:01,942 through dimensional ellipticities, if indeed ellipticals were simple, flattened 28 00:02:01,942 --> 00:02:07,536 or elongated prolate ellipsoids. But in reality, they can have three axis 29 00:02:07,536 --> 00:02:13,302 and therefore to flock, through axis ratio's, and that plays a little more 30 00:02:13,302 --> 00:02:16,653 complicated. So, the simplest case is if they 31 00:02:16,653 --> 00:02:20,228 spherical. There aren't many galaxies like that. 32 00:02:20,228 --> 00:02:25,554 Another case is that if one axis is equal to another And, those are bigger than a 33 00:02:25,554 --> 00:02:29,536 third one, than it's a football, it's a prolate ellipsoid. 34 00:02:29,536 --> 00:02:34,406 The other hand, if it's short one is actually shorter than the other or equal, 35 00:02:34,406 --> 00:02:38,216 we have oblate ellipsoid, flattened thing. Flying saucer. 36 00:02:38,216 --> 00:02:42,126 And more generally, we have three axes of different shapes. 37 00:02:42,126 --> 00:02:45,857 So, on average, the actual ratios are shown here. 38 00:02:45,857 --> 00:02:51,797 In fact ellipticals are fairly close to be oblate ellipsoids, flattened along one 39 00:02:51,797 --> 00:02:57,255 direction, but not perfectly so. In addition to the dynamical evidence for 40 00:02:57,255 --> 00:03:03,275 anisotropic velocities which I'll show you in the next module, we can actually tell 41 00:03:03,275 --> 00:03:09,140 this from pictures themselves. And this little tricky to envision but 42 00:03:09,140 --> 00:03:14,073 works like this. If you look at the set of nested triaxial 43 00:03:14,073 --> 00:03:21,561 ellipsoids from[UNKNOWN] you will in fact see that they're apparent projected major 44 00:03:21,561 --> 00:03:28,614 axis seems to move, rotate in the sky. And that is called the isophote twist. 45 00:03:28,614 --> 00:03:36,146 You can again see here, you can look from same[UNKNOWN] from different directions, 46 00:03:36,146 --> 00:03:41,013 and the regions of firedance will project in certain. 47 00:03:41,013 --> 00:03:45,916 Major axis erection at lower densities, you will see it from slide to different 48 00:03:45,916 --> 00:03:49,015 angle. An ellipse will look like if it's turning, 49 00:03:49,015 --> 00:03:54,317 and so that's exactly what's observed. Even in our immediate neighbor Andromeda 50 00:03:54,317 --> 00:04:00,077 Galaxy, Andromeda has two elliptical dwarf elliptical companions and if you stretch 51 00:04:00,077 --> 00:04:05,517 the picture at high contrast you can see that For one of them NGC205, the isophotes 52 00:04:05,517 --> 00:04:11,232 are twisted and actually a little boxy, which is another interesting question. 53 00:04:11,233 --> 00:04:17,472 What's shown on the lower left is set of isophotes for some elliptical galaxy and 54 00:04:17,472 --> 00:04:22,410 major minor axis are drawn. You can see as you go to ever larger 55 00:04:22,410 --> 00:04:28,582 radii, the Axis seemed to rotate. So this is still elliptical shapes. 56 00:04:28,582 --> 00:04:34,369 But actually, ellipticals are not purely elliptical in shape. 57 00:04:34,369 --> 00:04:41,203 Pretty close, but not always exactly. And the first deviation that you can 58 00:04:41,203 --> 00:04:47,803 quantify is, if there is an extra, harmonic if you will, around given 59 00:04:47,803 --> 00:04:52,204 isophote. You can think of an elliptical galaxy 60 00:04:52,204 --> 00:04:57,856 isophote as say single period wave as you go in the azimuth. 61 00:04:57,856 --> 00:05:05,142 But suppose that there is a, twice as high frequency component, then you will see two 62 00:05:05,142 --> 00:05:10,164 waves as you go around and there are 2 possibilities here. 63 00:05:10,164 --> 00:05:15,184 One is that they can be co-aligned with major axis and minor axis. 64 00:05:15,184 --> 00:05:21,670 In which case they're just going to bump out the isophotes along the axis and the 65 00:05:21,670 --> 00:05:25,851 thing would look like its got little lemon like shape. 66 00:05:25,852 --> 00:05:31,730 The, these are called disky galaxies because that's exactly what you get if you 67 00:05:31,730 --> 00:05:37,099 are to project together very thin disk on top of an elliptical isophote. 68 00:05:37,099 --> 00:05:40,918 The other possibility is that they're out of phase. 69 00:05:40,918 --> 00:05:45,950 And so then you get kind of bumps between the major and minor axis. 70 00:05:45,950 --> 00:05:51,692 Those are called boxy ellipticals, and we believe that their shape is due more to 71 00:05:51,692 --> 00:05:56,816 anisotropy and less to rotation. Of course, disks are always supported by 72 00:05:56,816 --> 00:06:02,324 rotation, even if the larger elliptical component is supported by anisotropy. 73 00:06:02,324 --> 00:06:07,609 And so here are a couple examples. One of, disky, elliptical and now we think 74 00:06:07,609 --> 00:06:13,060 that in fact most elliptical galaxies those originally classified as elliptical 75 00:06:13,060 --> 00:06:18,432 showless kind of isophotes lemon shaped, actually you have disks and there may be 76 00:06:18,432 --> 00:06:23,646 close to zeroes than elliptical sproper but there is a continuum of properties 77 00:06:23,646 --> 00:06:28,547 there are no sharp distinctions. The other one shown is also boxy galaxy. 78 00:06:28,548 --> 00:06:31,908 Some galaxies have both. A different radii. 79 00:06:31,908 --> 00:06:37,458 As you go up in radius, what looks in inner part is a box elliptical maze start 80 00:06:37,458 --> 00:06:43,090 looking like a disky one because there is a disk you see at large radius or, the 81 00:06:43,090 --> 00:06:46,758 other way around. And so this is not a fundamental 82 00:06:46,758 --> 00:06:51,016 distinction like. Pure spirals versus pure ellipticals. 83 00:06:51,016 --> 00:06:54,886 It's more of a na, nature of gradual change of the mix. 84 00:06:54,886 --> 00:07:00,502 There are some trends that support that there is physical and meaningful thing 85 00:07:00,502 --> 00:07:04,206 going on which has to do with velocity and isotropy. 86 00:07:04,206 --> 00:07:09,980 The boxy galaxies are more an isotropic. They also tend to be more luminous ones 87 00:07:09,980 --> 00:07:14,080 and more and, and also have higher extra luminosities. 88 00:07:14,080 --> 00:07:19,078 Those can be understood in terms of merging, random merging of pieces into an 89 00:07:19,078 --> 00:07:24,276 elliptical galaxy will both anisotropize/g its velocity dis-, dispersion. 90 00:07:24,276 --> 00:07:29,466 And, it will contribute to the mass, making then bigger, and can also heat the 91 00:07:29,466 --> 00:07:32,982 gas. And so there is a trend sometimes people 92 00:07:32,982 --> 00:07:38,878 overstate that, that boxy galaxies are products of mergers were the disky ones, 93 00:07:38,878 --> 00:07:42,493 the larger products of disks about to collapse. 94 00:07:42,493 --> 00:07:47,659 Neither is the case in reality. There is a mixture of those forming 95 00:07:47,659 --> 00:07:51,076 mechanisms and it's a question of a degree. 96 00:07:51,076 --> 00:07:56,072 Next time we'll talk about internal kinematics of ellipticals.