1 00:00:00,012 --> 00:00:05,410 So we have the disc in the center of the bulge, and around this we have a halo. 2 00:00:05,410 --> 00:00:10,762 The halo contains about 150 global clusters, typical masses of up to about a 3 00:00:10,762 --> 00:00:15,798 million solar masses and it turns out lots of field stars, so there are 4 00:00:15,798 --> 00:00:21,132 population 2 stars orbiting in a high inclination orbits that are not part of 5 00:00:21,132 --> 00:00:26,157 globular clusters, in fact we now know that 99% of the mass of the stars in the 6 00:00:26,157 --> 00:00:30,962 halo is outside the globular clusters. Globular clusters are very visible 7 00:00:30,962 --> 00:00:34,482 minority. furthermore about half of all these, 40% 8 00:00:34,482 --> 00:00:39,332 of orbit with inclinations larger than 90 degrees, which makes them retrograde 9 00:00:39,332 --> 00:00:43,042 orbits, so if the stars in the disc are orbiting one way. 10 00:00:43,042 --> 00:00:48,582 These stars are orbiting in a, by and large in the other direction so the 11 00:00:48,582 --> 00:00:51,727 angular momentum of the halo in net is about zero. 12 00:00:51,727 --> 00:00:56,562 In terms of size remember Shapley had a large halo of 100 kiloparsecs taking into 13 00:00:56,562 --> 00:01:01,717 account absorption and correct distance measurements, all but about less than 10 14 00:01:01,717 --> 00:01:06,082 of the globular clusters and most of the field stars are within 50 kiloparsecs. 15 00:01:06,082 --> 00:01:10,640 So typically one says, that the halo's sphere of radius 50 kiloparsecs. 16 00:01:10,640 --> 00:01:15,182 You'll notice this is twice the size of the radius of the disc, so the halo is 17 00:01:15,182 --> 00:01:19,492 larger and envelops everything. Of course, some of these objects are in 18 00:01:19,492 --> 00:01:23,117 the disc, we detected them first as population 2 stars in the solar 19 00:01:23,117 --> 00:01:26,757 neighborhood. That just happened to be moving with 20 00:01:26,757 --> 00:01:32,209 wrong velocity and all of the stars in the halo, as we said, are old between 11 21 00:01:32,209 --> 00:01:36,272 and 13 billion years. What else is out there? Well, we knew 22 00:01:36,272 --> 00:01:41,886 that the halo was permeated by warm gas, we could obeserve that because warm gas, 23 00:01:41,886 --> 00:01:47,961 by warm I mean up to 100,000 or a few hundred thousand Kelvin absorbs visible 24 00:01:47,961 --> 00:01:53,222 light, and so we could see the visible light absorption from the halo. 25 00:01:53,222 --> 00:01:59,466 some of the hydrogen atoms are excited, some of the other atoms are inexcited, 26 00:01:59,466 --> 00:02:05,464 are, are there and have lower excitation energy than hydrogen so we could track 27 00:02:05,464 --> 00:02:08,695 the gas in the halo, and there's not that much. 28 00:02:08,695 --> 00:02:13,565 It's not a significant mass. a new discovery from September this year 29 00:02:13,565 --> 00:02:17,476 so I'm putting it in there with an asterisk because I don't know how 30 00:02:17,476 --> 00:02:22,482 verified it's going to be, is that there is another gas component in the halo. 31 00:02:22,482 --> 00:02:27,782 This is hot gas with temperature of a million Kelvin which makes it transparent 32 00:02:27,782 --> 00:02:33,232 to a visible light because there are no electrons with that excitation energy but 33 00:02:33,232 --> 00:02:36,482 it has been absorbed through x-ray absorption. 34 00:02:36,482 --> 00:02:42,383 By oxygen highly ionized oxygen atoms in this gas and if the Chandler result 35 00:02:42,383 --> 00:02:48,284 holds, then this halo might extend as far as 100 kiloparsecs so that's more than 36 00:02:48,284 --> 00:02:52,688 300,000 light years. And that's past to give you a sense, the 37 00:02:52,688 --> 00:02:58,015 Large Magellanic Cloud, and they contain as much mass as the entire stellar 38 00:02:58,015 --> 00:03:04,626 population of the thin disk. So, 60 billion solar masses again, over again of 39 00:03:04,626 --> 00:03:10,532 the mass of the thin disk might be have been lost until recently in the form of 40 00:03:10,532 --> 00:03:15,083 this halo of hot gas. Note, that this is twice the radius of 41 00:03:15,083 --> 00:03:20,177 the globular cluster halo. So we're beginning to construct our 42 00:03:20,177 --> 00:03:22,796 model. I mentioned the large Magellanic Cloud. 43 00:03:22,796 --> 00:03:27,492 Indeed the Milky Way is not alone, there are satellite galaxies, a few dwarf 44 00:03:27,492 --> 00:03:31,194 satellites as well as the Magellanic Clouds orbit the Milky Way. 45 00:03:31,194 --> 00:03:35,903 I should point out that there are it's not quite clear whether the Magellanic 46 00:03:35,903 --> 00:03:40,587 Clouds have a positive, just positive or just negative Kinetic total energy 47 00:03:40,587 --> 00:03:44,960 relative to the Milky Way. In other words, are they inbound orbit or 48 00:03:44,960 --> 00:03:50,039 are they moving off and at some point will disappear, but not in my lifetime? 49 00:03:50,039 --> 00:03:54,374 in any event they are certainly, currently very near the Milky Way and we 50 00:03:54,374 --> 00:03:59,196 can see as we'll talk about the evidence of their tidal interactions with the 51 00:03:59,196 --> 00:04:02,491 Milky Way. Some other galaxies in the other some 52 00:04:02,491 --> 00:04:06,908 other of these dwarf galaxies are currently in the process of being ripped 53 00:04:06,908 --> 00:04:11,542 apart by the tidal forces of the Milky Way which are breaking them up into 54 00:04:11,542 --> 00:04:14,327 rings. The objects in the ring are being slowed 55 00:04:14,327 --> 00:04:18,927 down and eventually these dwarfs are merging to some extent with the Milky 56 00:04:18,927 --> 00:04:21,932 Way, the Milky Way gobbling up dwarfs and growing. 57 00:04:21,932 --> 00:04:27,152 this will see is part of the process of galactic dynamics so it pays to at least 58 00:04:27,152 --> 00:04:31,495 enjoy a look or two of our neighbors. To the left obviously the spectacularly 59 00:04:31,495 --> 00:04:35,416 beautiful Magellanic clouds. If you've never seen them and have a way 60 00:04:35,416 --> 00:04:38,233 to get to the southern hemisphere by all means do. 61 00:04:38,233 --> 00:04:42,378 It's a mind blowing experience, I've done it once and I can't forget it. 62 00:04:42,378 --> 00:04:46,573 to the right the Leo one dwarf. This is what a dwarf galaxy looks like. 63 00:04:46,573 --> 00:04:49,232 Of course, there's a long exposure picture, 64 00:04:49,232 --> 00:04:52,826 you probably would completely miss it and look right through it. 65 00:04:52,826 --> 00:04:57,449 The way that we identify these objects as being part of a separate galaxy is by 66 00:04:57,449 --> 00:05:02,266 tracking both their chemistry, their metallicity, and their proper motion, and 67 00:05:02,266 --> 00:05:05,812 radial motion we see a group of stars in a given direction. 68 00:05:05,812 --> 00:05:10,202 They all seem to be moving in concert, then we can study their random motion and 69 00:05:10,202 --> 00:05:13,674 see that they are in fact gravitationally bound to each other. 70 00:05:13,674 --> 00:05:18,225 And then this if the group is large enough dignifies them not as a star 71 00:05:18,225 --> 00:05:19,913 cluster, but as a small galaxy.