WEBVTT 1 00:00:03.399 --> 00:00:07.719 Welcome to Bedtime Astronomy. Explore the wonders of the cosmos 2 00:00:07.759 --> 00:00:12.279 with our soothing Bedtime Astronomie podcast. Each episode offers a 3 00:00:12.359 --> 00:00:16.320 gentle journey through the stars, planets, and beyond, perfect for 4 00:00:16.399 --> 00:00:20.239 unwinding after a long day. Let's travel through the mysteries 5 00:00:20.239 --> 00:00:22.440 of the universe as you drift off into a peaceful 6 00:00:22.480 --> 00:00:23.839 slumber under the night sky. 7 00:00:26.920 --> 00:00:33.960 I want you to visualize a really sprawling modern architectural marvel, 8 00:00:34.399 --> 00:00:37.679 like a massive glass and steel house pushed right on 9 00:00:37.719 --> 00:00:38.280 a cliff side. 10 00:00:38.320 --> 00:00:39.320 Okay, I can picture that. 11 00:00:39.600 --> 00:00:43.000 Yeah, And it has this incredibly complex foundation, right vaulted ceilings, 12 00:00:43.039 --> 00:00:46.799 a heavy slate roof. But there is a structural reality 13 00:00:46.840 --> 00:00:51.799 to this house that totally defies everyday intuition. Eighty five 14 00:00:51.840 --> 00:00:55.359 percent of the load bearing columns, the rebar, the absolute 15 00:00:55.479 --> 00:01:00.320 core of its structural integrity, is completely invisible. Okay, touch 16 00:01:00.359 --> 00:01:03.399 those columns. You can't bounce light off them, but the 17 00:01:03.439 --> 00:01:06.840 mathematics of tension and compression absolutely dictate they have to. 18 00:01:06.760 --> 00:01:09.599 Be there, right, because otherwise the whole thing comes down exactly. 19 00:01:09.680 --> 00:01:13.120 Yeah. If you were to somehow cleanly extract that invisible scaffolding, 20 00:01:13.519 --> 00:01:17.719 the entire cantilevered structure would just instantaneously succumb to gravity 21 00:01:17.760 --> 00:01:18.959 and collapse into the review. 22 00:01:18.959 --> 00:01:19.799 It'd be a disaster. 23 00:01:19.920 --> 00:01:23.280 Yeah, And that structural dynamic that is the baseline, non 24 00:01:23.359 --> 00:01:27.879 negotiable reality of how we understand our universe. Specifically, it's 25 00:01:27.920 --> 00:01:30.040 the foundational rule of galactic. 26 00:01:29.640 --> 00:01:34.359 Dynamics, because galaxies are held together by this invisible, non 27 00:01:34.439 --> 00:01:37.400 buryonic scaffolding, what we call dark matter. 28 00:01:37.560 --> 00:01:42.480 Right, But then imagine you are mapping out a seemingly 29 00:01:42.599 --> 00:01:48.480 quiet cosmic neighborhood and you stumble across an absolute architectural impossibility. 30 00:01:48.760 --> 00:01:51.680 You find a house floating perfectly intact. 31 00:01:51.439 --> 00:01:55.640 Yes, stable, entirely, fully built, without a single one of 32 00:01:55.680 --> 00:01:57.959 those invisible load bearing columns. 33 00:01:57.719 --> 00:02:00.640 Which I mean that forces a total re valuation of 34 00:02:00.680 --> 00:02:03.760 the engineering principles you thought govern the universe, right totally. 35 00:02:04.239 --> 00:02:06.959 Finding a structure that shouldn't exist doesn't just mean the 36 00:02:07.000 --> 00:02:10.639 structure is weird. It means your foundational equations regarding cosmic 37 00:02:10.719 --> 00:02:14.439 architecture have these spectacular, violent exceptions that we're well, we're 38 00:02:14.479 --> 00:02:15.960 only just beginning to comprehend. 39 00:02:16.080 --> 00:02:18.520 And that brings us to the monumental data published in 40 00:02:18.560 --> 00:02:21.639 April of twenty twenty six. Okay, let's unpack this because 41 00:02:21.719 --> 00:02:24.360 astronomers Mike l. A. Khin, Peter van Dockaman their collaborative 42 00:02:24.360 --> 00:02:28.080 team at Yale University just dropped an absolute bombshell. 43 00:02:28.159 --> 00:02:28.840 They really did. 44 00:02:29.000 --> 00:02:32.919 They confirmed the discovery of a third galaxy designated NNGC 45 00:02:33.039 --> 00:02:35.599 one oh five to two DF nine or just DF 46 00:02:35.719 --> 00:02:38.479 nine that appears to be entirely devoid of dark matter. 47 00:02:38.680 --> 00:02:39.719 Is just incredible. 48 00:02:39.879 --> 00:02:44.080 It is that cliff side House completely missing its invisible columns, 49 00:02:44.560 --> 00:02:46.439 just sitting out there, stable in the void. 50 00:02:46.639 --> 00:02:49.080 And you know, the scientific weight of this really cannot 51 00:02:49.120 --> 00:02:52.840 be overstated. I mean, finding a single galaxy missing its 52 00:02:52.919 --> 00:02:56.080 dark matter halo. Back in twenty eighteen, that was DF two, 53 00:02:56.240 --> 00:03:00.439 right DF two, that was treated by many as a 54 00:03:00.439 --> 00:03:03.599 statistical anomaly or perhaps even an observational error. 55 00:03:03.639 --> 00:03:05.280 People were definitely skeptical. 56 00:03:04.960 --> 00:03:07.919 Highly skeptical, and then finding a second one shortly after 57 00:03:08.000 --> 00:03:12.000 that elevated it to a real crisis for our theoretical models. 58 00:03:12.039 --> 00:03:13.400 Yeah, crisis is the right word. 59 00:03:13.520 --> 00:03:17.439 But confirming a third and critically finding that it perfectly 60 00:03:17.439 --> 00:03:20.199 aligns with the other two in a highly specific spatial 61 00:03:20.199 --> 00:03:23.599 and kinematic geometry, that isn't a glitch in the data anymore. 62 00:03:23.680 --> 00:03:24.400 No, definitely not. 63 00:03:24.560 --> 00:03:27.360 That is an entirely new class of object. It forces 64 00:03:27.439 --> 00:03:31.199 us to confront theories of galaxy formation that previously seemed 65 00:03:31.439 --> 00:03:35.280 well far too chaotic and hydrodynamically violent to be sustainable. 66 00:03:35.400 --> 00:03:38.840 So we have an incredibly complex physical puzzle to unpack. Today, 67 00:03:39.000 --> 00:03:41.560 we are going to look closely at the exact mechanics 68 00:03:42.000 --> 00:03:44.840 of how a galaxy manages to actually decouple from its 69 00:03:44.919 --> 00:03:45.800 dark matter halo. 70 00:03:46.120 --> 00:03:48.879 The uncoupling process is fascinating, it really is. 71 00:03:49.039 --> 00:03:51.520 And we're going to examine the extreme physics of a 72 00:03:51.599 --> 00:03:56.639 high speed supersonic cosmic collision that triggered this separation billions 73 00:03:56.639 --> 00:03:57.680 of years ago, right. 74 00:03:57.680 --> 00:03:59.800 The bullet dwarf scenario exactly. 75 00:04:00.240 --> 00:04:03.520 And perhaps the most beautiful paradox of this entire discovery, 76 00:04:03.879 --> 00:04:07.840 we're going to explore how this highly anomaloust galaxy, which 77 00:04:07.879 --> 00:04:11.120 is defined entirely by its missing dark matter, might actually 78 00:04:11.199 --> 00:04:15.439 serve as the final crushing mathematical proof that dark matter 79 00:04:15.719 --> 00:04:17.519 is a distinct physical. 80 00:04:17.199 --> 00:04:19.720 Entity because we found a place where it was left. 81 00:04:19.600 --> 00:04:22.120 Behind, precisely because we found the empty spot. 82 00:04:22.319 --> 00:04:25.240 It's an elegant paradox, isn't it to prove a substance 83 00:04:25.319 --> 00:04:29.040 is a physical reality rather than just a mathematical tweak 84 00:04:29.079 --> 00:04:31.759 to our understanding of gravity. You have to find a 85 00:04:31.800 --> 00:04:35.879 scenario where that substance has been dynamically separated from ordinary matter. 86 00:04:35.959 --> 00:04:37.319 You have to catch it in the act of not 87 00:04:37.439 --> 00:04:37.879 being there. 88 00:04:38.000 --> 00:04:42.319 Yeah, exactly. But to fully appreciate the mechanics of DF nine, 89 00:04:42.360 --> 00:04:44.680 we really need to look at the kinematic tension that 90 00:04:44.720 --> 00:04:49.519 defined the Lambda CDM model in the first place. Baseline expectation, right, 91 00:04:49.600 --> 00:04:52.000 we need to look at the baseline expectation for a 92 00:04:52.040 --> 00:04:53.680 galaxy's velocity dispersion. 93 00:04:53.959 --> 00:04:57.439 Let's focus on that kinematic tension then, because within the 94 00:04:57.439 --> 00:05:02.480 standard model of cosmology, lambda cold dark matter. Yeah, dark 95 00:05:02.519 --> 00:05:05.040 matter isn't just some fringe component, right. 96 00:05:04.959 --> 00:05:07.519 Not at all. It represents roughly eighty five percent of 97 00:05:07.560 --> 00:05:09.279 the universe's total matter content. 98 00:05:09.480 --> 00:05:13.879 So the baryonic matter, the gas, dust stars, that's just 99 00:05:13.920 --> 00:05:14.439 the frosting. 100 00:05:14.519 --> 00:05:16.079 Yeah, a very thin layer of frosting. 101 00:05:16.120 --> 00:05:17.600 And now the reason we know the cake is there 102 00:05:17.680 --> 00:05:20.240 even if we can't see it comes down to how 103 00:05:20.279 --> 00:05:21.600 fast that frosting is spinning. 104 00:05:21.680 --> 00:05:22.720 That's a great way to put it. 105 00:05:22.839 --> 00:05:25.680 Like if we look at the rotation curves of typical 106 00:05:25.720 --> 00:05:30.079 galaxies or the velocity dispersion of stars. Within dwarf galaxies, 107 00:05:31.240 --> 00:05:34.720 the math just fundamentally doesn't work if you only count 108 00:05:34.720 --> 00:05:35.600 the luminous matter. 109 00:05:35.720 --> 00:05:39.040 It fails catastrophically. The foundational tool we use here is 110 00:05:39.079 --> 00:05:40.000 the virial theorem. 111 00:05:40.120 --> 00:05:41.199 Okay, break that down for us. 112 00:05:41.399 --> 00:05:45.160 So, in a stable gravitationally bound system, the kinetic energy 113 00:05:45.199 --> 00:05:46.839 of the objects moving around inside it. 114 00:05:46.920 --> 00:05:47.920 The stars, right, the. 115 00:05:47.879 --> 00:05:50.759 Stars, their kinetic energy has to be balanced by the 116 00:05:50.759 --> 00:05:53.519 gravitational potential energy holding the whole system. 117 00:05:53.199 --> 00:05:54.920 Together, otherwise they just fly apart. 118 00:05:55.040 --> 00:05:57.879 Exactly, if we measure the kinetic energy, which we do 119 00:05:57.920 --> 00:06:02.079 by observing the velocity dispersion, or you know how fast 120 00:06:02.079 --> 00:06:04.600 the stars are buzzing around relative to each other, we 121 00:06:04.639 --> 00:06:09.360 can calculate exactly how much gravitational potential energy is required 122 00:06:09.639 --> 00:06:12.680 to keep those stars from exceeding escape velocity and just 123 00:06:12.839 --> 00:06:15.199 boiling off into intergalactic space. 124 00:06:15.360 --> 00:06:19.600 So the velocity dispersion is really the key metric here, absolutely, 125 00:06:19.680 --> 00:06:23.000 because if I'm looking at a typical dwarf galaxy, the 126 00:06:23.040 --> 00:06:26.160 stars are moving incredibly fast. It's like watching a merry 127 00:06:26.160 --> 00:06:27.720 go round spinning way too fast. 128 00:06:27.839 --> 00:06:30.279 We are violently fast married around the. 129 00:06:30.319 --> 00:06:34.160 Kids, or the stars should be flying off. This centripetal 130 00:06:34.199 --> 00:06:38.040 acceleration required to keep those stars in their orbits is massive. 131 00:06:38.560 --> 00:06:40.800 It is. And if we calculate the mass of the 132 00:06:40.839 --> 00:06:44.000 galaxy strictly by counting the starlight, you know, adding up 133 00:06:44.040 --> 00:06:45.800 the mass of the gas clouds. 134 00:06:45.600 --> 00:06:47.519 Luminous mass, if we can actually see. 135 00:06:47.519 --> 00:06:51.120 Right, the resulting gravitational binding energy from just that visible 136 00:06:51.160 --> 00:06:54.439 stuff is nowhere near enough to counter that kinetic energy. 137 00:06:54.600 --> 00:06:56.759 The kids should be flying off the merry go round. 138 00:06:56.639 --> 00:06:59.480 But they aren't, and the disparity is often an order 139 00:06:59.480 --> 00:07:03.720 of magnet or more. I mean, in typical dwarf spheroidals, 140 00:07:03.759 --> 00:07:07.279 we see mass to light ratios that are astronomically high. 141 00:07:07.439 --> 00:07:11.079 So you need invisible seat belts, exactly invisible seat belts. 142 00:07:11.399 --> 00:07:14.519 The visible matter might only account for ten percent or 143 00:07:14.560 --> 00:07:17.279 even one percent of the gravity required to keep the 144 00:07:17.319 --> 00:07:18.399 system virialized. 145 00:07:19.199 --> 00:07:20.879 That's wild one percent. 146 00:07:21.120 --> 00:07:24.600 Yeah, the stars are moving with such high velocity dispersions 147 00:07:24.639 --> 00:07:28.759 that absence some massive, unseen gravitational anchor, the galaxy should 148 00:07:28.800 --> 00:07:30.519 have dissolved billions of years ago. 149 00:07:30.759 --> 00:07:34.800 So the dark matter halo provides that deep gravitational potential. Well, 150 00:07:34.959 --> 00:07:37.040 it is the anchor, Okay, So the baseline rule is 151 00:07:37.079 --> 00:07:40.079 firmly established. If you have a galaxy of a certain 152 00:07:40.120 --> 00:07:43.480 stellar mass, you absolutely expect its stars to be moving 153 00:07:43.519 --> 00:07:47.040 at a relatively high velocity because they are surfing inside 154 00:07:47.079 --> 00:07:49.399 this massive unseen dark matter gravity. 155 00:07:49.399 --> 00:07:52.759 Well, that is the universal expectation, yes, which sets the 156 00:07:52.800 --> 00:07:56.560 state perfectly for the absolute kinematic shockwave the Yale team 157 00:07:56.600 --> 00:08:00.439 triggered when they aim their instruments at the constellation Seatus Oh. 158 00:08:00.519 --> 00:08:03.240 It was a massive disruption to the field. So we 159 00:08:03.399 --> 00:08:07.199 are looking at a field roughly sixty to seventy million 160 00:08:07.279 --> 00:08:10.879 light years away dominated by a massive elliptical galaxy called 161 00:08:11.040 --> 00:08:12.800 NNGC one oh fifty two. 162 00:08:13.160 --> 00:08:16.160 But the focus isn't that central elliptical galaxy. 163 00:08:15.759 --> 00:08:19.079 Right now, it's the highly unusual population of satellite galaxies 164 00:08:19.079 --> 00:08:22.040 surrounding it. The initial crack in the dark matter paradigm 165 00:08:22.120 --> 00:08:24.800 actually occurred a few years earlier, in twenty eighteen. 166 00:08:24.639 --> 00:08:28.000 Right when Van Dokam's team first identified NNGC one fifty 167 00:08:28.000 --> 00:08:28.959 two DF. 168 00:08:28.680 --> 00:08:31.079 Two exactly DF two and DF. 169 00:08:30.879 --> 00:08:33.799 Two is categorized as an alternate diffuse galaxy. And what's 170 00:08:33.799 --> 00:08:36.360 wild is they found it using an instrument that frankly 171 00:08:36.399 --> 00:08:39.039 sounds like it belongs in a commercial photography studio rather 172 00:08:39.120 --> 00:08:40.519 than a world class observatory. 173 00:08:40.559 --> 00:08:43.159 You're talking about the Dragonfly telephoto array. 174 00:08:43.279 --> 00:08:47.159 Yes, Dragonfly. It is a phenomenal piece of engineering, truly 175 00:08:47.399 --> 00:08:50.080 born out of a very specific observational necessity. 176 00:08:50.080 --> 00:08:51.840 Why did they need it? Why not just use Hubble 177 00:08:51.960 --> 00:08:55.559 right away? Well, traditional massive reflectors like the Hubble Space 178 00:08:55.600 --> 00:09:00.679 Telescope or the Keck observatory, they are unparallel at achieving 179 00:09:00.720 --> 00:09:04.120 high resolution, but they actually struggle when it comes to 180 00:09:04.320 --> 00:09:09.279 mapping incredibly faint, low surface brightness objects spread over a 181 00:09:09.320 --> 00:09:10.440 really wide field of. 182 00:09:10.480 --> 00:09:13.159 View because of the optics design, right, Yeah, largely, Like 183 00:09:13.200 --> 00:09:16.399 if you have a massive primary mirror and a secondary 184 00:09:16.399 --> 00:09:20.039 mirror suspended in front of it, you introduce diffraction exactly. 185 00:09:20.159 --> 00:09:23.080 You get light scattering off the microscopic imperfections in the 186 00:09:23.080 --> 00:09:26.159 mirror coatings or the struts holding the secondary mirror, which 187 00:09:26.279 --> 00:09:28.399 creates a kind of optical noise floor. 188 00:09:28.759 --> 00:09:30.440 So it's like trying to see a whisper in a 189 00:09:30.440 --> 00:09:31.120 noisy room. 190 00:09:31.279 --> 00:09:34.879 That's a perfect analogy. The internal scattering within a traditional 191 00:09:34.960 --> 00:09:40.559 reflecting telescope creates these broad wings on the point spread 192 00:09:40.559 --> 00:09:43.480 function of bright stars or central galaxies. 193 00:09:43.120 --> 00:09:46.840 And that scattered light effectively washes out the exceptionally faint 194 00:09:47.039 --> 00:09:48.679 diffuse glow of objects like. 195 00:09:48.679 --> 00:09:51.240 DF two, Right, it just drowns it out. But the 196 00:09:51.360 --> 00:09:55.879 Dragonfly telephoto array bypassed this entirely by using refractive. 197 00:09:55.320 --> 00:09:57.759 Optic refractive so lenses instead of mirrors. 198 00:09:58.200 --> 00:10:02.440 Right. They took commercially ava bill high end cannon telephoto 199 00:10:02.480 --> 00:10:05.679 camera lenses and literally arrayed them together. 200 00:10:06.159 --> 00:10:08.960 It literally looks like the compound eye of an insect 201 00:10:09.000 --> 00:10:09.799 pointing at the sky. 202 00:10:10.080 --> 00:10:12.200 It really does. And the reason it works so brilliantly 203 00:10:12.279 --> 00:10:16.919 is because those commercial lenses use proprietary nanocoatings. 204 00:10:16.320 --> 00:10:20.480 Coatings that were originally designed to reduce glare and ghosting 205 00:10:20.600 --> 00:10:22.960 for like sports and wildlife photographers. 206 00:10:23.080 --> 00:10:27.440 Yes, when you stack dozens of these lenses together, they 207 00:10:27.440 --> 00:10:31.159 mimic the light gathering power of a massive telescope. But 208 00:10:31.279 --> 00:10:35.559 because there are no secondary mirrors or central obstructions. 209 00:10:34.960 --> 00:10:36.919 Those struts to bounce light around. 210 00:10:36.759 --> 00:10:41.200 Exactly, the internal scattering is suppressed by orders of magnitude. 211 00:10:41.519 --> 00:10:44.840 The noise floor just drops out completely, revealing structures that 212 00:10:44.879 --> 00:10:47.399 are practically translucent, which is incredible. 213 00:10:47.440 --> 00:10:50.320 And DF two was one of those translucent structures. It was. 214 00:10:50.480 --> 00:10:53.840 It has a physical diameter roughly comparable to the Milky Way. Right, Yeah, 215 00:10:53.879 --> 00:10:56.639 it's huge, but it possesses only about one five hundredth 216 00:10:56.639 --> 00:10:57.720 of our stellar mass. 217 00:10:57.879 --> 00:10:59.120 It's basically a ghost. 218 00:10:59.399 --> 00:11:02.720 The stars are so sparse you can actually see background 219 00:11:02.759 --> 00:11:06.639 galaxy shining cleanly through the gaps in the galaxy itself. 220 00:11:06.279 --> 00:11:08.039 Which is so visually striking. 221 00:11:08.159 --> 00:11:11.279 But the diffuse nature of the galaxy wasn't the headline here. 222 00:11:11.600 --> 00:11:14.840 The headline was what happened when they pointed the massive 223 00:11:14.960 --> 00:11:18.879 KEC telescopes at DF two's globular clusters. 224 00:11:18.519 --> 00:11:21.440 To measure that velocy dispersion we were just talking about, right. 225 00:11:21.320 --> 00:11:26.320 Because globular clusters are incredibly dense, luminous spirical collections of 226 00:11:26.440 --> 00:11:29.519 older stars that orbit the galactic center. 227 00:11:29.440 --> 00:11:32.200 And because they are so bright and compact, they serve 228 00:11:32.240 --> 00:11:34.600 as excellent kinematic tracers. 229 00:11:34.320 --> 00:11:36.039 Like tracking lights on a dark highway. 230 00:11:36.159 --> 00:11:40.759 Exactly by taking specter of these clusters, astronomers can measure 231 00:11:40.840 --> 00:11:42.240 the Dopplay shift of their light. 232 00:11:42.480 --> 00:11:43.840 Okay, the Doppler shift. 233 00:11:43.759 --> 00:11:46.600 Yeah, So they can determine exactly how fast those clusters 234 00:11:46.639 --> 00:11:49.960 are moving within the gravitational potential of the galaxy. 235 00:11:49.639 --> 00:11:52.320 And the virial theorem dictates that for a galaxy of 236 00:11:52.399 --> 00:11:56.600 DF two's size, those globular clusters should be zipping around 237 00:11:57.159 --> 00:12:00.519 governed by the massive dark matter halo. We just assume there. 238 00:12:00.360 --> 00:12:03.279 They should be moving fast, but the spectral data told 239 00:12:03.320 --> 00:12:04.519 a completely different story. 240 00:12:04.759 --> 00:12:06.480 They weren't moving fast, not at all. 241 00:12:06.679 --> 00:12:10.000 The velocity dispersion of those clusters was staggeringly low. 242 00:12:10.240 --> 00:12:10.679 Wow. 243 00:12:10.720 --> 00:12:13.639 It was so low, in fact, that it perfectly matched 244 00:12:13.639 --> 00:12:16.840 the gravitational potential of the luminous matter alone. 245 00:12:16.679 --> 00:12:18.600 Just the visible stars, just the stars. 246 00:12:19.240 --> 00:12:22.279 The stars and clusters were moving slubbishly, exactly as you 247 00:12:22.279 --> 00:12:25.440 would expect if there was absolutely no dark matter halo present. 248 00:12:25.639 --> 00:12:28.279 And the reaction from the astrophysics community to that twenty 249 00:12:28.320 --> 00:12:33.240 eighteen paper was well, it was not exactly a quiet 250 00:12:33.320 --> 00:12:33.960 round of applause. 251 00:12:34.039 --> 00:12:36.559 Oh no, it was intense scrutiny. 252 00:12:36.279 --> 00:12:40.440 Bordering on outright hostility from certain corners of the theoretical community, 253 00:12:40.480 --> 00:12:41.559 I imagine, definitely. 254 00:12:41.679 --> 00:12:44.519 And the primary vector of attack was the distance measurement, 255 00:12:45.000 --> 00:12:45.759 Which makes sense. 256 00:12:46.039 --> 00:12:49.279 That is the most rigorous and appropriate scientific response. 257 00:12:48.919 --> 00:12:53.120 Right it is. In astronomy, distance is the foundational variable 258 00:12:53.159 --> 00:12:56.799 for almost all derived properties. If you get the distance wrong, 259 00:12:56.960 --> 00:12:59.000 your entire physical model collapses. 260 00:12:59.159 --> 00:13:02.559 Right, because OFF two isn't sixty five million light years away, 261 00:13:02.639 --> 00:13:05.799 but say only thirty million light years away. 262 00:13:06.000 --> 00:13:08.480 Then it fundamentally changes what the object is. 263 00:13:08.799 --> 00:13:13.279 It would be intrinsically much smallertrinsically much less luminous, and 264 00:13:13.320 --> 00:13:16.399 therefore its stellar mass would be drastically lower than they 265 00:13:16.480 --> 00:13:17.480 originally calculated. 266 00:13:17.600 --> 00:13:20.639 Exactly. And if the stellar mass is lower, then the 267 00:13:20.720 --> 00:13:24.720 sluggish velocity dispersion might actually perfectly align with a normal 268 00:13:24.799 --> 00:13:28.080 standard dark matter halo for a dwarf galaxy of that 269 00:13:28.240 --> 00:13:29.159 much smaller size. 270 00:13:29.600 --> 00:13:32.399 I see, So the missing dark matter would just be 271 00:13:32.440 --> 00:13:34.679 an artifact of putting the decimal point in the wrong 272 00:13:34.720 --> 00:13:37.639 place on the distance calculation precisely. 273 00:13:37.799 --> 00:13:41.799 The distance debate absolutely dominated the literature for over a year. 274 00:13:42.000 --> 00:13:47.200 I remember that teams ran independent analyzes utilizing different calibration methods, 275 00:13:47.440 --> 00:13:50.480 specifically looking at surface prightness fluctuations. 276 00:13:50.519 --> 00:13:51.080 Well, what's that. 277 00:13:51.320 --> 00:13:54.320 It's a statistical method for estimating distance based on the 278 00:13:54.360 --> 00:13:57.440 pixel to pixel variance in the galaxy's light. And some 279 00:13:57.519 --> 00:14:01.600 of those initial independent studies suggested DFI was indeed much closer. 280 00:14:01.759 --> 00:14:04.519 But the Yale team didn't just back down or stand 281 00:14:04.519 --> 00:14:06.960 by their initial data blindly, did they. 282 00:14:07.240 --> 00:14:11.399 No, they didn't. They secured highly contested observation time on 283 00:14:11.600 --> 00:14:14.720 the Hubble space telescope to execute what is really the 284 00:14:14.720 --> 00:14:17.320 gold standard of cosmic distance measurement. 285 00:14:17.039 --> 00:14:19.679 The tip of the red giant branch TRGB. 286 00:14:19.960 --> 00:14:24.000 Yes, the TRGB method is incredibly elegant. 287 00:14:24.759 --> 00:14:26.039 Walk us through how that works. 288 00:14:26.200 --> 00:14:29.279 So, when a medium mass star exhausts the hydrogen in 289 00:14:29.320 --> 00:14:32.639 its core, it expands into a red giant. Right, it 290 00:14:32.679 --> 00:14:36.879 starts burning hydrogen in a shell around an inert helium core. Okay, 291 00:14:37.000 --> 00:14:39.720 As the core contracts and heats up, it eventually hits 292 00:14:39.759 --> 00:14:42.879 a critical temperature and pressure where the helium rapidly ignites. 293 00:14:43.039 --> 00:14:44.080 That's a helium flash. 294 00:14:44.159 --> 00:14:47.879 Yes, the helium flash, and that flash, that specific evolutionary 295 00:14:47.919 --> 00:14:53.440 phase happens at a highly predictable, standardized intrinsic luminosity. 296 00:14:52.879 --> 00:14:54.720 So it functions as a standard candle. 297 00:14:54.559 --> 00:14:58.840 Precisely because we know the exact intrinsic absolute magnitude of 298 00:14:58.840 --> 00:15:00.840 a star at the tip of the red giant branch, 299 00:15:01.080 --> 00:15:04.159 if we can individually resolve those specific stars in a 300 00:15:04.200 --> 00:15:05.600 distant galaxy. 301 00:15:05.240 --> 00:15:07.120 And measure their apparent brightness from. 302 00:15:07.039 --> 00:15:10.240 Earth, then the inverse square law gives us a highly precise, 303 00:15:10.480 --> 00:15:12.399 unambiguous geometric distance. 304 00:15:12.759 --> 00:15:15.960 But it takes an incredible amount of resolving power to 305 00:15:16.200 --> 00:15:20.120 isolate individual red giant stars in a galaxy millions of 306 00:15:20.200 --> 00:15:20.879 light years away. 307 00:15:20.960 --> 00:15:23.320 It's incredibly difficult, but Hubble did it. 308 00:15:23.480 --> 00:15:24.200 Of course it did. 309 00:15:24.279 --> 00:15:27.799 They isolated the TRGB for DF two and the results 310 00:15:27.799 --> 00:15:29.080 were absolutely definitive. 311 00:15:29.120 --> 00:15:30.919 The distance was correct, It was correct. 312 00:15:31.000 --> 00:15:35.000 It was roughly twenty two megaparsex away. It was exactly 313 00:15:35.039 --> 00:15:38.399 as large and as massive as the Yale team originally claimed, 314 00:15:38.639 --> 00:15:42.120 which meant the velocity dispersion was genuinely anomalous. 315 00:15:42.279 --> 00:15:44.000 The dark matter was truly missing. 316 00:15:44.120 --> 00:15:47.399 It was really missing, and that confirmation was immediately followed 317 00:15:47.399 --> 00:15:49.480 in twenty nineteen by the discovery of. 318 00:15:49.480 --> 00:15:51.960 DF four, a virtual twin to DF two. 319 00:15:51.879 --> 00:15:55.000 Exactly located in the same NGC one off fifty two group, 320 00:15:55.240 --> 00:15:58.720 exhibiting the exact same missing dark matter profile. 321 00:15:58.360 --> 00:16:01.919 Two impossible structures, which brings us to the April twenty 322 00:16:01.960 --> 00:16:05.559 twenty six breakthrough. DF nine, the newcomer right the third 323 00:16:05.759 --> 00:16:09.120 ultraed diffuse galaxy in this specific system devoid of dark matter. 324 00:16:09.639 --> 00:16:12.399 But what elevates DF nine is the sheer precision of 325 00:16:12.399 --> 00:16:15.480 the kinematic data they acquired and the spatial relationship it 326 00:16:15.519 --> 00:16:16.320 shares with the other two. 327 00:16:16.440 --> 00:16:19.320 The data on DF nine is just beautiful Let's look 328 00:16:19.320 --> 00:16:20.639 at the instrumentation first. 329 00:16:20.679 --> 00:16:25.080 Because they utilize the Keck Cosmic Web Imager or CASEWI 330 00:16:25.480 --> 00:16:29.120 on the Kekit telescope. How did this instrument pull the 331 00:16:29.200 --> 00:16:32.159 velocity dispersion out of DF nine so precisely? 332 00:16:32.440 --> 00:16:36.279 KCWI is a spectacular piece of technology. It is an 333 00:16:36.320 --> 00:16:39.679 integral field spectrograph. Or ifs, how. 334 00:16:39.600 --> 00:16:41.600 Is that different from a normal spectrograph. 335 00:16:41.639 --> 00:16:45.480 Well, traditional spectrographs require you to place a narrow slit 336 00:16:45.600 --> 00:16:48.360 over a specific part of a galaxy, so you only 337 00:16:48.399 --> 00:16:51.840 get kinematic data for that tiny sliver. Oh I see, Yeah, 338 00:16:52.000 --> 00:16:53.720 if you want to map the whole galaxy, you have 339 00:16:53.759 --> 00:16:55.919 to move the slit, take another long exposure, move the 340 00:16:55.919 --> 00:16:58.799 slit again, and basically piece it together over countless nights of. 341 00:16:58.720 --> 00:17:02.519 Observation, which is incredile inefficient for a massive diffuse object 342 00:17:02.559 --> 00:17:04.119 where the light is already so spread. 343 00:17:03.919 --> 00:17:06.519 Out exactly it would say forever. But an integral field 344 00:17:06.519 --> 00:17:09.200 spectrograph like KCWI uses an image. 345 00:17:08.880 --> 00:17:10.200 Slicer and image slicer. 346 00:17:10.440 --> 00:17:13.000 Yeah, it takes the two dimensional image of the entire 347 00:17:13.039 --> 00:17:17.640 galaxy optically slices it into dozens of thin strips aligns. 348 00:17:17.640 --> 00:17:20.359 Those strips end to end into one giant pseudo slit 349 00:17:20.839 --> 00:17:24.359 feeds that light through the spectrograph grading, and then computer 350 00:17:24.440 --> 00:17:27.240 software reconstructs it into a three D data cube. 351 00:17:27.359 --> 00:17:28.720 The data cube right. 352 00:17:28.759 --> 00:17:31.680 Two axes represent the spatial X and Y coordinates of 353 00:17:31.680 --> 00:17:34.640 the galaxy, and the third axis represents wavelength. 354 00:17:34.839 --> 00:17:37.799 So for every single pixel in the image of DF nine, 355 00:17:38.119 --> 00:17:39.319 you get a full spectrum of light. 356 00:17:39.440 --> 00:17:41.799 Every single pixel you can see exactly what elements are 357 00:17:41.839 --> 00:17:44.920 absorbing light and how fast that specific patch of stars 358 00:17:44.960 --> 00:17:45.319 is moving. 359 00:17:45.440 --> 00:17:46.519 That's amazing it is. 360 00:17:46.799 --> 00:17:50.160 And they are looking very closely at absorption lines, specifically 361 00:17:50.200 --> 00:17:52.920 things like the Calciumer two H and K lines or 362 00:17:53.160 --> 00:17:54.759 the Bomber series of hydrogen. 363 00:17:55.240 --> 00:17:58.720 Okay, so when we talk about measuring velocity dispersion, here 364 00:17:59.039 --> 00:18:01.039 we are just looking at the overall red shift of 365 00:18:01.079 --> 00:18:03.799 the galaxy moving away from us due to cosmic expansion. 366 00:18:03.920 --> 00:18:04.119 No. 367 00:18:04.119 --> 00:18:06.720 No, we are looking at the Doppler broadening of those 368 00:18:06.759 --> 00:18:08.160 specific absorption. 369 00:18:07.839 --> 00:18:10.240 Lines, the smearing of the fingerprint, basically. 370 00:18:10.119 --> 00:18:15.000 Exactly the smearing in a galaxy with a high velocity dispersion. 371 00:18:15.480 --> 00:18:18.759 Some stars are orbiting rapidly toward our line of sight 372 00:18:18.799 --> 00:18:21.440 and their light is blue shifted. Other stars are orbiting 373 00:18:21.519 --> 00:18:23.960 rapidly away from us, and their light is red shifted. 374 00:18:24.400 --> 00:18:28.240 Because we can't resolve every single star individually, all their 375 00:18:28.359 --> 00:18:29.519 light blends together. 376 00:18:29.720 --> 00:18:33.759 So the sharp, narrow absorption line of calcium becomes mathematically broadened. 377 00:18:33.759 --> 00:18:35.000 It gets fat and smeared out. 378 00:18:35.160 --> 00:18:37.680 Yes, so if you measure the width of that absorption line, 379 00:18:37.680 --> 00:18:40.440 it tells you the spread of velocities within that population 380 00:18:40.519 --> 00:18:40.960 of stars. 381 00:18:41.160 --> 00:18:44.599 The fatter the line, the higher the velocity dispersion, the 382 00:18:44.599 --> 00:18:46.720 more mass of the dark matter halo must be to 383 00:18:46.759 --> 00:18:47.279 contain them. 384 00:18:47.319 --> 00:18:50.039 You've got it perfectly. And the data from KCWI on 385 00:18:50.119 --> 00:18:53.920 DF nine was stunning. They calculated the total stellar mass 386 00:18:53.960 --> 00:18:56.880 of DF nine to be approximately one point four times 387 00:18:56.960 --> 00:18:59.160 ten to the eighth solar masses. 388 00:18:58.880 --> 00:19:00.720 Around one hundred and forty million. 389 00:19:00.440 --> 00:19:05.079 Suns right, and based on standard LAMBDAICDM scaling relations, a 390 00:19:05.160 --> 00:19:08.160 dwarf galaxy of that stellar mass residing in a typical 391 00:19:08.240 --> 00:19:12.519 dark matter halo should exhibit a velocity dispersion approximately twenty 392 00:19:12.559 --> 00:19:14.079 seven kilometers per second. 393 00:19:14.240 --> 00:19:17.839 So the absorption line should have a very specific calculated 394 00:19:17.880 --> 00:19:20.680 with corresponding to twenty seven kilometers per second. 395 00:19:20.839 --> 00:19:24.519 Yes, But when they process the data cube from KEK, 396 00:19:24.720 --> 00:19:28.079 what do they see the absorption lines were incredibly narrow. 397 00:19:28.480 --> 00:19:31.920 The measured velocity dispersion was approximately six point four kilometers 398 00:19:31.960 --> 00:19:32.400 per second. 399 00:19:32.440 --> 00:19:35.279 Six point four compared to the expected twenty seven kinematic 400 00:19:35.319 --> 00:19:36.880 energy is just gone. 401 00:19:36.920 --> 00:19:40.000