1 00:00:00,012 --> 00:00:06,540 So far, we looked at the observations in visible light, which also corresponds to 2 00:00:06,540 --> 00:00:11,808 the rest, red shifted rest frame ultraviolet light in galaxies. 3 00:00:11,808 --> 00:00:16,956 In other words, what you just see directly in stellar surfaces. 4 00:00:16,956 --> 00:00:20,931 However, we do know that galaxies contain dust. 5 00:00:20,931 --> 00:00:24,756 And, likely they did so from very early on. 6 00:00:24,756 --> 00:00:32,112 The dust absorbs ultraviolet and visible radiation and remits it in fiery infrared 7 00:00:32,112 --> 00:00:37,169 or some[UNKNOWN). Because there has to be a obscured star 8 00:00:37,169 --> 00:00:41,901 formation component involved in galaxy evolution. 9 00:00:41,901 --> 00:00:47,665 And indeed as we aquired ability to observe deep universe on these wave 10 00:00:47,665 --> 00:00:54,261 lengths it was discovered that there are new sources appearing in the sky that are 11 00:00:54,261 --> 00:00:58,091 simply not visible at all in, in visual light. 12 00:00:58,091 --> 00:01:04,163 Here we see a composite in the upper left. Visble light image from Hubble And the 13 00:01:04,163 --> 00:01:08,226 median infrared image from the Spitzer Space Telescope. 14 00:01:08,226 --> 00:01:12,885 As you can see in the upper right, there isn't a trace of the thread source. 15 00:01:12,885 --> 00:01:17,299 In the lower left, you can see it just beginning to show up, zero to near 16 00:01:17,299 --> 00:01:20,624 infrared. And in the lower right, it's pure mid 17 00:01:20,624 --> 00:01:26,722 infrared image and source is very obvious. So sky would look different on these 18 00:01:26,722 --> 00:01:32,572 wavelengths, and there could well be a previously unaccounted population of 19 00:01:32,572 --> 00:01:35,552 sources. And this is indeed the case. 20 00:01:35,552 --> 00:01:42,038 The first observations that uncovered such a population of sources were done at James 21 00:01:42,038 --> 00:01:47,388 Clerk Maxwell telescope in Hawaii. It is a submillimeter telescope. 22 00:01:47,388 --> 00:01:52,872 It is equipped with a barometer array measuring submillimeter radiation from 23 00:01:52,872 --> 00:01:57,177 sources in the sky. These are very difficult observations to 24 00:01:57,177 --> 00:01:59,564 make. This is why it took so long. 25 00:01:59,564 --> 00:02:05,265 And they looked at a couple of the deep fields, the Hubble Deep Field, and another 26 00:02:05,265 --> 00:02:10,286 one called Lockman Hole. And found that there are resources there. 27 00:02:10,286 --> 00:02:16,611 The resources whose nature at the time was unknown but were of surely obscured star 28 00:02:16,611 --> 00:02:21,650 forming galaxies far away. The reason why they look so bloppy is the 29 00:02:21,650 --> 00:02:27,801 poor resolution of these telescopes. You may know that the angular resolution 30 00:02:27,801 --> 00:02:33,341 of telescope is proportional to its diameter divided by the wave length. 31 00:02:33,341 --> 00:02:36,545 So for optical, this is a very high number. 32 00:02:36,545 --> 00:02:42,184 There are many wavelengths of photons stretched over, say, the mirror of the 33 00:02:42,184 --> 00:02:45,738 Keck telescope. Not so in radial or submillimeter. 34 00:02:45,738 --> 00:02:51,052 In there you have very low resolution. Nowadays, we have new inter therometers/g 35 00:02:51,052 --> 00:02:56,519 like alma/g and sheila that will actually produce optical light resolution in these 36 00:02:56,519 --> 00:03:00,026 wavelengths. But back then, this wasn't the case. 37 00:03:00,026 --> 00:03:05,155 Now here is a really nearby example of what we might expect. 38 00:03:05,155 --> 00:03:10,088 This is the galaxy M8, which is a nearby starburst galaxy. 39 00:03:10,088 --> 00:03:14,868 The picture shown here combines radio or visible light. 40 00:03:14,868 --> 00:03:18,601 In the purple is ionized hydrogen emission. 41 00:03:18,601 --> 00:03:25,107 What we see here is a intensely star forming this galaxy obscured in the middle 42 00:03:25,107 --> 00:03:29,391 but, the supernovae exploding pushed the gas out. 43 00:03:29,391 --> 00:03:35,448 Th, they drive a galactic wind. Expelling it into intergalactic space. 44 00:03:35,449 --> 00:03:41,548 Now if we take a broad-band spectrum of M82, we see that there are two bumps. 45 00:03:41,548 --> 00:03:47,665 There is one, the optical, which corresponds to a sort of quasi-black body, 46 00:03:47,665 --> 00:03:54,552 sum of all stellar, photospheres, and the bigger one in five red, which is thermal 47 00:03:54,552 --> 00:03:59,438 emission from dust that was heated up by these young stars. 48 00:03:59,439 --> 00:04:05,722 In this particular galaxy, there is actually more energy emerging in the form 49 00:04:05,722 --> 00:04:09,569 of[INAUDIBLE] radiation than visible light. 50 00:04:09,569 --> 00:04:15,719 But on average, in this part of the universe, there is roughly equal amount of 51 00:04:15,719 --> 00:04:21,959 obscured and unobscured star formation The same turns out to be true at higher 52 00:04:21,959 --> 00:04:25,247 redshifts. You may recall the concept of 53 00:04:25,247 --> 00:04:31,545 K-corrections which means that as you observe in some stationary instrument on 54 00:04:31,545 --> 00:04:36,793 planet Earth, you're looking at different parts of the spectrum. 55 00:04:36,793 --> 00:04:41,911 For sources of different redshifts. Now by and large for galaxies, this makes 56 00:04:41,911 --> 00:04:47,441 them dimmer because galaxy spectra tend to be redder, more energy in, in the red part 57 00:04:47,441 --> 00:04:50,928 of the spectra. It turns out that for sub-millimeter 58 00:04:50,928 --> 00:04:56,340 sources it's exactly the opposite. You are redshifting into the plank curve 59 00:04:56,340 --> 00:05:01,247 is climbing from the inside. And so even though sources get further 60 00:05:01,247 --> 00:05:06,989 away, and therefore should be dimmer, you're sampling a brighter part of their 61 00:05:06,989 --> 00:05:11,036 intrinsic spectrum. And so some of those k corrections 62 00:05:11,036 --> 00:05:15,870 actually become negative. The upshot is that for many of these 63 00:05:15,870 --> 00:05:20,948 sources The brightness almost does not change with red shift. 64 00:05:20,948 --> 00:05:25,819 And therefore if you can reach a flux level, you can see very far away. 65 00:05:25,819 --> 00:05:31,293 This is what SCUBA sources turn out to be. Which also means that we can do fairly 66 00:05:31,293 --> 00:05:36,321 deep source counts in these wave links. And here are some examples of it. 67 00:05:36,321 --> 00:05:41,647 They can be used then to constrain directly the contribution of these sources 68 00:05:41,647 --> 00:05:45,651 to the overall star formation history in the universe. 69 00:05:45,651 --> 00:05:50,875 And say something about their evolution. There is one difficulty, however. 70 00:05:50,875 --> 00:05:56,081 The poor angular resolution of these submillimeter observations means that 71 00:05:56,081 --> 00:06:01,441 optical counterparts, which we need in order to measure the redshifts, are hard 72 00:06:01,441 --> 00:06:04,516 to guess. In some cases there is an obvious 73 00:06:04,516 --> 00:06:10,381 counterpart, but in many cases, there are many faint galaxies inside the There was 74 00:06:10,381 --> 00:06:16,035 circle of a sub-millimetre source. It's not clear which one, if any of them, 75 00:06:16,035 --> 00:06:21,596 is the actual counterpart. So it took a while to get radius, measured 76 00:06:21,596 --> 00:06:25,935 for these objects. The trick that was used is that the radio 77 00:06:25,935 --> 00:06:31,670 measurements can affect some of them. And radio measurements have precision that 78 00:06:31,670 --> 00:06:35,466 is perfectly good to, to match to optical observations. 79 00:06:35,466 --> 00:06:40,432 Then the spectra we're taking of those, and in fact may turn out to be strong 80 00:06:40,432 --> 00:06:45,072 emission sources, where that blind emission could be powered by star 81 00:06:45,072 --> 00:06:49,756 formation or quite simply by an active nucleus in those galaxies. 82 00:06:49,756 --> 00:06:52,749 Possibly a little bit of both is happening. 83 00:06:52,749 --> 00:06:58,048 And the predictions from fitting the source counts, where many of these sources 84 00:06:58,048 --> 00:07:02,649 will be around redshifts 2 or 3 and that exactly turn out to be the case. 85 00:07:02,649 --> 00:07:07,959 So now that we've seen how we can detect both obscurred and unobscurred component 86 00:07:07,959 --> 00:07:14,057 of evolbing galaxies. We'll, we'll look into the overall star 87 00:07:14,057 --> 00:07:18,130 formation history of the universe.