University Place  Aleks Diamond Stanic Problems with the Cosmic Fuel Supply  March 11, 2014 UP994   cc >> Good evening. Welcome to UW Space Place. This is our monthly guest speaker night and we have tonight Aleks Diamond-Stanic from the UW Astronomy Department who is going to talk to us about why the universe needs feedback. Aleks?  >> Thank you, Jim. It's a pleasure to be here. Thanks to all of you for coming out on this Tuesday evening. Thanks again to Jim for organizing this guest presentations and for inviting me to give this particular presentation. Thanks to Wisconsin Public Television for being here and broadcasting this presentation beyond the confines of this room here at UW Space Place. My name is Aleks Diamond-Stanic. I'm a Grainger Fellow in the Department of Astronomy at UW-Madison. 

The main goal of this talk is to convince you that you should care about galaxies and in particular, what galaxies do with their supply of gas which is the cosmic fuel for star formation. After I do that, if I'm successful, my next goal will be to frame some of the questions that I think about on a regular basis and why I think they're interesting and why I think you should think that they're interesting. Then I'll give you a little bit of a flavor for how we can use data from telescopes both on Earth and in space to start answering some of these questions. To motivate why you should care about the galaxy, I'm starting with this image taken by an amateur astronomer from Romania with a relatively modest sized eight-inch optical telescope and he took exposures, added them together. The integrated exposure time was I think about several dozen hours. This is what nearby galaxies look like at optical wavelengths that our eyes are sensitive to. This here is what often a spiral galaxy will look like. This is more of an edge-on galaxy that actually is going through what's called the starburst phase. It's forming lots of stars but this is what galaxies look like on optical wavelengths. An interesting to know is that these galaxies are often named after Charles Messier who's someone who didn't care about galaxies. 

This is the galaxy Messier 81. This is the galaxy Messier 82. It turns out Charles Messier was only interested in comets and he saw these spiral looking things, these nuisance objects that he wanted to get rid of so he made a catalog of them so he'll never look at them ever again. Now, of course I'm making the argument that you should care deeply about what these objects actually are. Again, to motivate this, I'd like to play a video and this video is going to motivate why you should care about galaxies by putting in the context the size scale of the galaxy that you live in. This video starts in Venice, Italy. It starts with a hula hoop that is a meter in size and then the circle of people around that is about 10 meters in size and as we play the video forward, we're going to go out powers of 10. Each time a circle is drawn, it's going to be a factor of 10 larger in scale. We'll play the video.  We get outside the size of the Earth, we're already at 10,000 kilometers, 100,000 kilometers. 

I'll just stop it here as we see the moon. This is the farthest that we've ever been able to take an actual image looking back towards the Earth. It's actually a little bit farther than where astronauts in space have actually traveled. The rest of this is going to be a visualization based on what we think the distribution actually is but it's going to be sort of an artist conception of what things would look like. Our next circle will be...  You see orbits of the planets within our own solar system...  Getting up to scales of a billion kilometers...  100 billion, a trillion kilometers, 10 trillion and then we start to get out to the closest star, our sun, so 50 trillion kilometers. We'll switch the unit of five light-years, the distance that light can travel in one year... 10 light-years, 100 light-years, 1,000, 10,000, and then you get up to the scale of our own Milky Way galaxy at 100,000 light-years across. 

Now, this is taken from an IMAX Cosmic Voyage video. There are longer versions of this narrated by Morgan Freeman that go into more detail that you can look up. I'll go out just one more power of 10.  Once you get out to these scales of millions of light-years, it seems like everything you see is a galaxy itself. Now we're seeing both our own Milky Way galaxy and the nearest galaxy similar to it which is the Andromeda spiral galaxy two million light-years away. Now, the galaxies we typically think about are either millions of light-years away or I'll talk about galaxies, they're up to 10 billion light-years away. For me, a million light-years away is relatively close whereas you get to billions of light-years away, you're starting to get to reasonable distances. 10 billion is actually starting to get quite far. I've used this term galaxy. I want to go ahead and define what I mean by galaxy. 

Here I'm showing an image of actually the same galaxy that we saw on the first slide, this galaxy Messier 81. This is again how it's seen at optical wavelengths that are visible to our own eyes. I'm also going to adopt a definition of what I mean by galaxy and that is gravitationally bound collection of stars and the properties of that collection of stars cannot be explained by combination of the normal matter that we see and Newton's laws of gravity. What that means is that when we measure actually the motions of the stars and gas within these galaxies, there is a direct implication of this hidden mass that is referred to as dark matter. That dark matter is an essential ingredient in the galaxies that we observed, something that we can't see directly but we can infer indirectly based on the motions of stars and gas within galaxies. Now, one of the themes that I'd like to put forward is that the information you can learn depends strongly on what wavelength that you're observing at. This is what you can see with visible wavelengths. This is with Hubble Space Telescope. I'm now going to show you what the same galaxy looks like in the infrared. In the infrared, the dust lanes that we saw on the previous image that were blocking the star light, we're actually now starting to see that dust in emission. 

For example, if I just switch back and forth, I can see dust lanes through here that are now popping up as hot dust emission that you can see at infrared wavelengths and this was taken by the Spitzer Space Telescope. Now this is not the only wavelength that we can take an image of a galaxy like Messier 81. This is looking at ultraviolet wavelengths so photons with energies higher than the visible light photons that we can see with our own eyes and showed at wavelengths. Also, you start to see this population of stars that have recently formed that you couldn't really quite see in the first image that we saw at optical wavelengths. In particular, this extended emission out here that we're seeing from these recently formed stars producing these energetic photons. Going back to the first image, again, you didn't really see them very well. You could see the stars at the center of the galaxy but you can't see the stars recently formed at the edge as well. The point of showing this is just to make the point that we can learn a lot about the content of galaxies by observing them at multiple wavelengths. Now something I'd also like to talk about are black holes. Now, there are two flavors of black holes in the universe that we know of and we differentiate them by their masses. 

There's a flavor of black holes that are around the mass of our sun or may be 10 times the mass of our sun that form when a mass of star will explode as a supernova and then collapse. The most massive stars will collapse to these black holes. The kinds of black holes that we're interested in the centers of galaxies, we refer to as supermassive black holes. The term supermassive means that they are millions to billions of times the mass of our sun. Again, my definition of a black hole, a region of space that has so much mass concentrated in it that nothing that comes to close can escape its gravitational pull. Now, why should you care about black holes? I'm going to motivate this first by just showing an animation here on the left. This is an animation but it's based on real data taken with the Keck 10 meter telescope on Mauna Kea in Hawaii by a group at UCLA. What they've done is over the span of about 15 years, they've measured the precise positions of stars very close to the center of our galaxy. In the span between, this video runs from about 1995 up until the present day and you actually can map the orbits of stars by going back every year applying for telescope time, measuring the position of each one of these stars and one that's particularly interesting is this star they call S0-2 which has actually completed the full orbit. When you have that information about the orbit of this particular star, you can measure very precisely and very accurately the mass within that orbit. 

When you do that, you infer a mass four million times the mass of our sun. The only thing that can be to be in such a small volume with that much mass is a black hole. The best direct evidence we have for the existence of a supermassive black hole comes from measurements like this of the orbits of stars around our own Milky Way's black hole. In addition, our galaxy is not particularly unique. Essentially, every massive galaxy has one of these supermassive black holes at its center. Something that was a discovery basically in the 1990s, in the previous decade, people started to realize that the masses of those black holes were related to the masses of the galaxies that they lived in. That tells us that the process that leads to the growth of the galaxy also seems to be important to the growth of a black hole at the center of those galaxies. We're able to observe galaxies that are forming stars. We're also able to observe galaxies that have black holes that are actively accreting new material and shining as a term called active galactic nuclei which basically just means a growing black hole. I told you a little bit about galaxies, a little bit about black holes. 

I actually want to stop now and put this in the context of what you may or may not already know in part to get some information about where we should go as we move forward. I'm going to give you some specific instructions. I'm going to put up on the next slide a multiple choice question. There's no pressure about getting it right or wrong but I would like you to read it and think about it. I'm going to give you enough time to read it and then prepare an answer. Now, if I brought something like this, we could actually have little voting cards. I want you to do more analog voting or this is A, one finger, B, two fingers, C, D and E and if you really don't like this process, when it comes the time to vote, you can put your hand in front of your face to show your objection. I'm going to ask you to read the question, prepare your answer. If you want to indicate to me that you're ready, you can maybe just put your hand like this but don't show your answer yet. I want you to think by yourself. 

I don't want your neighbor to be  looking over and figuring out what you voted. Then we're going to vote simultaneously and I will give you a countdown and we'll do that together. Without further ado, here's the question.  All right, looks like we're on the same page. Here's what we're going to do. On the count of three, we're going to vote simultaneously. I want you to show me your answer in a way that only I can see it that your neighbor can't see it. [laughter] Then I may give you the opportunity to talk to your neighbor if there's motivation to do that. Let's vote on three. One, two, three. 

 There seems to be some consensus here so I'm going to prove that you are the source of the information so I'm actually going to ask you to shout out your answer. What is the answer on three? One, two, three. >> D. >> D, that is right. The correct answer is D, that about 5% is the fraction that's in the form of normal matter. Thank you. I think this is actually a really important point which is why I wanted to pause to emphasize it that the vast majority of the universe falls in the category of we don't really know what it is. I'm being somewhat provocative with that statement. The caveat is we have some pretty good ideas of what it is. In fact, we've measured quite precisely what we think the breakdown of the different components of the universe is at the present day. 

Now, this is a pie chart taken from the Planck collaboration from European Space Agency. They've measured to three significant figures what fraction is dark matter which we referred to in the context of galaxies, normal matter made up of things like protons and neutrons primarily, also electrons, other forms of baryons and leptons and then there's this other mysterious thing we refer to as dark energy that's taking up the vast majority of this pie. We measured this expansion of the universe quite well from the early phases of the universe today. This particular model provides an excellent fit to the data but the problem is we don't quite know what the dark matter is and we certainly don't know what the dark energy is but when we think about how galaxies form, they're forming from this backbone of dark matter that we understand quite well how it behaves gravitationally. Even though we don't understand what it is, we know how it behaves quite well. When we think about how galaxies form, we're really thinking about how they're forming within what's called halos of dark matter. This dark energy, we still don't really know what it is but we know how it behaves quite well and that it causes this accelerated expansion of the universe. I think it's actually the fact that we don't really know what the vast majority of the universe actually is, is one of the reasons why I think it's a pretty exciting time to be working in this field, working in astrophysics and cosmology. Now, I'd like to show you a simulation, a dark matter simulation of what we think the universe looks like on large scales. It's a movie from almost a decade ago called The Millennium Simulation. 

This sized scales here, this is a gigaparsec. A parsec is about three light-years and giga means 10 to the 9th, so it means a billion light-years. One gigaparsec would be three billion light-years. It's going to zoom in from this large scale. It has 10 billion particles in this simulation from a decade ago. It'll give you a sense of what this dark matter backbone looks like.  It's zooming in on a particular intersection of these cosmic filaments in a particular dark matter halo that would host something like a galaxy cluster or cluster of galaxies.  I'll stop it here and just make the point that we can't directly observe this dark matter but one thing we can measure is we can measure quite well the three-dimensional positions of galaxies in the sky and when we do that, you can see how clustered galaxies are. It's not a random distribution. When you measure the position, you see that galaxies like to be near each other. 

When you make those measurements, it turns out that it matches quite well at least on large scare what you'd expect from this kind of dark matter movie that we've just seen. One point that I'll make is that this is a very impressive simulation from a decade ago, simulations have gotten in order of magnitude better since then but in this particular simulation, it models dark matter's individual particles. Each one of those particles is actually about 100 million times the mass of our sun. A particle physicist might tell you, "We think a dark matter particle might be, we think about massive proton." A massive proton is 10 to the minus 24 of a gram compared to something that's 100 million times the mass of our sun so it's actually, you know, the simulations today get on these large volumes can get down to about a million sort of mass particles but that's still a factor of 10 to the 63 difference between what this simulation has for a dark matter particle and what you might expect from a physics perspective what the actual dark matter particle might be. Now, we don't actually need to improve by a factor of 10 to the 63 to model the effects of dark matter on large scales but I think it's interesting to keep in mind the fact that we don't quite understand what's happening with these individuals. What we think might be the particles that make up dark matter. With that, you got your instructions and here is another question.  We did great last time. [laughter] Let's try again. Let's vote simultaneously on three. 

One, two, three.  This is actually great, because there's not a consensus the way there was last time. I'd actually like you to take, say a minute and I want you to find someone next to you. If there's no one next to you, I want you to turn around to some person behind you or the person three chairs over and I want you to ask them what their answer was and I want you to argue why you think you're right and listen to them why they think they're right. Ready? Go. 60, 59...  If you're interested, I would encourage people say in the corner if you want to find out what someone at the end of your row said. I would like us, now that we had a little conversation to see to what extent answers have changed they may or may not have but does anyone need more time to prepare a vote. Let's vote again simultaneously. One, two, three. 

 Let's see what you shout out. What is the answer? There was some disagreement here. Now, this is an interesting kind of conversation because I haven't really given you any information about this except I don't know if anyone read the abstract for the talk. The answer was actually in the abstract of the talk and the answer is again, about 5%. Again, I've taken this time. I'm sorry if I made anyone uncomfortable to try to nail home this point that what we know about what we think we understand normal matter that's in the form of stars, dust, black holes, planets and people, that's only 5% of the 5%. Only 5% of the universe is normal matter and only 5% of that normal matter is in the form that we can see directly. What's the rest? It's gas. It is the gas around galaxies. This is why you should care about what galaxies do with their supply of gas and the fact that they're so bad at converting that gas in the things like stars and planets. 

Here's a way to show that visually in a pie chart that I'd taken from a group at Purdue. Again, their version of this dark energy, dark matter, normal matter plot and then they divided it into the fraction that they've estimated to be in stars, I think the summary is a little big high but again, it's a small sliver of this 5% that is normal matter. Our understanding of how galaxies form, how they evolve through cosmic time is essentially a question of what they do with this supply of gas in the universe and how they convert that gas into stars and how efficiently they're able to do that. We really want to know why we're only able to convert 5% of the 5% into stars and galaxies. Now, let's go back to that image I showed on the title slide. Again, this is a spiral galaxy Messier 81, Messier 82 and a local group of galaxies about 10 million light-years away. What we see at visible wavelengths is the emission dominated by starlight, by these stars that I've told you are only 5% of the normal matter. Now, I'd like to show you again emphasizing a multi-wavelength perspective another image of the same region taken with a radio telescope. This radio telescope is sensitive to a particular transition of hydrogen gas. It's able to probe neutral hydrogen gas both in these galaxies and outside of these galaxies. 

I've highlighted a few regions of hydrogen gas that are essentially, going back to the previous image, you didn't see anything there. We can see these spiral arms and this galaxy Messier 81 extending out further, much further in this neutral hydrogen gas that we can't see in the light that we can see with our eyes. We're seeing all this material. This is only hydrogen that's neutral. Most of the hydrogen is actually going to be ionized. It means it lost its electron and going to be a much hotter phase, more ionized phase. This is only tracing one phase of that gas but just a simple of comparison of showing that this is the dominant component of the normal matter of the universe, this neutral hydrogen plus the more highly ionized hydrogen. This is the same now with a deeper radio image where they're trying to essentially detect about an order of magnitude fainter. Essentially, what you see is there are neutral hydrogen gas everywhere in between these galaxies. Again, we want to understand what this gas is doing, how it gets into the centers of dark matter halos where galaxies themselves form and how it's converted into stars. 

Any questions so far about any of these topics?  We can also defer these to the end so I'm also happy to answer them then. On the next slide, what I'd like to do, and I'll try to motivate why you should care about galaxies and why you care about what they do with their gas, the kinds of questions that I think about on a regular basis and that guide my own research. Those questions are the following. How does gas get into galaxies? I'm showing a simulation of how that process might work. This is a simulation that's not just dark matter. It's also trying to include the gas component and gas seems to want to come in along these dark matter filaments and galaxies in the form of these intersections of dark matter filaments so we think that's what's happening. How does gas form stars and fuel black holes? I'm showing an image here from the Hubble Space Telescope at the center of a nearby galaxy Messier 51 and we think that stars form in regions of high gas density where the gas can actually get quite cold and collapse and gravitationally form stars. Some fraction of that gas also has to make it down to the very center of the galaxy if we actually want to fuel the growth of a supermassive black hole at the center. Finally on the right, how does feedback, I'm getting back to this feedback word that was on my title slide, how does feedback regulate this gas supply? Here I'm showing an image of a cluster of stars and the dust that surrounds them and the ultraviolet photons from these young stars can push on the surrounding dust and gas and essentially blow it away from the region that was forming stars. When I use this term feedback, what I mean is injecting energy and momentum from stars that have recently formed into the medium that surrounds those stars and then influencing how the next generation of stars can be formed. 

Essentially, if you inject a lot of energy and momentum into the region where you just formed lots of stars, basically that gas isn't going to be able to cool down anymore and isn't going to be able to form the next generation of stars in that particular location. That's one way that we can try to reduce this efficiency with which galaxies convert their gas into stars. Again, to make these points, we think gas gets in the galaxies by accreting along dark matter filaments. We think stars form in regions of high gas density and we think that massive stars will be able to heat up and blow out the surrounding gas that might otherwise form the next generation of stars. Now what I'd like to show you is a movie here on the left. This is again a simulation that includes both dark matter and gas.  What this movie is trying to show is how we think galaxies form within these dark matter halos. What we're actually seeing here is only the gas, what the gas is doing.  This is actually a very impressive and I think beautiful movie making something that actually looks like a population of real galaxies.  It will focus on a particular group of galaxies here. 

They're undergoing this gravitational interaction.  I'll stop it here and just make a point that the simulations are getting much, much better as basically building bigger computers, you're able to simulate not just the dark matter but also the gas and try to understand how it's going to collapse and form stars within these dark matter halos but one problem that these simulations tend to have almost generically is that one measurement you can make is just count up how many galaxies you form in a given volume of the universe and bin them up as a function of their mass. This red line here is roughly what you get from a simulation like this on the left. You expect more low mass galaxies and not as many high mass galaxies. The main point is that this is almost a straight line on this particular plot that you're predicting these many galaxies that a mass of about a trillion times the mass of our sun and these many galaxies out here at about a billion times the mass of our sun. Now, I'm contrasting that with a line here which is the actual, if you go with your telescope and you do a survey and you measure the masses of galaxies, the black line is what you get. This is the plot, number of galaxies and function of the mass of those galaxies. The point is the black line and the red line look nothing like each other. Do you agree in a sense that you check more low mass things than high mass galaxies but the shape of these mass functions in the data is quite different than what you expect from simulations? In particular, you're vastly over predicting the number of things here at the lowest masses and at the highest masses. Now the important point, gas is important. 

What can be frustrating about it is that a lot of the relevant physics that's happening is occurring on scales that aren't very well resolved by the simulations. You're not getting on to the point where you're forming individual stars and resolving that kind of scale. Basically, it's just a problem that galaxies are so big and you have such a range of scales that you're trying to simulate. If you want to simulate a representative volume, there's only so much resolution you can get. What we're trying to do observationally is try to understand what processes would make galaxies be so inefficient at forming stars, essentially all masses but in particularly the most massive galaxies in the universe and the lowest mass galaxies in the universe and we think it's related to what's happening with this process called feedback, the way the gas is shocked, the way it heats, if heated the way it cools and basically we're trying to understand that better by observing galaxies that are going through these kinds of processes and comparing the simulations that are trying to get right this gas physics. I'd like to show what this looks like, what feedback looks like, this injection of energy and momentum from star formation. This is again, the galaxy we saw on our first slide, Messier 82. This is what it looks like at visible wavelengths again with the Hubble Space Telescope. What you see here is emission from stars and you also see basically how those stars are obscured by dust in the disc of the galaxy. This is sometimes called Cigar galaxy because that's sort of the shape that it looks like at least to this kind of wavelengths. 

These are just the stars and the dust in front of it. I'm going to show this image which is, we still have this component of the stars but then we're also looking at this ionized hydrogen gas that you can trace by a particular emission feature. What you see is this sort of biconical outflow of material that's coming perpendicular to the plane of the disc that's forming stars. Basically what the signature of it, is gas being blown out of the galaxy by the massive stars that have recently formed. As massive stars produce lots of photons, those photons can push on the surrounding gas and dust. They also explode as supernovae injecting lots of heat, lots of thermal energy into the surrounding medium and can dry gas out perpendicular to the disc. Now, this is with the Hubble Space Telescope. This is actually an image with the WIYN Telescope that Wisconsin is a partner in. Colleagues at Wisconsin were responsible for making this image. You can go much deeper actually from a telescope like WIYN is bigger than the Hubble Space Telescope. 

It doesn't have as good as spatial resolution but you're able to trace this ionized hydrogen gas out too much, you're able to get a much deeper image essentially and you can see how far it's extending. Basically, this galaxy is forming stars and then the gas that didn't form stars is being ejected out into the region between galaxies, the region outside the galaxies where we think is where most of these gas resides. Finally, I'm going to show-- This is actually, you can directly image the dust in this galactic wind. This is an image with the Spitzer Space Telescope. There's a particular kind of dust molecule that radiates by infrared fluorescence that you can then actually directly observe. We're seeing this gas being ejected out from this galaxy. This is stars and its dust. Finally, I wanted to show an image of the very hot gas that you can observe with an X-ray telescope. This is gas that's millions of degrees. It's had the very hot wind fluid that maybe the dominant component of the wind that's blowing this gas out. 

This is a super vision of our initial optical image from the Hubble Space Telescope, the dust seen by the Spitzer Space Telescope and finally this very hot gas seen by the Chandra X-ray Observatory. This hot gas probes this biconical outflow reasonably well so we think that the stars here, the center of this disc, of this galaxy are basically blowing out large amounts of gas and dust into the region between galaxies. Now, getting back to how we can try to start to answer some of these questions, how does gas get into galaxies, how does it form stars and fuel the growth of black holes and how does this feedback from star formation regulate the supply of gas for subsequent star formation. I just want to highlight one particular project that I'm involved in that I have a leadership role in which is called MaNGA. It's called Mapping Nearby Galaxies at Apache Point Observatory. This is a telescope at Apache Point Observatory in New Mexico. It's a telescope from the Sloan Digital Sky Survey. This particular survey is part of the Sloan Digital Sky Survey and basically, what we're doing is we are observing galaxies not just as considering them as a point source. Oftentimes you can disperse the light according to wavelength and take a spectrum of the galaxy. People often do that but you only get one spectrum from one galaxy. 

With this particular survey, what we're doing is each one of these holes here is its own optical fiber and for a galaxy, say like the one shown here, we're going to get information for I believe it's 127 different locations on that galaxy. What's really unique about the Sloan Digital Sky Survey telescope is that it has very sensitive instruments on it and it has a very large field of view so if you put sort of thumb and pinky out in the sky that's about 10 degrees across, what is incredibly large for the field of view of a telescope, if you think about the moon, the moon is about a half a degree across. The Sloan telescope has a field of view that's in fact would be six moons across which then if you think of a circular area, it's about seven square degrees. What we're going to do with this survey MaNGA is we're going to put these bundles of optical fibers on galaxies and we're actually going to observe 17 galaxies at once because once you have a field of view that's this large, you have enough galaxies that you can put bundles on just-- so, we're not just taking one spectrum on one galaxy. We're taking hundreds of spectra of each of 17 galaxies at once. By doing that, we can learn about how gas is getting into these galaxies. We can look at say if there's something different about the outskirts of the galaxy compared to the center. You can understand what's the history of star formation at different locations within a galaxy. Essentially what we're going to do is observe 17 galaxies at once for a few hours at a time and repeat that 600 times over the course of six years and build up a sample of about 10,000 galaxies where we can map their properties as a function of position in the galaxy and learn about all three of these questions. This is a survey that Wisconsin is very intimately involved in and the full survey operations are actually going to start this summer. 

Stay tuned for results from this MaNGA survey. Now, I have this one particular research project in addition to that and it has to do with observing this feedback process in the distant universe. Now, it turns out these outflows of gas that we can observe directly in a galaxy like Messier 82, if you move it from just 10 million light-years to 10 billion light-years away, all of a sudden that gas becomes too diffused to see directly in emission but something that you can do is actually to detect by gas an absorption towards a bright background source and one of the ways you can do that is to use a galaxy itself as a background source. When you do that, you see all the gas along your line of sight. Now, I'll make a point that this drawing of a telescope and distance to a galaxy is vastly not to scale. This distance here is billions of light-years whereas the distance you might picture my telescope is of orders, you measure it nanolight-seconds. I think it's about the factor of 10 to the 24 or so often scale but it's designed to be illustrative. This is a telescope that Wisconsin is a partner in to South African Large Telescope that we can use to measure that gas in absorption. When we see this gas, we can actually measure the velocity of that gas very precisely relative to its whole galaxy. You can see if that gas is inflowing or outflowing using the Doppler shift of those individual spectra lines. 

If they're shifted towards red, longer wavelengths, that means the gas is moving away from you. If they're shifted towards shorter wavelengths in the blue wavelengths, that means the gas is moving towards you. The signature of gas that's outflowing would actually be gas moving towards bluer wavelengths similar to analogy in terms of when a police car drives by the pitch as it's coming towards you, it's a little bit higher and the pitch if it's going away is a little bit lower. Pitch is actually frequency. It has to do with the frequency is changing basically based on how the material is moving. I want to just show an example of how we can do this. Now, in addition, this is one technique to study gas right in front of a galaxy. It turns out there will be other galaxies along your line of sight and you can also detect the gas around them again looking towards some distant background object so that's another technique that I've been using with telescopes like the South African Large Telescope. You can detect these halos of gas where we think most of the gas actually is. I'm just going to show one particular results. 

What I'm showing here is as I said you can take the light of the galaxy and disperse it according to wavelength and take a spectrum. This is actually four individual spectra of four different galaxies that we had observed because we recently formed lots of young stars and the question is can we see this signature of gas that's outflowing if we take a spectrum like this? Basically, this is what galaxies look like when you're not looking at images, when you look at spectra. You actually see individual features. It's actually coming from absorption from hydrogen gas in the atmospheres of stars so from this, you know exactly the properties of the stars in this galaxy. I said what we're looking for is absorption lines. What absorption line looks like in a spectrum is a dip. I'll just highlight these dips here. This is actually from a particular, it's a magnesium gas that we're seeing, ionized magnesium gas. On the right, I'm basically zooming in on this individual absorption line for magnesium gas. Basically, if the gas were where it should be, it would be right at the center of these panels but basically the only thing I want to draw your eye to if nothing else is this blue arrow. 

The fact is that these absorption lines that we're measuring of magnesium gas are shifted towards bluer wavelengths. From that shift, we can measure the velocity of that gas. It turns out what we measured for these particular galaxies were a thousand to 2,000 kilometers per second gas coming out at velocities where you might expect they've actually didn't even escape the dark matter halo in which the galaxy is residing. This was interesting because this was the highest velocity outflows that had ever been observed for stars from galaxies at any epoch in the universe. The question is how can you do that? This process might be quite important for explaining why say the most massive galaxies stopped forming stars. They stopped converting gas into stars. Maybe you need this kind of process to just basically blow the gas out of the galaxy. Let me show a movie here that shows a theoretical expectations for how you might blow gas out of galaxies. We're starting with two disc galaxies, again focusing on the gas and they're going to merge so basically, they're interacting gravitationally. They have dispersed passage that starts to disturb this gas supply. 

It actually starts to sort of push some of that gas together so it can more easily form stars. The galaxies then continue to feel each other gravitationally similar to the way our Milky Way actually feels our neighbor galaxy Andromeda which we are likely to merge with in a few billion years. Then in this particular simulation, there's a prescription for how the gas accrete onto a black hole and then they take a small fraction of that since the accretion of energy and they coupled it with the gas. Basically right now, you blow this very powerful high velocity outflow of gas. One hypothesis is that maybe we're seeing the signature of this kind of feedback, this kind of outflow gas driven by this process of this growing black holes, again, wonderful simulation. It turns out we've done more detailed measurements of these galaxies and in particular, you can take images of them with the Hubble Space Telescope. Now, these may not look as impressive as the images of galaxies like Messier 82 that I showed earlier but I think they're beautiful. These are basically showing that a lot of these sources almost just look like kind of a point source but they're very compact. The fact that when something is very compact, that means basically gas launched from very close to the center of the galaxy will be accelerated as it goes out by this feedback energy from star formation and that you actually expect the galaxies that are most compact to have the highest velocity outflows. Essentially, you're in a deeper, you can think of it you have a deeper potential well that you're in where you're able to accelerate over a larger range of scales out to a large radius. 

Basically, we quantified what this would mean and actually saw that we could explain these kinds of outflows as being driven by this feedback from very compact star formation such that you would necessarily need this kind of feedback from a growing black hole but this is the kind of feedback from star formation that's not currently well resolved by simulations so it would actually be quite difficult for simulations at least of a few years ago to resolve these kinds of scales to produce these particular kinds of outflows. As simulations get better, it will be interesting to compare to see if this is actually a dominant and an important process for driving gas out of galaxies. With that, I'd like to move towards a conclusion and just make a few summary points. I tried to get across the point that gas is actually incredibly important in the formation and evolution of galaxies and their supermassive black holes across cosmic time. Simulations almost generically over predict the efficiency with which stars will be able to form from that gas that you expect to form lots of low mass galaxies and lots of high mass galaxies. We really just don't observe that many and that when I say the universe needs feedback, what I'm saying is that we need some process that keeps that gas from forming stars and that a feedback process taking the energy from massive stars and also supermassive black holes can be an important ingredient in resolving this discrepancy. With that, I'd like to leave with this image of Messier 82 as seen the disc of the galaxy, invisible lights, the outflowing dust from the galaxy in red and the hot gas in blue and finally comparing both what you see with your eyes and stars and what you're able to see when you actually can try to measure the gas directly. Thank you very much. [applause]