Wisconsin Public Television University Place  Shedding Light on Distant Galaxies  Britt Lundgren  7/9/13 UP896  cc >> Welcome to UW Space Place. I'm Jim Lattis and this is our guest presentation for this month. Tonight's speaker is Dr. Britt Lundgren who's an astronomer in the UW-Madison astronomy department. Britt got her PhD in astronomy at the University of Illinois and then spent some time at Yale, and is now here in Madison doing research. She is going to tell us about that tonight. Britt?  >> Okay. There we go.  Okay. Is the sound okay? Can you hear me? Okay, great. I'm really happy to be here and very happy to be giving my first talk at Space Place. 

I've been meaning to come by here quite a bit and get involved. This is really exciting. I'm glad you could all come out. What I'll be talking about today is probing the distant universe using a special tool called quasars. Some of you may know what they are. You may have never heard of them before. Hopefully by the end of my talk today you'll all be co-experts on them. [laughter] I'll start out really, really simple. If you went out and looked at the night sky, and if it were less cloudy than it is now and substantially darker, you might see something like this. Also if there were mountains that would help, and a comet. 

This is just a beautiful image I found online showing a number of things you might find in the night sky, if you were in Australia. [laughter] What you can see here are some things you'll all recognize, a lot of stars. The cloudy thing that's going kind of diagonal across the screen, hopefully some of you recognize as our home galaxy, the Milky Way. We see it edge on from our place in the galaxy. There's also a comet, and those cloudy things over to the right, those are some neighboring satellite galaxies that live close by. They're associated with our own galaxy and interacting with it as we speak. Those are things that you could see with your naked eye if were in the southern hemisphere. Unfortunately, we're not, so you may never have seen those yourself. But all of these things are made of stars except for the comet which is made of a lot of rock and ice. But all of these things are visible with the naked eye. 

Now if you had a very strong ground-based telescope like this one, the Canada-France-Hawaii Telescope, which is located in Hawaii at the top of the mountains to get the best possible seeing conditions. Then you might be able to see something a little more spectacular even than the last image. What your looking at here is a distant galaxy. Actually it's pretty close to home, but you still need a powerful telescope to see it. It's about 100 million light years away. This is the Persieus Cluster. Some of the brightest things that you see on the screen are really massive galaxies all living pretty close by to each other. They're actually sharing some of the same neighborhood. What you see in the background, if you squint a little bit, are some spiral galaxies and some other smaller galaxies that are maybe further away or actually smaller than the brightest ones you see. You also see a sprinkling of stars, which is pretty typical if you're looking through our galaxy to get at something further away. 

Again, all these things are made of luminous matter. You can see them with your eye if you had a strong enough telescope. In order to get an even deeper picture of the universe you might need something like the Hubble Space Telescope. What I'm showing here is the deepest ever image taken of the universe. This is something called the Hubble Ultra Deep Field. Basically, what the team of astronomers who made this image did was they picked a spot on the sky that looked completely blank. All of our best images of the sky showed nothing in that particular region. They just pointed the telescope there for a very long time and integrated. They were able to get a really deep image of the universe. What they found was that it's just littered with galaxies. 

As far as you can see there are more and more galaxies. This particular picture contains some galaxies that are as much as ten billion light years away. The light from those galaxies has been traveling for ten billion light years to reach us. It might surprise you to learn that of all those things that I just showed you, it's really just the tip of the iceberg. Normal matter only five percent of everything in the universe. Of that, 90% of normal matter is completely invisible. It's too cold and too dark, too far away. We just can't see it with our naked eye. We can't see in even with our strongest telescope, not even with the Hubble Space Telescope. Everything that you saw in that image of the ultra deep field taken by the Hubble Space Telescope is really just a fraction of a percent of everything that's contained in the universe. 

If you look at this pyramid which shows the relative contributions of various types of matter and energy to our total censes of matter and energy in the universe, you'll see that the foundation of the pyramid which is about 70% of everything that we can account for. It's this mysterious thing called dark energy. We don't really understand very much about it. We've studied its effects pretty substantially at this point. Dark energy is what we think is responsible for the acceleration of the universe. I should say, the expansion, and today's present acceleration of the expansion of the universe. Making up the next 25%-- or I should say-- I need to get my glasses. I think 20% of what's in that pyramid is-- 25%. Thanks. --is something called cold dark matter. 

Cold dark matter we know exists because we study the way that galaxies rotate. We can tell that there's some kind of hidden matter that we can't account for with everything we know in our periodic table. There's some additional particle that we still have yet to identify. But we know from the ways that we model the universe and compare with our observations that that particle must only react to gravity and nothing else. It behaves very differently from everything else that makes up our bodies and the planet, and everything else that we see in the universe. I'll from now on be calling that normal matter. I've talked about this a little bit already. What I'll be talking about for the rest of the talk is that dark matter is not the mysterious dark matter, but normal matter that we just can't see. I'll call it invisible. This is matter that's made up of everything you've heard about, hydrogen, helium, carbon, oxygen, all those things that sound really familiar. 

Except it's cold enough and far enough away that we can't detect it with our eyes. It doesn't glow, you can't see it in the way that you would conventionally think of seeing something. But this cold gas is actually fundamental to everything that's going on in the universe and the way the universe has evolved to the present day. Cold gas is fundamental to forming stars. Stars can't form without it. If you have gas that's too hot it won't collapse. You just can't compress it to the point where it ignites and starts burning hydrogen into heavier elements. Those stars become the seeds for galaxies which are some of the largest structures in the universe. And of course those stars, ultimately at the end of their lives, explode. If they're massive enough it's a supernova. 

Then you get all kinds of heavy elements which are produced during those explosions that you would otherwise not have. So all of this is dependant on the existence of cold gas in the universe. Of course, knowing how critical it is to producing stars, you can understand how its own supply and recycling within galaxies is determining how they grow and evolve in time. It's really quite fundamental even though it's hard to see. How do we see it? Well, there's one trick that I'll really be selling hard in this talk and that's backlighting. You've seen it before. This is a great example with the moon backlighting some other things that you couldn't see in the dark. The bat's not glowing. The castle has no lights on, and the cloud itself is pretty dark. If you have a large enough and bright enough light behind it you can pick up on what's there without looking at it directly. 

Believe it or not, you've already seen this. You've probably seen it today if you walk outside because it was pretty cloudy. If you look at our own Milky Way, if it's dark enough, you can see some dark patches in the disc. Those are dust lanes. Parts of the galaxy that are littered with lots of clouds and gas and dust. What they do is actually absorb some of the light from the stars behind them and they make them slightly darker. Some of the densest part of our own galaxy, to the naked eye, look quite dim, simply because the gas and dust is really hiding some of the stars behind them. Ironically, the brightest light in the universe, I'm going to tell you, are black holes. They are notoriously not bright. I'll get to it in a second. 

But we'll just to start out with a fundamental understanding of black holes, which some of you may already be aware of. Black holes are defined to be objects that are so massive and so dense that nothing can escape their gravity, not even light. Anything that falls within a certain radius of the black hole, depending on how much mass it contains, cannot escape. That includes light, which is why it's so hard to see them. In fact, anything can turn into a black hole. If you took the Earth for instance. The Earth could be a black hole if you just found a way to squish it down to the size of a grape. It's extremely dense, almost to the point where we can't really imagine how dense something like that would be. These are really exotic phenomena that are maybe hard for us to create in a laboratory, but happen really naturally in nature. They're produced by massive stars when they die in a violent death known as supernovae. 

If a star is sufficiently massive, when it dies it will leave behind a black hole. We imagine that the universe is almost certainly littered with these kind of single-star massed black holes. But when it gets really interesting is when you look at the center of a galaxy where a number of black holes have maybe combined and coalesced as gravity has acted on the system through time. As far as we can tell, the center of every massive galaxy, every galaxy that's sufficiently bright that it has millions or billions of stars, if you look towards the center you'll see one of these supermassive black holes. The mass in these black holes is equal to millions of stars of masses similar to our own sun. These are just incredibly massive. If you think about it, it's even crazier. You take millions of stars, squish them down, and the black hole itself is left in an area that's smaller, approximately, than our solar system. This is a really, really tiny, dense, region of, we think, every galaxy. You might wonder, well, if they're in every galaxy, have we looked at our own? It turns out we have. 

If you look towards the very center of our galaxy and take a series of photos over the course of about ten years, what you can do is start measuring how the stars are moving around the center of the galaxy. This was a project undertaken by a team in Europe using a very powerful telescope. This is actual data. What you're looking at-- Let me just play it one more time. This is a zoom in on Sagittarius Star A, which is the region that we know to be the center of our galaxy. If you just trace out the orbital paths of the stars in the region surrounding that location, what you can see is that they're whipping around something that seems totally invisible. It turns out, with some relatively simple math, you can use the orbital path of those stars and the speeds at which they orbit, to determine the estimated mass of the black hole that's living in that region. If you do the math it turns out that to be about three million times the mass of our sun. That's pretty big considering we can't see any other trace of it. But I told you that black holes can be used as backlights. 

It turns out there's special kinds of supermassive black holes that can be extremely luminous. In fact, when they light up they're among the most luminous objects in the entire universe. These are known as quasars. The way quasars work is you have this supermassive black hole that's otherwise very difficult to detect unless something starts falling into it. When that happens it can become as luminous as about 1,000 galaxies. Here's an example. If you have a star that gets maybe too close to a supermassive black hole, the gravitational potential around that black hole will actually tear that star apart and shred it. As the gas spiraled in towards the accretion disc surrounding the black hole which is rapidly spinning, you see that it actually gets extremely hot and ultimately succumbs to the black hole. As that happens you may often see a production of a jet, but that very small region becomes super luminous. In quasars that super luminous state can last for a very long time. 

What we can then do is use them. Because they're 1,000 times more luminous than an entire normal galaxy we can use them as probes of very distant parts of the universe, because we can see them from much, much farther away. We basically use them as cosmic flashlights. What we want to do is look at something similar to the shadows left on the light from those quasars from things in the foreground. They could be clouds of gas, or they could be big clouds of gas that also contain stars. I would call those galaxies. Any of those things can be picked up as a shadow left on the light. What I'm showing here is basically the light coming from the quasar. This is flux, so it's the amount of light as a function of wavelength. If you were to take a spectrum. 

I'll talk about it more later. But what's important to see is just those dips where the light goes down to zero. Those are spots where those galaxies basically cast a shadow. They've absorbed the light and it's not longer coming towards us. Just a really quick physics lesson. And this, I promise, will be it. Just to give you an idea of how this is really happening. It's not really a shadow. If you have a gas cloud, or any kind of gas, and it's made up of a single type of atom, say it's neon gas. You may have noticed neon lamps tend to have a very particular color. 

If you take neon gas and excite it using electricity, for instance, you'll get it to glow at a really specific color. That's what happens with all gas when it's excited by any kind of energy. If you look at something like the sun, or a star which our sun is, or a quasar, these things produce a continuous spectrum of light. That means if you were to look at the sun, which you shouldn't do, but if you were to look at light from the sun and spread it out with a prism, you would see every color of the rainbow. The sun is actually emitting all of those colors. We just see it as one single color with our eye. The trick is, if you were to put a gas cloud like the one shown at the top, in front of the sun what you would see is that the light coming from the sun would be passing through the gas cloud. You'd get that rainbow spectrum, but it would be missing a color. And that color would be precisely the fingerprint that that gas cloud leaves based on what its atomic structure is. Every kind of atom, every molecule, has its own preferred fingerprint that we can see in light. 

This means that when we see colors missing in a certain pattern it tells us exactly what a cloud is made of. It can also, based on other things we observe about it, we can tell things like temperature and density of the gas, and all kinds of things. The important thing to realize is that every kind of cloud has its own fingerprint. That becomes extremely useful going forward. Edwin Hubble actually made his name on this phenomenon. You've probably heard his name, but maybe you don't know the story. If you were to look at a bunch of galaxy spectra, and I showed you color versions before of a continuous spectrum, but back in Hubble's day they only had black and white film. We're going to show his actual data. Here are some black and white spectra of galaxies with the blue being on the left and the red on the right. These squares are just the actual image that you would get of the galaxies that are being investigated. 

If you look at the spectrum for each of these galaxies hopefully you can see two little lines that are in different places for each of the galaxies. Those lines are the chemical fingerprint of calcium 2. We'll just call it calcium. It's something that's very typical. It's a biproduct of stars. We see a lot of calcium gas and dust in galaxies. When it gets between us and the stars in the galaxies we get this little shadow. It becomes really useful because Hubble realized you could, from the shift, tell how fast the galaxy you're looking at must be moving away from us on Earth. The reason for this is it's something akin to the Doppler Effect. If something like a siren from a truck is making it's siren noise and it's moving away from us, the tone will go down. 

If it's coming towards us, you'll hear it getting higher in pitch. The same is true for light. If you have something emitting light but it's moving away from you very quickly, it will stretch the wavelengths of the light that you see into the red. We call this red shifting. That you've probably heard of. As something is going faster and faster the shifting of the light will get greater and greater. That's what you're seeing here with those two little calcium lines as they're shifting towards the red side, which on here is the right side of the figure. Where this gets really interesting is that Hubble made a quick plot and said, let's guess at what the distances of what these galaxies are. Let's say we don't know exactly what the distances are, but we know how bright they are. We can tell that from the images. 

Virgo, which is really big and really bright is very close to us relative to these other large galaxies that are also well known. If you plot a distance estimate for each of those verses the velocity that you get from that Doppler Effect type shifting of the calcium features, what you get is a really, actually surprisingly nice, relation between velocity and distance. This has become the fundamental tool for everything we've learned about the universe since. What you'll see next is something you've probably also all heard of, that the universe is expanding. The very first interpretation of this result was that space itself must be expanding between galaxies. How is it that you go from that plot to an understanding that the universe is expanding? Think about the initial-- I wish I could get this to stop. Okay. The initial state of this movie is three galaxies. Let's say we're on the red one in the middle. The green galaxy starts out being a million light years away from us, and the blue galaxy starts out being two million light years away from us. 

If you take the space in between and expand it by a factor of two, as happens here, the green galaxy after the expansion will move from one light year. Or sorry, on million light years to two million light years away. The blue will double also, moving from two million light years to four million light years. In the same amount of time with the same expansion factor of the universe, the green one has only moved half as far as the blue. The blue, if we were to measure velocity from the distance traveled in the same amount of time we would see that it's moving twice as fast. That's what's key. In every direction we see galaxies receding from us. The further away from us they are, the faster they seem to be receding. If we go back to this chemical light fingerprint idea then what you can see is nearby stars will have, say, a pattern of lines that we recognize. As you move to more and more distant galaxies you can see the red shifting of those lines. 

That's just fundamental for the work that I'll be talking about. It really helps us extremely to map out the positions of distant galaxies and also distant gas in the universe. I told you that quasars are excellent for this, as they are very, very bright. We can see them from very, very far away. If we take our new understanding that we can recognize patterns in the light and see how much they've moved, and from that determine how far away gas or galaxies along the line of sight have to be. Then you can take even just a single quasar and map the positions of multiple object along the line of sight. If you have 200,000 quasars as now do from some recent surveys, you can image that you're sampling the universe 200,000 times across the night's sky. You can start to build up a three-dimensional map of all the gas and galaxies that you couldn't otherwise see because they're too dim. I like to also think about this, not to drown you in analogies, but if you think about ice cores in the arctic. If you drill down through a huge glacier and arctic sheet the stuff on the top that's closest to us in that core, that's very new ice. 

It's very recent. As you go farther and farther down you're moving down thousands of years in ice. What that can tell you, if you look at the stuff that's trapped in the ice, is something about the chemical history of the Earth. Because the speed of light is finite and constant, we can also use quasar sight lines to tell us something about the universe at a wide range of ages. If you have a quasar that's ten million-- Sorry, ten billion light years away and you're looking at its light, then everything you sample along the way is not only a different distance from us, but its light has taken a different amount of time reach us. That means, as you go to further and further galaxies that are producing these shadows on the light, you're actually looking further and further back in time also. I think this is an additional really exciting tool. I should say, ten billion years, that's most of cosmic history. Quasars, we can see them from pretty much the beginning of the observable universe. They're great tools. 

What I'm showing here is not real data. This is a movie produced by some theorists that I'm friends with. It's really beautiful because it shows a big chuck of the universe as theorists expect it to look evolving from the beginning of time to the present day. I'll start over. Let me just pause. What we're seeing here is the universe about 13-1/2 billion years ago. At that time it wasn't so long after the big bang. Most of the universe was just hydrogen and helium. In fact, even most of that was hydrogen. There were very simple products from the big bang. 

Only a few trace elements are also produced. What you'll notice is that everything seems pretty evenly distributed. There's not a lot of clumping. You don't see any stars. It's all just a lot of gas with some minor inhomogeneites, a little bit of clumping that it starts out with. Now if you move forward in time, what you'll see is that gravity starts acting on the few places that are slightly more dense to begin with than others. What that does is it kind of snowballs. The dense places get even denser. They pull in the gas from their surrounding areas. What you're left with is this filamentary cosmic web. 

It's a lot of dense structures that are in this filamentary pattern, and where they meet, where the filaments join, you'll get some of the densest parts in the universe. Those will be the birth places of galaxies and maybe even clusters of galaxies. What you're seeing in this animation, which I think is so interesting too, is that these guys have added in the explosions that you'll get from star formation in young galaxies. You can see, it's blowing gas out. And here the colors represent temperature. So black means very, very cold. You can see the voids are black. As you move in towards the red filamentary structures, that's what we would call warm gas. Warm being about 100,000 degrees Kelvin. It's warmer than you'd want to be enjoying today, I think. 

Then if you get in towards the really hot gases, they're many millions of Kelvin. Those are produced by shocks and supernova feedback from galaxies. Basically, stars start forming, they live, then at the end of their lives they produce these huge explosions, and that blows gas out. And the gas is hot. Okay, so if we want to use quasars to not just have theoretical maps, but start getting real pictures of what the universe looks like, we can do that. This is how it works. We take our cosmic web and what's shown here in green, that's all hydrogen. What you see maybe in dots in red and blue, those are just different galaxies. Blue means they're forming stars, red means they're not. It's not really too important, but you can get the idea. 

You have a quasar that's backlighting all of this structure, and we have a telescope here. What you'll get is a spectrum of light. If you take, not an image, but a spectrum of the quasar. You spread out the light with a very expensive prism. Then look at how much light there is as a function of wavelength of color. On the left side is the blue end of the spectrum. On the right side is the red. Each of these pixels is telling you how much light we're seeing from the quasar as a function of those wavelengths. Each one of these dips that you see in the yellow region that's highlighted here is caused by a different cloud of hydrogen gas. Where there's a big chuck of light missing, where it goes all the way down to zero, what you're getting there is a really, really dense section of the universe that's probably home to a galaxy. 

Let me just give you one more visual to really drive home what we're doing. This is what a quasar would look like if you were right up next to it and you could measure its light spectrum. The quasar is here on the left. We're going to follow it as it's light moves through a bunch of intergalactic space, and eventually a galaxy. We'll follow through a number of clouds. You can see every time it hits a cloud we lose some of the light. It goes down to zero. If you come up on a galaxy you'll see a big effect. You'll get a big chunk out of the light where the hydrogen would usually be eating up. On the right, those are the fingerprints of heavier metals, things that are produced by stars. 

We pick those us in a separate part of the quasar spectrum and those are useful in a different way. When we see those we know we're looking through a part of the universe that's been processed by stars. The Baryon Oscillation Spectroscopic Survey is underway. This is something I've been working on for a number of years. It's a later generation of a survey you may have heard of already, the Sloan Digital Sky Survey which was the most ambitious digital survey ever tried at the time. It successfully mapped a quarter of the night's sky starting in 1999 and going on for, I think, six years. We have really extensive maps of the sky. Those maps, those images, were used to select galaxies and quasars that we would observe in the Baryon Oscillation Spectroscopic Survey. I'll call it BOSS so I don't have to keep saying that. BOSS takes spectra of all those objects. 

Not just images, but these special diagnostic plots where we see how the light is distributed as a function of color. The purpose of BOSS is to make a huge survey of quasar sight lines, and basically get a map of all the gas that we can see in the middle. Because of the fact that it's ground-based, I won't go into too much detail, but that means we're sensitive to only a special time in the universe if we're observing this kind of hydrogen absorption. The BOSS survey is particularly looking at the universe as it was in gas 11 billion years ago. That gives us a nice map of the gas, but also we can use the structure and the clustering and the way that gas is clumpy and it's actual distribution, to tell us something about dark energy. That would be a talk in and of itself, but this is actually a dark energy experiment which has been pretty successful. I'll talk about the result in a couple of slides. Just to give you a little bit more background, the BOSS survey is located in Apache Point which is in New Mexico. It's an observatory with multiple telescopes, but this particular telescope was built just for the Sloan Digital Sky Survey and it only works for the survey. So it's every night observing, not just for BOSS, but for other sub-surveys that are contained within the Sloan. 

By the end we'll have mapped a quarter of the night's sky and obtained spectra for one million galaxies and 200,000 quasars. To give you some idea, maybe 10 or 15 years ago we knew of hundreds of quasars. So having 200,000 is just unprecedented in terms of the number of probes that we have of the distant universe. This is just some of the collaboration, which includes 48 different universities worldwide. I don't think everyone would fit in this picture if I could actually include everyone, but there are a couple hundred people that are involved in this effort. These are just some of them. These, I should say, are the astronomers. But the real astronomers, people who know how to use telescopes, work at the site. These are the people doing the lion's share of the work. The real trick with the BOSS survey is that the spectragraph is actually built-- It can take 1,000 different fibers at once, each taking a spectrum of a different object. 

I don't know if you can see this, but this is one of the recently retired plates. The data that was taken with this plate is not even published yet. It becomes public later this month. You're seeing it first here. Each one of these holes would line up on the sky with a galaxy or a quasar, or maybe a star that we're interested in. Then every single one gets own fiber optic cable. These people plug them all by hand in the way that you can see there on the right. 1,000 for every plate and multiple plates per night. They're very, very busy. In fact, we've had such great weather and such long nights over the winter that they set a record obtaining the greatest number of individual spectra ever taken in a month, 103,000 just in March. 

It's crazy. We're really fortunate that the survey has worked to do what we wanted it to do in terms of testing the structure of the gas, and measuring the expansion of the universe at the particular point in time. What you're seeing here, and I maybe haven't explained it properly, this is just supposed to give you an idea of what a chunk of what the BOSS survey would look like. Those red dots are distant quasars. They're of course not all at the same distance in reality, but they're collapsed for the sake of the animation. You can see those blue lines probing through the purple gas in the middle, which would be hydrogen and large scale structure. Those are all moving towards the telescope. We're getting measurements of those beams of light for all of 160,000 very distinct quasars in 200,000 total quasars. For the first 45,000, as you'd hope, it was enough to make a first stab at the measurements everyone was interested in. What we're seeing here is a map. 

This is just a tiny slice of the data. Imagine that the telescope is right here at the very bottom of what we call a pie diagram. As you move out you can see each one of those black dots is a separate galaxy, just one in the million that we've obtained spectra for. You can kind of squint your eyes and see, maybe there's a web there too. There really is. They're probing that same filamentary structure that I showed you earlier. Now if you move out to where the quasars kick in at 11 billion light years away, what you're seeing is an actual map of the distant universe as it looks in hydrogen gas. That's most of what it is anyway. The red spots are areas of high density of gas and the blue are regions of low density gas. It may not look too spectacular even when you zoom in on it. 

Keep in mind, this is the first time we've ever made a real, legit map of the universe as it looked at this early age. If we go back to this animation, I can actually stop it at where we're looking when we look at the gas. You that see that there are some galaxies that are already forming. Those white spots where star formation has already happened and it's heating the gas and driving it out. But for the vast majority of the region you're still seeing a dominant structure that's caught up in those gaseous filaments. I told you the point of the survey was to measure dark energy and that's what we've done. I won't go into it in detail because there are even astronomers who have trouble wrapping their mind around this, but if I can just convince you that we measure the structure in such a way that it's quantitative. We can make a measurement of the way the gas clusters and how clumpy it is, and on what scales it's more clumpy than others. What that gives us a clue to is the rate of expansion of the universe. Hubble's known for awhile, and we've all known along with him, that the universe is expanding. 

In recent times, it was kind of a surprise in the last 20 years, we've learned that not only is it expanding, but it's accelerating in its expansion. For some reason things are speeding up. It's expanding faster now than it ever has. That won a Nobel Prize. It's pretty well established. This white line that you're seeing here that kind of curves is how the rate of expansion should change as a factor of time. On the right, that's today on the dotted line, it's constantly moving into the future. If you trace back in time, going to the left, you can see how we're fitting through other measurements of what the expansion rate of the universe would be at those different times. The red dot is exactly what we've measured using the structure that we see in the hydrogen at 11 billion years ago. It lies right on the curve. 

The curve is not a fit to the data. The curve is what you expect from theory based on what we think we know about the way dark energy acts on the expansion history of the universe. It's so far looking very, very good. No surprises, unfortunately I think, for some physicists. I think we're closing the arguments in any case. Something I'm more interested in-- I wouldn't consider myself a cosmologist. I like to use quasars for something else. I like to look at the dense parts of the universe, things like galaxies, and see what they're doing. Quasars are actually a perfect tool for that too. Galaxies, although we can see them in their light when they're close to us, they become very, very faint when you get far away. 

In fact, you can see that with this pie diagram. Very quickly we run out of the ability to see those distinct galaxies even if they're extremely massive and extremely luminous. What quasars allow us to do is to pick up gas that is nearby to a galaxy that maybe we can't otherwise see. From that you can, just from the spectrum of the quasar, learn a lot about the physical environment of the gas. The chemical composition, you know, what it's made of, what atoms are in that gas, what it's temperature and density are. Or maybe if you know that you're close to a galaxy, maybe you can use it to better understand outflows, winds that are being driven from the explosions of stars dying. You can even learn a little bit about dark matter. I could convince you of that, but that's an entire different talk also. I'll just go on to the exciting stuff. So galaxy evolution, a whole different topic from cosmology, but I think equally exciting right now. 

A lot of progress is being made. The fundamental question in galaxy evolution is why modern galaxies look like this. If you look at galaxies in the local universe at our day and age, they're close enough that the light is relatively modern coming from them. You see they're basically two kinds of galaxies, these really blue, spirally, star-forming galaxies and then these other ellipticals that look pretty boring, I think. These are lovingly referred to as red and dead. We call them red and dead because stars when they're born are blue and very hot and very young. Over the course of their lifetime they cool down and become more red. When you're looking at a galaxy that looks like the one on the left, you can tell there's not a lot of young stars in that galaxy. If you look at the one on the right you might notice something else. You'll see those dust lanes, like what you see in the Milky Way if you look out at night. 

There's some spots that look dark. That's just dense gas and dust along the line of sight. That gas and dust is really critical to forming stars. For some reason we don't see it so much in these massive, dead galaxies. It's maybe been blown out. This is one of the ideas. But something happened when the universe was-- Well, about six billion years ago something put this dichotomy, you know, these two populations of galaxies into place. In fact, if you look into the more distant and earlier universe, like in this zoom in on the Hubble ultra deep field, you're looking at galaxies that are about ten billion years old. They look very different. They look very diverse. 

They're all different colors. Some of them you can see, just in the center and towards the bottom right, there are galaxies that are actually interacting with each other. That's what's producing those tails. There's gravity from all those galaxies pulling on the others. They're frequently merging. They're rapidly evolving and forming stars. It's very different from what we see locally. If you make measurements, this is I think the only plot I'm putting in this whole talk. What's shown here are a huge combination of measurements by a number of groups which have independently measured how fast stars are being formed if you averaged across the entire universe as a function of time. In this plot, unlike the last one I showed you, the current time is on the far left. 

If you start from the right, that's a very early time. It's like 10 billion years ago, 12 billion years ago. You can see that the universe is really gearing up. It's forming more and more stars more quickly, and then when it's about middle-aged it kind of sputters. Something goes wrong and we stop making stars at the rates that we were making them before. We really want to understand why galaxies start to die. Why do they stop forming stars about six billion years ago. I had a spoiler alert earlier in the talk. Gas is key to these things. You need cold gas to form stars. 

This is something you may also recognize. This is the Eagle Nebula as viewed by the Hubble Space Telescope. Those are huge, millions of light years long columns of gas and dust. They are actually stellar nurseries. There are brand new stars being born right in the middle of those clouds. They wouldn't be able to be born were it not for the fact that those clouds are very cold and allowed to become very dense. Anything that's happening in a galaxy to drive the gas out or to heat it up, or both, will ultimately cause the galaxy to die. This is what a galaxy like our Milky Way would look like in terms of it's gas life. You have a lot of things going on. You have clouds coming in from intergalactic space. 

You have gas being driven out by stars that are going off in supernovae. That produces this massive explosion, right? And it drives the gas out. If the galaxy is sufficiently massive then what will happen to that gas as it's being driven out is that eventually it will slow down. It's being pulled back in by the gravity of that massive galaxy. In that way, a lot of the gas can be recycled. You get gas that's no longer pristine because it's been in a star, it's been enriched with metals. That will eventually fall back down and maybe start forming stars again. So there's a lot going on in our own Milky Way. If you look at a converse galaxy you might see something that's forming stars so quickly, or it's so low mass, that the gas it's driving out in big winds from all of these supernovae, is not ever going to come back. We think that that happens a lot in the early universe. 

The galaxies aren't very massive, stars are very, very big, and they die very, very violently. But there may also be galaxies as recently as six billion years ago that are doing this. We know of some galaxies in our local universe that are star-bursting, they're driving out gas so fast that it's probably ultimately starving that galaxy. Despite the fact that they're rapidly forming stars, they're very soon going to be starved of that gas. Again, this is something that I'm really interested in working on now. When you look at a background quasar you can pick up all of the gas as a shadow on that light and you can start to really understand what's going on around these galaxies. The type of fingerprint that I like to use, and most people who are working on this right now use, is magnesium. There's nothing special about it at all, it's just that stars make a lot of it. You see it frequently all over the universe where gas has been processed by stars. It also makes a really convenient, beautiful set of two lines with a difference in wavelength that we can pick up really easily. 

They're very obvious when you see them. It means for sure looking at gas that's been processed through a star, and probably blown out. They're also really easy to see in optical wavelengths. That's what the Sloan Telescope works in, just visible light that you would see with your eye. Magnesium at those wavelengths is seen at very large distances. You can use it to pick up on galaxies that are getting to the edge of what we can see, and going well beyond. I've actually "mined" the magnesium out of all the quasars in the Sloan. This is going to give you just a really quick visualization of how big that data set looks like. What's shown here are galaxies in the Sloan Survey. We're rotating around the center which is where we are on Earth. 

This is the northern sky, and then there's a little bit of southern sky that we pick up too. All of these green lines are those beams of light to distant quasars. They all converge at the central point which is we are. They look out into the universe along all of the region of sky that we've surveyed. All the blue dots along those beams of light are individual detections of magnesium that we've been able to extract because we see that nice feature of two lines really clearly separated. Because we know what the pattern looks like, we can see where it is on the spectrum and read off the red shift. Of course, the red shift gives us the three-dimensional aspect. We know where it is on the sky because the quasar is there, then we know how far away it is based on how much those two lines have been shifted away. That's what allows us to make these beautiful three-dimensional maps. When it gets really useful is when you can start comparing those detections to the actual galaxies that are producing them. 

Getting the detections is one thing. It tells you a little bit about how gas has been enriched since the early universe. But it doesn't really tell you much about the galaxies themselves. The galaxies that produce them, of course, are very faint. We're starting out at a pretty large distance because of where we detect the magnesium to begin with. Finding the galaxies takes a more sensitive telescope. I've been using the Hubble Space Telescope recently. This is showing you as image taken with the Hubble Space Telescope that it's centered on a quasar labeled QSO, Quasi- Stellar Object, which is a historical naming convention. What you see are a number of galaxies labeled one through four which are nearby. The little inset shows you what this would look like if you used the most powerful ground-based telescope. 

That's taken with the Keck Telescope in Hawaii. You can see, you really can't make out those little, bitty galaxies surrounding it very well at all. In fact, one of them is completely smudged out by the quasar. With new upgraded instruments on the Hubble Space Telescope which were put in by astronauts about four years ago, we have improved sensitivity, very high resolution. We can see in the infrared which allows us to see things that are red shifted even further away. That allows us to really chase down these extremely distant galaxies. That's what I've been working on, comparing those to the gas we see. This is just-- I guess I won't explain this too much. You can tell, using different instruments on Hubble you can get different information. That can tell you the red shift, basically how far away the galaxy is. 

You can get its shape just from the normal images you see on the top. You can measure the size by a combination of knowing how far away it is and how big it looks in the image. Then, using the spectrum which is shown in the blue, you can look at those emission lines. I won't go into it, but this can tell you how quickly those galaxies are forming stars. That tells us a lot about the health of the galaxy. Then using things like the inclination of the galaxy with respect to our quasar where we're seeing the gas, you can start to actually make a picture of how the gas must be outflowing from the galaxy. The most recent result is that it looks like earlier in time galaxies are pretty disordered, Their outflows are kind of all over the place. They're rapidly forming stars and they're very low mass. As you evolve towards the modern universe, when it's about middle-aged and onward. You see galaxies are pretty well formed into things that look like spirals. 

When spirals are forming stars the winds that they blow out are very well columnated. This all helps us really better understand the models that we've been working on with theorists, which is exciting. I'll just summarize it here. These new large quasar surveys really are of unprecedented size, and they're allowing us to make beautiful pencil-beam surveys of the gas distribution in the distant universe. They're key to understanding the evolving chemical history of the universe, and also providing insight into galaxy evolution. And of course, everything is better when you put the Hubble on it too. This is a big bonus. When you take those huge surveys you can pick out the most interesting quasars out of all 200,000, and point to Hubble at those. Everyone love a really efficient use of Hubble time. Results for that will be forthcoming. 

I'm happy to take any questions, and thanks again. [applause]