 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to discuss supermassive black holes, the gigantic black holes that exist at the centers of galaxies and that are the power source for quasars and other types of active galaxies. So let's go ahead and get started here. And what we see is that some of the, just review some of the properties of quasars and other AGNs or active galactic nuclei, first of all we know that they are extremely luminous. They are brighter than an entire galaxy, so extremely bright and again luminous means not only visible light but also other forms of electromagnetic radiation as well. We know that they are very small about the size of our solar system, so they have to be extremely compact. They emit jets of material in narrow beams that are very close to the speed of light, so another something else that needs a very high energy source. So this much energy emitted from such a small region needs an energy source much more powerful than something like the nuclear fusion that powers the stars. And we also see that quasars were much more common in the early history of the universe, that's when they were around, than they are today, so now they're not around. So let's look for some observational evidence for these supermassive black holes. Do they really exist? And some things that we see, we have some indirect evidence, first of all, we have a large mass in a small volume, what else can it be? We have very short time period variations in brightnesses, again meaning that it has to be very small. These are both indirect evidence for black holes, that it is a supermassive black hole. But to really determine that we have to be able to determine the mass. So we need Kepler's third law, remember our good way of being able to determine the mass. And we can look near our center of our galaxy, near the center of the Milky Way. And we look at some of the orbits of various stars that are very close. And the central object is right down here at the center. And for example, stars like S14 follow very elliptical orbits. They come out and they whip right down and they turn around and go right back out the opposite direction. We can use the orbits of S14 and any of these other stars. We can figure out their semi-major axis A, and we can figure out their period P. And the mass of that central object is going to be equal to the cube of the semi-major axis, dividing by the square of the period. So that will tell us the mass of that central object. When we do that, we find 4.1 million solar masses as a good estimate for that central object. The size is about the diameter of the orbit of Neptune. So it is about the size of our solar system. That would be about 60 astronomical units. That's the planets there, from Neptune's orbit from one side to the other. Is that big, would be that far apart? And that is one way that we can use to be able to determine the masses of these black holes and get some observational evidence that they exist. Now another way to do this is to look at the motions at the center of a galaxy. And this is the elliptical galaxy known as M87. And we can do something similar. We can look at the gases very close to the center. This isn't the galaxy itself. We're looking at just the very central portion here. But if we take a spectrum on this side, we get a red peak. It is red shifted. If we take a spectrum over here, it is blue shifted. And what that means is that it is rotating very rapidly, swirling around that central object. And again, we can use those motions to be able to determine the mass of that central object and find that it is a supermassive black hole, much larger than the 4.1 million solar mass black hole at the center of our galaxy. Now let's try to look a little bit and understand how this works. How can we produce energy near a black hole? Black holes themselves can't give off anything. As we know, nothing can escape from a black hole. Once you cross the event horizon, you are done. So how can this black hole produce energy at all? Well, energy isn't produced by the black hole itself. It's produced in what we call the accretion disk. And that is this disk of material around the black hole. So the event horizon and the black hole itself are right there at the very center. But material spiraling around it can give off energy. It has not yet crossed the event horizon. So material, for example here, may be taken from a star, spirals in and around the black hole. And it accelerates to very high speeds and heats to very high temperatures, hundreds of thousands of degrees. That means that we can convert in this 10% or even more of this mass is converted directly to energy. So we can convert a large portion of that to compare nuclear fusion and a star is less than 1% conversion. So this is far more efficient conversion, meaning that we can convert a little bit of mass to a lot of energy and that can be what powers this black hole and powers the quasars. So let's go ahead and look at what else we have here. And we have that we can also form jets of material. So how can we get a jet of material? We have material spiraling in and there are intense pressures trying to push material outward. And that is trying to push material away. Now it can't move in all directions. As we have the black hole at the center here, we also have material, so material trying to exit out in the same direction that material is spiraling in in the accretion disk will not work. It can't go back out the way it came in. The only way it can is through these jets. And you can get a jet of material going out around the poles away from the accretion disks. This will depend on how thick the disk is. We can see here a very thin disk will not collimate the material very well and it will spread out into a much wider jet. Whereas a much thicker disk will be able to give us the jets that we actually see in a lot of these active galaxies. And one example that we can look at here is this is an example of M87 and this is a large elliptical galaxy and it has a large jet coming out from it. So again here we are looking at the very central portion, the center part of the galaxy here. And we can see this very highly collimated jet coming out and slowly spreading out as it moves out into space. So this would be seeing one side of that material. But we see these very often, so this type of outflow is very common in astronomy. Now when we go back and look at quasars they are a way to be able to study the early history of the universe. Remember when we look in astronomy the more distant an object is the longer ago we see it. So a quasar that is 10 billion light years away is seen not as it is today but as it was 10 billion years ago. So what does it look like today? And this is a really good question. What does it look like today? The number of quasars has decreased over time having reached a peak here, a billion or two years after the Big Bang and their number has declined very quickly to the point where after about 10 billion years ago we don't see any. There are no more quasars. Here we had tens of thousands of them early on in the history of the universe. Here we still had hundreds of them. By 10 billion years we were down to just the last handful. So what that means is that long ago these quasars had fuel for their engines. They were able to feed the central black hole and cause it to give off this energy. Now black holes just don't go away so the black holes are still here today at the centers of these galaxies possibly powering active galaxies today just not as intense as the quasar era. Right now a lot of them are just dormant. So where do we get this fuel? Where do quasars get this fuel to be able to get all of this energy? Well, we can get the occasional passing star or gas cloud meaning that a star passing too close to the black hole we saw some of those that orbited if one were to get too close the tidal forces would rip the star apart. Remember how tidal forces work. The gravitational force on one side is stronger than the gravitational force on the other and if it's close enough the gravitational force here can be enough to actually pull the star apart and rip it apart and then it will be absorbed into the accretion disk and finally give off energy. So that will give off all that energy that we see for these active galaxies and quasars. Another way to do this is through collisions of galaxies. This is another way to provide material for the black hole. When galaxies collide they pretty much pass through each other but if something passed close to the black hole it could be collected and could be added into its accretion disk meaning that it is giving off more and more energy in the past and we also know that collisions were much more frequent in the past than they are today and we can see that in a couple of images here when we look at some of these very distant galaxies so when we look at some very distant galaxies a lot of them are very distorted and disturbed have tidal features here and here very distorted galaxies they don't look like the nice pretty spiral and elliptical galaxies that we talk about today. So galaxies billions of years ago ten billion years ago did not look the same and as they collided material would be added to feed the central black holes of these galaxies possibly producing the quasars. So let's finish up as we do with our summary here and what we've looked at in this lecture first of all active galaxies are believed to be powered by a supermassive black hole at their core this is what is producing all of the energy we can infer the existence of these black holes because of the amount of energy being produced from such a small area and the motions of stars near the center and studying these quasars tells us about the early history of the universe since this is when they were most active and common since for the first few billion years they were around but for the last ten billion we do not have any quasars so nothing has been able to feed a massive black hole enough to make a quasar since the universe expanded up to a certain size and material started to spread out more so that concludes this lecture on supermassive black holes we'll be back again next time for another topic in astronomy so until then have a great day everyone and I will see you in class