 Greetings, and welcome to the Introduction to Astronomy. In this video we are going to talk about galaxy evolution and mergers, and how galaxies change over time. So let's go ahead and get started here, and what we know is that how we can observe changes in galaxies, well one of the big problems in observing changes is something similar to what we had when we looked at stars. All of the changes occur far too slow to be observed over a human lifetime. However, we do also know that because light travels at a finite speed we get to see galaxies at different stages in the past, meaning that we can look at a galaxy 12 billion light years away and see it as it was when the universe was only 2 billion years old. This allows us to study the evolution of galaxies over time, so we can't follow any one galaxy but we can look at distant galaxies at various stages of their evolution. So what do we see? Well we find first of all the spectra of galaxies that we look at, and one of the problems is that it is difficult to get the spectrum of a galaxy because the galaxies are so faint. It makes them very hard to be able to get enough light to get a nice clear spectrum of. However, we can observe the colors of galaxies, and we do know that by looking at the colors that wherever the galaxy is, a blue galaxy must have had recent star formation and that a red galaxy will not have had recent star formation. So just looking at this image we can see that there are very red galaxies that have very little star formation recently, and bluer galaxies that have had much more star formation in the recent past from when they formed. So what that tells us, and what we also learn is about the early generations of stars, and it tells us that heavy elements formed in the first billion years of the history of the universe. It did not take a long time for heavier elements to develop. We know that they did not form in the Big Bang, but we also know that they formed relatively quickly once they did. And we know this because even very distant galaxies show lines of heavy elements, again heavy elements, anything that is not hydrogen or helium, and these were not created in the Big Bang, so we know that even these very distant galaxies had time for some heavier elements to form. So how do galaxies evolve? Well, in a very different manner from stars. What were these early galaxies like? What did galaxies look like billions of years ago? We have an advantage in that we can see them. We get to see what galaxies looked like a billion and five billion and ten billion and thirteen billion years ago. And what we find is that these galaxies do not look like the galaxies today, and what we mean is those is the nearby galaxies. They have changed. Galaxies very early in the history of the universe were much smaller than the galaxies we see nearby, and they were bluer. Now remember, when we look at those galaxies that are nearby, we are seeing them pretty much as they are today. And when we look back many billions of years, we are seeing galaxies as they were long ago. So over those billions of years, galaxies have changed from things that were very small and very blue to galaxies like the spiral and elliptical galaxies that we see today. We also see that galaxies were more chaotic, showed less structure very early on in the history of the universe. And the Hubble classification that we use, differentiating between spiral and elliptical galaxies, only works for the galaxies today. So what do we know about the mergers of galaxies? How do galaxies merge together? And we have seen that this does occur and has occurred very often in the past. We can look at galaxies and see multiple examples, as shown here, of galaxies that are colliding. And these are certainly not all of them. Very many galaxies collide together. And that includes even 5% of galaxies close to us, but these mergers were very important early in the past. And this is the primary way that galaxies evolve over time is through these mergers. So galaxies collide together and may merge together to form a larger galaxy. And that would explain why distant galaxies were much smaller, because they had not yet had time to merge with other galaxies. Now we talk about galaxies colliding. Why do we not talk about stars colliding? Well, galaxies are very large compared to the distances between them. And that means that they will eventually collide together. For example, you could put about 20 or 30 galaxies like our Milky Way between us and the Andromeda Galaxy, the nearest large galaxy to us. In terms of stars, how many billions of stars could we suns could we put between the sun and the Alpha Centauri, the next large similar star, to our sun? So we can put lots and lots and lots of stars there, but only a couple of dozen galaxies between us and the nearest galaxy. And that means that galaxies are much more likely to collide in a galaxy collision. And the stars are more likely to just pass right by each other, never coming close enough to collide. So when we look at galaxy collisions, what do we mean? Well, a galaxy collision is not like an ordinary collision. That you think about if you think of a collision of cars or anything else, galaxy collisions are not like that. They can take hundreds of millions of years, very long time for them to occur. And the individual stars do not collide. What does collide is examples of the gas clouds. That will enhance star formation. So this gives rise to what we call a star burst galaxy, in which galaxies will have a burst of star formation because their gas clouds collided together, enhancing the rate at which the stars form, and giving them a great burst of star formation. We will also see streams of material that will be expelled as the gravitational fields interact. And that serves to distort the shape of the galaxies. So galaxies that are colliding like these do not look like the ordinary spiral and elliptical galaxies that we are used to looking at. Now, galaxies can collide, they can merge, they can also cannibalize each other. And many times when galaxies collide together, it will take that collision and become a merger. And what we say is that large galaxies will cannibalize smaller galaxies, incorporating them into themselves. Similar sized galaxies will merge together. So as we know, the Andromeda galaxy and our galaxy are on a collision course, they will eventually merge together to form one larger galaxy. However, our galaxy also has smaller galaxies orbiting it, and those galaxies over time will eventually be cannibalized by our galaxy and pulled in and become a part of our galaxy. We see many galaxies with significant signs of this having occurred in the past. And in fact, we can look at a number of galaxies and see, and we can see that we sometimes see within a galaxy multiple nuclei, meaning that we have the main nucleus of the galaxy and then maybe one from a smaller galaxy that it has merged with or cannibalized. So we can see that occurring from previous mergers that have occurred. So what kind of timeline do we have for this? Well, galaxy collisions today are very rare, about 5% of galaxies, so not completely uncommon. But most galaxies are not in the process of collision. However, they were much more common in the past. Distant galaxy clusters show multiple collisions. So when we look at these distant galaxies, we see lots of galaxies that are colliding far more than the 5% that we count today. We see multiple collisions, sometimes multiple galaxies colliding together at the same time. So what this has done is that we had has built up galaxies, the ones we see today, the great spirals, the great ellipticals, all formed from material that we saw that formed from these smaller galaxies that existed long ago. They slowly built up each other over the time of collision and cannibalization, making the much larger galaxies that we see today. Now, how does this relate to active galactic nuclei that we have looked at? Well, those collisions are what feed the black hole. That is, feeding the black hole, making it an active galactic nucleus, or AGN. These AGN were much more common in the early universe. Why? Because collisions were more common. There were more chances to feed the black hole at the center back 12 billion years ago than there were just a billion or two years ago. There were far more collisions. And you could collide multiple times, giving that black hole significant sources of fuel, causing it to give off all of this excess energy. And what we find also is that they might have had an effect on the formation in the remainder of the galaxy. Forming an active galactic nucleus with its core and the jets may also impact star formation in the remainder of the galaxy. So it might have led to the galaxies that we see today, not only by the collisions, but also by impacting the star formation rates that we see. So let's finish up here, as we do with our summary. And what we find is that galaxy evolution is driven by collisions. Those collisions lead to either mergers of galaxies or cannibalization of galaxies by one another. Similar sized galaxies will merge together to form a larger galaxy, and smaller galaxies can be cannibalized by those larger galaxies. These collisions take a long time to occur, and the individual stars do not collide when galaxies collide. These galaxy collisions were much more common in the early history of the universe and are relatively rare today. So that concludes this lecture on galaxy collisions and mergers. 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.