Chapter 15 Galaxies and the Foundation of Modern Cosmology Copyright 2009 Pearson Education Inc What is Hubble s law Copyright 2009 Pearson Education Inc What have we learned What is Hubble s law The faster a galaxy is moving away from us the greater its distance velocity H0 distance Copyright 2009 Pearson Education Inc What have we learned How do distance measurements tell us the age of the universe Measuring a galaxy s distance and speed allows us to figure out how long the galaxy took to reach its current distance Measuring Hubble s constant tells us that amount of time about 14 billion years Copyright 2009 Pearson Education Inc 15 3 Galaxy Evolution Our goals for learning How do we observe the life histories of galaxies How did galaxies form Why do galaxies differ Copyright 2009 Pearson Education Inc How did galaxies form Copyright 2009 Pearson Education Inc Our best models for galaxy formation assume that Matter originally filled all of space almost uniformly Gravity of denser regions pulled in surrounding matter Copyright 2009 Pearson Education Inc Denser regions contracted forming protogalactic clouds H and He gases in these clouds formed the first stars Copyright 2009 Pearson Education Inc Supernova explosions from the first stars kept much of the gas from forming stars Leftover gas settled into a spinning disk Conservation of angular momentum Copyright 2009 Pearson Education Inc M101 M87 But why do some galaxies end up looking so different Copyright 2009 Pearson Education Inc Why do galaxies differ Copyright 2009 Pearson Education Inc Why don t all galaxies have similar disks Copyright 2009 Pearson Education Inc Conditions in Protogalactic Cloud Spin Initial angular momentum of protogalactic cloud could determine the size of the resulting disk Copyright 2009 Pearson Education Inc Conditions in Protogalactic Cloud Density Elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk Copyright 2009 Pearson Education Inc Distant Red Ellipticals Observations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe Copyright 2009 Pearson Education Inc We must also consider the effects of collisions Copyright 2009 Pearson Education Inc Collisions were much more likely early in time because galaxies were closer together Copyright 2009 Pearson Education Inc Many of the galaxies we see at great distances and early times indeed look violently disturbed Copyright 2009 Pearson Education Inc The collisions we observe nearby trigger bursts of star formation Copyright 2009 Pearson Education Inc Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical Copyright 2009 Pearson Education Inc Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical Galaxy Collision Animation Copyright 2009 Pearson Education Inc Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together Copyright 2009 Pearson Education Inc Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies Copyright 2009 Pearson Education Inc Starburst galaxies are forming stars so quickly that they will use up all their gas in less than a billion years Copyright 2009 Pearson Education Inc The intensity of supernova explosions in starburst galaxies can drive galactic winds Copyright 2009 Pearson Education Inc X ray image The intensity of supernova explosions in starburst galaxies can drive galactic winds Copyright 2009 Pearson Education Inc What have we learned How do we observe the life histories of galaxies Deep observations of the universe are showing us the history of galaxies because we are seeing galaxies as they were at different ages How did galaxies form Our best models for galaxy formation assume that gravity made galaxies out of regions of the early universe that were slightly denser than their surroundings Copyright 2009 Pearson Education Inc What have we learned Why do galaxies differ Some of the differences between galaxies may arise from the conditions in their protogalactic clouds Collisions can also play a major role because they can transform two spiral galaxies into an elliptical galaxy Copyright 2009 Pearson Education Inc 15 4 Quasars and Other Active Galactic Nuclei Our goals for learning What are quasars What is the power source for quasars and other active galactic nuclei Do supermassive black holes really exist Copyright 2009 Pearson Education Inc What are quasars Copyright 2009 Pearson Education Inc If the center of a galaxy is unusually bright we call it an active galactic nucleus Quasars are the most luminous examples Active Nucleus in M87 Copyright 2009 Pearson Education Inc The highly redshifted spectra of quasars indicate large distances From brightness and distance we find that luminosities of some quasars are 1012LSun Variability shows that all this energy comes from a region smaller than the solar system Copyright 2009 Pearson Education Inc Thought Question What can you conclude from the fact that quasars usually have very large redshifts A B C D They are generally very distant They were more common early in time Galaxy collisions might turn them on Nearby galaxies might hold dead quasars Copyright 2009 Pearson Education Inc Thought Question What can you conclude from the fact that quasars usually have very large redshifts A B C D They are generally very distant They were more common early in time Galaxy collisions might turn them on Nearby galaxies might hold dead quasars All of the above Copyright 2009 Pearson Education Inc Galaxies around quasars sometimes appear disturbed by collisions Copyright 2009 Pearson Education Inc Characteristics of Active Galaxies Luminosity can be enormous 1012LSun Luminosity can rapidly vary comes from a space smaller than solar system They emit energy over a wide range of wavelengths contain matter with wide temperature range Some drive jets of plasma at near light speed Copyright 2009 Pearson Education Inc What is the power source for quasars and other active galactic nuclei Copyright 2009 Pearson Education Inc The accretion of gas onto a supermassive black hole appears to be the only way to explain all the properties of quasars Copyright 2009 Pearson Education Inc Energy from a Black Hole The gravitational potential energy of matter falling into a black
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