Lec27 - Chapter 21 Galaxy Evolution 21.1 Looking Back...

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Unformatted text preview: Chapter 21 Galaxy Evolution 21.1 Looking Back Through Time Our goals for learning: •  How do we observe the life histories of galaxies? •  How did galaxies form? How do we observe the life histories of galaxies? Deep observations show us very distant galaxies as they were much earlier in time (old light from young galaxies). Observing galaxies at different distances shows us how they age. How did galaxies form? We still can’t directly observe the earliest galaxies. Our best models for galaxy formation assume: •  Matter originally filled all of space almost uniformly. •  Gravity of denser regions pulled in surrounding matter. Denser regions contracted, forming protogalactic clouds. Hydrogen and helium gas in these clouds formed the first stars. Supernova explosions from the first stars kept much of the gas from forming stars. Leftover gas settled into a spinning disk due to the conservation of angular momentum. NGC 4414 M87 But why do some galaxies end up looking so different? What have we learned? •  How do we observe the life histories of galaxies? – Deep observations of the universe show 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 in the early universe that were slightly denser than their surroundings. 21.2 The Lives of Galaxies Our goals for learning: •  Why do galaxies differ? •  What are starbursts? Why do galaxies differ? Why don’t all galaxies have similar disks? Conditions in Protogalactic Cloud? Spin: The initial angular momentum of the protogalactic cloud could determine the size of the resulting disk. 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. 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. We must also consider the effects of collisions. The Antennae: Galaxies in collision Computer simulation of colliding galaxies http://www.galaxydynamics.org/spiral_metamorphosis.html Andromeda galaxy Role of Galaxy Clusters •  Galaxy clusters suggest some galaxies shaped by interactions: •  •  •  •  elliptical galaxies more common in cluster centers collisions occur more often in crowded cluster centers central dominant (CD) galaxies are gigantic ellipticals found in cluster centers grow large by consuming other galaxies •  These CD galaxies often contain tightly bound clumps of stars. •  Probably leftover cores of galaxies cannibalized by the CD. •  Some CD galaxies are more than 10 times as massive as the Milky Way. •  making them the largest galaxies in the Universe! ...
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