AST201H1 Lecture 13: Lecture 13-Chapter 21-Galaxy Evolution
4 Aug 2010
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Chapter 21 ± Galaxy Evolution
x Models for galaxy formation:
x Hydrogen and helium filled all of space more or less uniformly when the universe was
very young
x The distribution of matter in the universe was not perfectly uniform ± some regions
denser than the others.
Why Galaxies Differ
x Protogalactic Spin: a galax\¶VW\SHPLJKWEHGHWHUPLQHGE\WKHVSLQRIWKH
protogalactic cloud from which it formed.
x If the original cloud had a significant amount of angular momentum, it would have
rotated quickly as it collapsed, resulting in a spiral galaxy.
x If the protogalactic cloud had little or no angular momentum, its gas might not have
formed a disk at all, resulting in an elliptical galaxy.
x Protogalactic density: DJDOD[\¶VW\SHPLJKWEHGHWHUPLQHGE\WKHGHQVLW\RIWKe
protogalactic cloud from which it formed.
x With high gas density would have radiated energy more effectively and cooled more
quickly, allowing more rapid star formation.
x If the star formation proceeded fast enough, all the gas could have been turned into
stars before any of it had time to settle into a disk, resulting in lacking a disk, making
it an elliptical galaxy.
x A lower-density cloud would have formed stars more slowly, leaving gas to form disk
of a spiral galaxy.
Galactic Collisions
x Galaxies rarely evolve in perfect isolation.
x Collisions between galaxies are constantly occurring, in which a collision between two
spiral galaxies can create an elliptical galaxy.
x Tremendous tidal forces between the colliding galaxies tear apart the two disks,
randomizing the orbits of their stars.
Galaxies in Clusters
x Elliptical galaxies dominate the galaxy populations at the cores of dense clusters of
galaxies.
x Central dominant galaxies are gigantic elliptical galaxies that apparently grew to a
huge size by consuming other galaxies through collisions.
x They contain tightly bound clumps of stars, the center of an individual galaxy that was
swallowed by the giant.
x The process of galactic cannibalism can create central dominant galaxies more than 10
times as massive as the Milky Way.
x When a spiral galaxy cruises through the center of a galaxy with very hot gas, the hot
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x Its disk will eventually fade, while its bulge and halo will remain prominent.
x If the disk has already formed a large number of stars when its gas is stripped, the
remaining galaxy will look like a spiral galaxy without its gaseous disk, a lenticular
galaxy.
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Document Summary
N hydrogen and helium filled all of space more or less uniformly when the universe was. N the distribution of matter in the universe was not perfectly uniform some regions denser than the others. If the original cloud had a significant amount of angular momentum, it would have rotated quickly as it collapsed, resulting in a spiral galaxy. If the protogalactic cloud had little or no angular momentum, its gas might not have formed a disk at all, resulting in an elliptical galaxy. N protogalactic density: ,,,895029-0/0907230/-90/0389419e protogalactic cloud from which it formed. N with high gas density would have radiated energy more effectively and cooled more. If the star formation proceeded fast enough, all the gas could have been turned into stars before any of it had time to settle into a disk, resulting in lacking a disk, making it an elliptical galaxy.
