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Lecture 13

Lecture 13-Chapter 21-Galaxy Evolution

Astronomy & Astrophysics
Course Code
Stefan Mochnacki

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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\¶VW\SHPLJKWEHGHWHUPLQHGE\WKHVSLQRIWKH
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: DJDOD[\¶VW\SHPLJKWEHGHWHUPLQHGE\WKHGHQVLW\RIWKe
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
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
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
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