01:750:109 Chapter Notes - Chapter 12.3: Galaxy Formation And Evolution, Before Present
Observing Galaxy Evolution
●Relationship among distance, cosmological redshift, and lookback time is helpful to modern
astronomers bc it means that powerful telescopes can be used like time machines to observe
life histories of galaxies
●Overview of how the ability to look back through time enables astronomers to test models of
galaxy evolution
What do we see when we look back through time?
Observing Galaxies at Different Ages
●A galaxy's lookback time is linked directly to its age
●Older and younger galaxies are visibly different
○Youngest galaxies look more disrupted, suggesting that they are in the midst of colliding
and merging
○Mature galaxies generally looked more organized
Models of Galaxy Evolution
●Assumptions of galaxies building up gradually over time through collisions and mergers with
other galaxies and protogalactic clouds:
○Hydrogen and helium gas filled all of space more or less uniformly when the universe
was very young, in the first million years after its birth
○The distribution of matter in the universe was not perfectly uniform-- certain regions of
the universe started out ever so slightly denser than others
●Regions of enhanced density originally expanded along w the rest of the universe but the slightly
greater pull of gravity in these regions gradually slowed their expansion
○Within about a billion yrs, the expansion of these denser regions halted and reversed,
and the material within them began to contract into protogalactic clouds
, much like
those that eventually formed our Milky Way
●Early in the universe's history, when all the galaxies were much closer together, collisions were
frequent but later in time, after the average distances between galaxies grew, collisions became
less common, allowing galaxies to settle into the spiral and elliptical shapes typical on the
present-day universe
Document Summary
Relationship among distance, cosmological redshift, and lookback time is helpful to modern astronomers bc it means that powerful telescopes can be used like time machines to observe life histories of galaxies. Overview of how the ability to look back through time enables astronomers to test models of galaxy evolution. A galaxy"s lookback time is linked directly to its age. Older and younger galaxies are visibly different. Youngest galaxies look more disrupted, suggesting that they are in the midst of colliding and merging. Assumptions of galaxies building up gradually over time through collisions and mergers with other galaxies and protogalactic clouds: Hydrogen and helium gas filled all of space more or less uniformly when the universe was very young, in the first million years after its birth. The distribution of matter in the universe was not perfectly uniform-- certain regions of the universe started out ever so slightly denser than others.
