Lecture 20
April 22, 2008
Formation and Evolution of Galaxies
Key Questions:
- Why are there different galaxy types? (nature vs nurture)
- How did galaxies form and evolve over time?
- How can astronomers "see" the birth and growth of galaxies over time?
- What happens when galaxies collide?
Spirals vs. Ellipticals
|
Spiral Galaxies |
Elliptical Galaxies |
| Morphology |
disk-like, spiral arms |
elliptical, smooth and featureless |
| Stars |
young and old |
old stars |
| Gas and Dust |
lots and cold |
little and hot? |
| Kinematics |
rotation and random motions |
random motions |
| Environment |
low density (e.g. field) |
high density (e.g.clusters) |
Galaxy Formation Scenarios
1. "Monolithic Collapse" Scenario
A. Angular Momentum Difference
Difference in angular momentum content may account for the differences
between spiral and elliptical galaxies. Protogalactic gas clouds born with
more angular momentum may have formed a pancake-like disk first before most of
stars were formed.
B. Density/Cooling Difference
Difference in gas density (denser gas could radiate away energy
more quickly and condense into stars) may account for the differences
between spiral and elliptical galaxies. Protogalactic gas clouds with a
denser, clumpy structures may have formed many of its stars before
the gravitational collapse formed a pancake-like disk of gas.
Review: MW formation and evolution - galaxy formation and evolution is far more complex because galaxies do not form in isolation.
2. "Hierarchical Growth" Scenario
Viewing the Young Galaxies as They Were
Galaxy Evolution by Collisions and Mergers
- Galaxy interactions or collisions are quite common, even in galaxies that seem isolated.
- Major Cosmic Train Wrecks are also seen!
- Radical transformations
- collisions and mergers involving a satellite companion
- collisions and mergers involving two large galaxies
- Interactions and
Mergers are the main drivers of the galaxy evolution?
- collisions re-arrange the distribution of stars and dark matter
- collisions re-arrange the angular momentum of stars and dark matter
- collisions re-arrange the distribution of gas, where collisions and
concentrations lead to starbursts (formation of new stars)
- collisions can heat the gas and expel some of the gas
Gas and Stars Respond Differently in Galaxy Collisions
Stars
A shotgun blast in slow motion: the buckshots are moving bunched together because they started together as a group, not necessarily because they are held together by any force. Tidally stripped material from galaxies form spatially coherent structures ("phase coherent" structures) and travel together, like these buckshots.
Gas
Unlike stars (or buckshots), collisions involving gas clouds are inelastic, sticking and falling in, piling up in the central regions of merging galaxies and subsequently fueling intense starburst activities or feeding the central black holes.
Active Galaxies: Starbrust Galaxies
How do we know galaxies are built up by collisions and mergers?
Reading assignment for next lecture: Unit 77
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