16.3%20Structure%20Formation

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16.3 Structure Formation
Our Goals for Learning
What is the role of dark matter in galaxy
formation? What are the largest structures in
the universe?
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Clicker QuestionWhat is the role of dark matter
in galaxy formation?
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Gravity of dark matter is what caused
protogalactic clouds to contract early in time
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WIMPs (Weakly Interacting Massive Particles)
cant contract to center because they dont
radiate away their orbital energy
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Dark matter is still pulling things
together After correcting for Hubbles Law, we
can see that galaxies are flowing toward the
densest regions of space (arrows at left show
galaxies' velocities after this correction)
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What are the largest structures in the universe?
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Maps of galaxy positions reveal extremely large
structures superclusters and voids
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Largest scales distribution of galaxies seems
almost uniform
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Models show that gravity of dark matter pulls
mass into denser regions universe grows lumpier
with time
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Models show that gravity of dark matter pulls
mass into denser regions universe grows lumpier
with time
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Structures in galaxy maps look very similar to
the ones found in models in which dark matter is
WIMPs
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What have we learned?

• What is the role of dark matter in galaxy
  formation?
• Because most of a galaxys mass is in the form of
  dark matter, the gravity of that dark matter is
  probably what formed protogalactic clouds and
  then galaxies from slight density enhancements in
  the early universe.

• What are the largest structures in the
  universe?
• Galaxies appear to be distributed in gigantic
  chains and sheets that surround great voids.
  These giant structures trace their origin
  directly back to regions of slightly enhanced
  density early in time.

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Activity 46 part II, pages 172-175
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4A What does a solid-body rotation curve look
like?

1. Rises quickly (within 5 kpc), then levels off
2. Straight line from lower left to upper right

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4B Which of the curves in Fig. 2 does the Milky
Ways rotation curve look like?

1. Point Mass
2. Disk Mass
3. Solid Mass
4. None of the above

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5A If solid-body rotation is depicted using
arrows on a disk (a la Fig. 3), then

1. The shortest arrow is at the center, and the
   longest at the edge
2. The longest arrow is at the center, and the
   shortest at the edge
3. All the arrows are the same length

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6B The Sun is 7.5 kpc from the center of the
Galaxy. The rotational velocity of the Sun
around the center of the Galaxy is

1. 250 km/s
2. 210 km/s
3. 190 km/s
4. None of the above

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7ABC Which rotation curve in Fig. 2 is most
similar to the real curve in Fig. 6?

1. Point Mass
2. Disk Mass
3. Solid Mass

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7D Whats wrong with Xaviers reasoning?

1. Spiral arms do wind up or unwind
2. Spiral arms are made of gas and dust its the
   stars that orbit as a solid object
3. The matter in spiral arms doesnt stay there
   the arms are traffic jams where different stars
   and gas clouds keep piling up

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7F It appears there is a major component of mass
that we cannot see, called

1. WIMPs
2. MACHOs
3. Dark matter
4. Dark energy
5. Baryonic matter