15.3 Galaxy Evolution

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15.3 Galaxy Evolution

• Our Goals for Learning

How do we observe the life histories of
galaxies? How did galaxies form? Why do
galaxies differ?
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How do we observe the life histories of galaxies?
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Deep observations show us very distant galaxies
as they were much earlier in time (Old light
from young galaxies)
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How did galaxies form?
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We still cant directly observe the earliest
galaxies
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• Our best models for galaxy formation assume
• Matter originally
• filled all of space
• almost uniformly
• Gravity of denser
• regions pulled in
• surrounding
• matter

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Denser regions contracted, forming protogalactic
clouds H and He gases in these clouds formed
the first stars
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Supernova explosions from first stars kept much
of the gas from forming stars Leftover gas
settled into spinning disk Conservation of
angular momentum
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NGC 4414
M87
But why do some galaxies end up looking so
different?
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Why dont all galaxies have similar disks?
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Conditions in Protogalactic Cloud?

• Spin Initial angular momentum of protogalactic
  cloud could determine size of resulting disk
• Density Elliptical galaxies could come from
  dense protogalactic clouds that were able to cool
  and form stars before gas settled into a disk

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Collisions also a factor
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Collisions were much more likely early in time,
because galaxies were closer together
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The collisions we observe nearby trigger bursts
of star formation
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Modeling such collisions on a computer shows that
two spiral galaxies can merge to make an
elliptical
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Modeling such collisions on a computer shows that
two spiral galaxies can merge to make an
elliptical
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Collisions may explain why elliptical galaxies
tend to be found where galaxies are closer
together
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Giant elliptical galaxies at the centers of
clusters seem to have consumed a number of
smaller galaxies
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Intensity of supernova explosions in starburst
galaxies can drive galactic winds
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X-ray image
Intensity of supernova explosions in starburst
galaxies can drive galactic winds
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Why do galaxies differ?

• Angular momentum may determine size of disk
• Density of protogalactic cloud may determine how
  fast a galaxy forms
• Collisions shape galaxies early on
• Mergers of small objects make halo bulge
• Mergers of larger objects make elliptical
  galaxies
• Relatively undisturbed galaxies can still have
  disks

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What have we learned?

• How do we observe the life histories of
  galaxies?
• Todays telescopes enable us to observe galaxies
  of many different ages because they are powerful
  enough to detect light from objects with lookback
  times almost as large as the age of the universe.
• We can therefore assemble family albums of
  galaxies at different distances and lookback
  times.

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What have we learned?

• How did galaxies form?
• The most successful models of galaxy formation
  assume that galaxies formed as gravity pulled
  together regions of the universe that were ever
  so slightly denser than their surroundings. Gas
  collected in protogalactic clouds, and stars
  began to form as the gas cooled.

• Why do galaxies differ?
• Differences between present-day galaxies probably
  can arise both from conditions in their
  protogalactic clouds and from collisions with
  other galaxies. Slowly rotating or high-density
  protogalactic clouds may form elliptical rather
  than spiral galaxies. Ellipticals may also form
  through the collision and merger of two spiral
  galaxies.

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• To understand the spectra of galaxies, we have to
  understand the spectra of stars.
• The spectrum of a galaxy is the sum of the
  spectra of all the stars in it (possibly modified
  by dust, and plus the spectra of all the nebulae
  in the galaxy)

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Blue light from young stars, orange light from
old stars Dust causes reddening of any light.
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Activity 28, page 95-97

• To understand the spectra of galaxies, we have to
  understand the spectra of stars.
• The spectrum of a galaxy is the sum of the
  spectra of all the stars in it (possibly modified
  by dust, and plus the spectra of all the nebulae
  in the galaxy)
• Stars have spectral types OBAFKM different
  spectral types have different properties.

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Stars of which spectral type are the hottest?

1. O
2. B
3. A
4. F
5. G
6. K
7. M

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Activity 28, page 95-97

• Stars have spectral types OBAFKM
• O stars are the hottest M stars are the coolest
• The peak emission wavelength ?max of a star (in
  nm or nanometers) tells you its temperature T in
  Kelvin. This is Wiens Law ?max(3 million)/T
• Absorption lines of neutral hydrogen (H I) and
  neutral sodium (Na I) are also shown in the seven
  simulated stellar spectra on pages 95-96
• How are those absorption lines helpful? Think
  what happens to neutral atoms as a star heats up?

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You have two blue stars. In one, the neutral
hydrogen absorption is strong. In the other,
its weak. Which star is hotter?

1. Blue star with strong neutral hydrogen
2. Blue star with weak neutral hydrogen

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Activity 28, page 95-97

• Stars have spectral types OBAFKM
• O stars are the hottest M stars are the coolest
• The peak emission wavelength ?max of a star (in
  nm or nanometers) tells you its temperature T in
  Kelvin. This is Wiens Law ?max(3 million)/T
• Absorption lines of neutral hydrogen (H I) and
  neutral sodium (Na I) are also shown in the seven
  simulated stellar spectra on pages 95-96
• Using those absorption lines and the peak
  emission wavelengths, find the seven stars
  spectral types.

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Which of the seven stars are hottest and coolest?

1. 1 is hottest, 6 is coolest
2. 2 is hottest, 6 is coolest
3. 5 is hottest, 6 is coolest
4. 1 is hottest, 4 is coolest
5. 2 is hottest, 4 is coolest
6. 5 is hottest, 4 is coolest
7. 2 and 5 tied for hottest, 6 coolest
8. 2 and 5 tied for hottest, 4 coolest

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Activity 28, page 95-97

• Stars have spectral types OBAFKM
• O stars are the hottest M stars are the coolest
• The peak emission wavelength ?max of a star (in
  nm or nanometers) tells you its temperature T in
  Kelvins This is Wiens Law ?max(3 million)/T
• Absorption lines of neutral hydrogen (H I) and
  neutral sodium (Na I) are also shown in the seven
  simulated stellar spectra on pages 129-130
• Using those absorption lines and the peak
  wavelengths of each star, answer questions 3-5.

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3. Which of the 3 students is correct?

1. Ilias
2. Ole
3. Patricio
4. Ilias and Ole
5. Ilias and Patricio
6. Ole and Patricio
7. All three
8. None of them

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4. Which star is hotter, one in which lines of He
II (ionized helium) are strong or one in which
lines of He I (neutral helium) are strong?

1. Star with He II lines
2. Star with He I lines

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5. What is the spectral type of this real star?

1. O
2. B
3. A
4. F
5. G
6. K
7. M