Galaxies

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Galaxies

• Tuesday, February 26

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What is a galaxy?
Examples of galaxies
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It took centuries for astronomers to deduce the
true nature of galaxies.
Whirlpool Galaxy as seen with telescope (Rosse,
AD 1845)
Andromeda Galaxy as seen by naked eye (Al
Sufi, AD 964)
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What is a galaxy?
A gravitationally bound assembly of
many stars ( associated planets)
interstellar gas ( dust)
and dark matter.
many 10 million
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The Milky Way Galaxy (our galaxy!) is surrounded
by orbiting dwarf galaxies.
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The dwarf galaxies (even distant ones) are
orbiting rapidly.
This shows that our dark halo is (1)
massive (2) large in radius.
Andromeda
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500,000 parsecs
50,000 parsecs
Luminous galaxy
Dark halo
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Luminous part of a galaxy (made of electrons,
protons, neutrons) is much smaller than the
dark part (made of Weakly Interacting
Massive Particles).
Whats special about electrons,
protons, neutrons that concentrates them at
the center of dark halos?
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Tootsie pop hypothesis central luminous
galaxy forms 1st, then is dipped in dark matter.
Twinkie hypothesis outer dark halo forms 1st,
then luminous galaxy is injected.
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During the first years of the universe,
Dark matter (WIMPs) ordinary matter (electrons,
protons, neutrons) were mixed together.
What can ordinary matter do that dark matter
cannot?
Emit light!
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Consider a gas of electrons, protons, helium
nuclei, and WIMPs all mixed together
e
e
e
e
e
and all moving in random directions.
e
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Initially, the particles move rapidly. They have
a high temperature
e
e
e
e
e
and therefore a high pressure.
e
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However, the ordinary particles emit photons,
which carry away energy
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e
e
e
e
so ordinary particles (but not WIMPs) slow
down. .
e
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Ordinary particles, no longer supported by
pressure, flow where gravity takes them
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e
e
e
e
to the densest clumps of dark matter
e
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Astronomy jargon falling down the
gravity well
Since ordinary stuff, made
of electrons, protons, neutrons, can
easily dump its excess energy, it
falls toward dense regions.
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Modified Twinkie Hypothesis originally (dark
matter) sponge cake (ordinary matter) creme
filling coexist.
Gravity injects the ordinary matter to the
center of the dark matter.
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Computer simulation of galaxy formation ordinary
matter on left, dark matter on right.
small box eventual size of galaxy
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We expect luminous ordinary matter (which loses
energy by radiating light) to form denser
knots than dark matter.
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Galaxies have been seen that have a redshift z
10.
The light we see from these galaxies was emitted
at t 500 million years.
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The time from t 350 thousand years to t 500
million years is known as the Dark Ages (no
galaxies yet).
From t 500 million years to t NOW is the Age
of Galaxies.
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All we can see of a galaxy is the luminous part
(duh!)
dwarf galaxy
giant galaxy
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Galaxies have a range of luminosities. (Milky Way
Galaxy, with a luminosity of 1011 Suns, is a
bright galaxy.)
Very few galaxies are brighter than twice the
Milky Ways luminosity.
Why is there an upper limit on the size of
galaxies?
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Why is there an upper limit on the size of land
animals?
Animals bigger than 200 tons cant support their
own weight still be flexible enough to move.
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Why is there an upper limit on the size of
superclusters?
Dense regions more than 50 Mpc across havent had
time to collapse.
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Why is there an upper limit on the size of
galaxies?
Coma Cluster of galaxies
Bigger clumps of gas havent had time to cool
down.
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It takes time for an object to cool off by
radiating away photons.
A huge turkey cools off more slowly than a small
french fry.
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It takes time for an object to cool off by
radiating away photons.
A huge gas cloud cools off more slowly than a
small gas cloud.
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Bright galaxies tend to have one of two shapes.
1) Spiral galaxies, like the Milky Way Galaxy and
the Andromeda Galaxy
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Stars in a spiral galaxy go around on neat
(almost) circular orbits.
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2) Elliptical galaxies, like the galaxy known as
M87.
Spiral galaxies are more photogenic, so they
appear in all the coffee table books.
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Stars in an elliptical galaxy are on disordered,
randomly oriented orbits.
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Spiral galaxy stars are good citizens,
traveling on orderly orbits, all moving in the
same direction.
Elliptical galaxy stars are individualists,
traveling on chaotic orbits, all in different
directions.
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Why are some galaxies orderly (spiral) others
chaotic (elliptical)?
Lets see what happens when the Milky Way and
Andromeda galaxies collide.
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When 2 orderly spiral galaxies collide, they
become a chaotic elliptical galaxy.
(When 2 orderly cars collide, they dont become
an orderly truck they become a chaotic heap of
metal.)
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Spiral galaxies exist mainly in lower-density
regions (such as the Local Group).
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Elliptical galaxies exist mainly in high-density
clusters (such as the Coma
Cluster).
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Thursdays Lecture
Formation and Evolution of Stars
Reading
none