Feb_26 - Galaxies Tuesday, February 26 What is a galaxy?...

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Galaxies Galaxies Tuesday, February 26 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. Andromeda Galaxy as seen by naked eye (Al Sufi, AD 964) Whirlpool Galaxy as seen with telescope (Rosse, AD 1845)
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What is a galaxy? A gravitationally bound assembly of many stars (+ associated planets) interstellar gas (+ dust) and dark matter 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. Andromeda This shows that our dark halo is (1) massive & (2) large in radius.
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500,000 parsecs 50,000 parsecs Dark halo Luminous galaxy
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Luminous Luminous part of a galaxy (made of electrons, protons, & neutrons) is much smaller than the dark dark part (made of Weakly Interacting Massive Particles). What’s special about 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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Dark matter (WIMPs) & ordinary matter (electrons, protons, neutrons) were mixed together. What can
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Feb_26 - Galaxies Tuesday, February 26 What is a galaxy?...

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