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Need a supply of gas to feed to the black hole
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(Black holes from 1 million to >1 billion solar
masses!
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Scales as a few percent of galaxy bulge mass.)
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Collisions disturb regular orbits of stars and gas
clouds
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Could feed more gas to the central region
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Galactic orbits were less organized as galaxies were
forming,
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also recall the “hierarchical” galaxy formation
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Expect more gas to flow to central region when
galaxies are young =>
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Quasars (“quasar epoch” around z=2 to z=3)
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Most galaxies may have massive black holes in them
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They are just less active now because gas supply is less
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