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Abstract: One of the most dramatic signatures of the reionization era may be theenormous ionized bubbles around luminous quasars with radii reaching ~40comoving Mpc, which may survive as -fossil- ionized regions long after theirsource shuts off. Here we study how the inhomogeneous intergalactic mediumIGM evolves inside such fossils. The average recombination rate declinesrapidly with time, and the brief quasar episode significantly increases themean free path inside the fossil bubbles. As a result, even a weak ionizingbackground generated by galaxies inside the fossil can maintain it in arelatively highly and uniformly ionized state. For example, galaxies that wouldionize 20-30% of hydrogen in a random patch of the IGM can maintain 80-90%ionization inside the fossil, for a duration much longer than the averagerecombination time in the IGM. Quasar fossils at z<10 thus retain theiridentity for nearly a Hubble time, and will appear -gray,- distinct from boththe average IGM which has a -swiss-cheese- ionization topology and a lowermean ionized fraction, and from fully-ionized bubbles around active quasars.More distant fossils, at z>10 have a weaker galaxy-generated ionizingbackground and a higher gas density, so they can attain a swiss-cheese topologysimilar to the rest of the IGM, but with a smaller contrast between the ionizedbubbles and the partially neutral regions separating them. AnalogousHeIII-fossils should exist around the epoch of HeII-HeIII reionization at z~3,although rapid recombinations inside the HeIII-fossils will be more common. Ourmodel of inhomogeneous recombination also applies to -double reionization-models and shows that a non-monotonic reionization history is even moreunlikely than previously thought.

Author: Steven Furlanetto UCLA, Zoltan Haiman Columbia, S. Peng Oh UCSB


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