Is Double Reionization Physically Plausible?

Recent observations of z ∼ 6 quasars and the cosmic microwave background imply a complex history to cosmic reionization. Such a history requires some form of feedback to extend reionization over a long time interval, but the nature of the feedback and how rapidly it operates remain highly uncertain. Here we focus on one aspect of this complexity: which physical processes can cause the global ionized fraction to evolve non-monotonically with cosmic time? We first examine how galactic winds affect the transition from metal-free to normal star formation. Because winds expand much more slowly than ionization fronts, we find that this mechanism cannot be responsible for double reionization given plausible parameters for the winds. We next consider photoheating, which causes the cosmological Jeans mass to increase in ionized regions and hence suppresses galaxy formation there. Double reionization is more plausible in this case, but only if small halos form stars efficiently and if the suppression from photoheating is strong relative to current expectations. Finally, we consider H2 photodissociation, in which the buildup of a soft UV background suppresses star formation in small halos. This too can cause the ionized fraction to temporarily decrease, but only during the earliest stages of reionization. For all of these mechanisms, double reionization appears less likely than a long, but still monotonic, ionization history. Finally, we briefly consider the effects of some of these feedback mechanisms on the topology of reionization. Subject headings: cosmology: theory – galaxies: evolution – intergalactic medium