The Thermal Memory of Reionization History

The recent measurement by WMAP of a large electron scattering optical depthtau_e = 0.17 +- 0.04 is consistent with a simple model of reionization in whichthe intergalactic medium (IGM) is ionized at redshift z ~ 15, and remainshighly ionized thereafter. Here, we show that existing measurements of the IGMtemperature from the Lyman-alpha forest at z ~ 2 - 4 rule out this ``vanilla''model. Under reasonable assumptions about the ionizing spectrum, as long as theuniverse is reionized before z = 10, and remains highly ionized thereafter, theIGM reaches an asymptotic thermal state which is too cold compared toobservations. To simultaneously satisfy the CMB and forest constraints, thereionization history must be complex: reionization begins early at z >~ 15, butthere must have been significant (order unity) changes in fractions of neutralhydrogen and/or helium at 6 < z < 10, and/or singly ionized helium at 4 < z <10. We describe a physically motivated reionization model that satisfies allcurrent observations. We also explore the impact of a stochastic reionizationhistory and show that a late epoch of (HeII --> HeIII) reionization induces asignificant scatter in the IGM temperature, but the scatter diminishes withtime quickly. Finally, we provide an analytic formula for the thermalasymptote, and discuss possible additional heating mechanisms that might evadeour constraints.Comment: 10 pages, submitted to ApJ, new references, additional discussion on earlier work and partial HeII reionizatio