He II REIONIZATION AND ITS EFFECT ON THE INTERGALACTIC MEDIUM

Observations of the intergalactic medium (IGM) suggest that quasars reionize He II in the IGM at z approximate to 3. We have run a set of 190 and 430 comoving Mpc simulations of He II being reionized by quasars to develop an understanding of the nature of He II reionization and its potential impact on observables. We find that He II reionization heats regions in the IGM by as much as 25,000 K above the temperature that is expected otherwise, with the volume-averaged temperature increasing by similar to 12,000 K and with large temperature fluctuations on similar to 50 Mpc scales. Much of this heating occurs far from quasars by photons with long mean free path. We find a temperature-density equation of state of gamma - 1 approximate to 0.3 during He II reionization, but with a wide dispersion in this relation having sigma(T) similar to 10(4) K. He II reionization by the observed population of quasars cannot produce an inverted relation (gamma - 1 < 0). Our simulations are consistent with the observed evolution in the mean transmission of the He II Ly alpha forest. We argue that the heat input from He II reionization is unable to cause the observed depression at z approximate to 3.2 in the H I Ly alpha forest opacity as has been suggested. We investigate how uncertainties in the properties of QSOs and of He II Lyman limit systems influence our predictions.