Possible evidence for an inverted temperature-density relation in the intergalactic medium from the flux distribution of the Lyα forest

We compare the improved measurement of the Ly alpha forest flux probability distribution at 1.7 < z < 3.2 presented by Kim et al. to a large set of hydrodynamical simulations of the Ly alpha forest with different cosmological parameters and thermal histories. The simulations are in good agreement with the observational data if the temperature-density relation for the low-density intergalactic medium (IGM), T= T-0 Delta(gamma-1), is either close to isothermal or inverted (gamma < 1). Our results suggest that the voids in the IGM may be significantly hotter and the thermal state of the low-density IGM may be substantially more complex than is usually assumed at these redshifts. We discuss radiative transfer effects which alter the spectral shape of ionizing radiation during the epoch of He II reionization as a possible physical mechanism for achieving an inverted temperature-density relation at z similar or equal to 3.