Possible evidence for an inverted temperature-density relation in the intergalactic medium from the flux distribution of the Lyman-alpha forest
J. S. Bolton, M. Viel, T. -S. Kim, M. G. Haehnelt, R. F. Carswell
TL;DR
The study investigates whether the Lyman-α forest flux PDF at $1.7<z<3.2$ can be reconciled with hydrodynamical simulations by varying the thermal history and cosmology. Using GADGET-2 simulations with a uniform ultraviolet background and a family of temperature-density relations $T=T_{0} Δ^{γ-1}$, they generate synthetic spectra and compare to Kim et al. (2007)'s metal-cleaned PDF. They find that the observed PDF is best reproduced if the low-density IGM is near isothermal ($γ≈1$) or inverted ($γ<1$), implying hotter voids and more complex thermal states than standard assumptions. Radiative-transfer effects during He II reionization are discussed as a plausible physical mechanism to produce an inverted temperature-density relation. These results impact interpretation of the Lyα forest and the derivation of the matter power spectrum from flux statistics.
Abstract
We compare the improved measurement of the Lya forest flux probability distribution at 1.7<z<3.2 presented by Kim et al. (2007) to a large set of hydrodynamical simulations of the Lya 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 ionising radiation during the epoch of HeII reionisation as a possible physical mechanism for achieving an inverted temperature-density relation at z~3.