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

TL;DR

This study compares observational data of the Lyman-alpha forest with hydrodynamical simulations, suggesting the low-density intergalactic medium may have an inverted temperature-density relation, indicating complex thermal states at redshifts 1.7 to 3.2.

Contribution

It provides evidence that the temperature-density relation in the IGM may be inverted or nearly isothermal, challenging standard assumptions about the thermal state of the low-density IGM.

Findings

Simulations agree with observations if gamma<1.

Voids in the IGM may be hotter than expected.

Radiative transfer effects during HeII reionisation could cause the inversion.

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.