Abstract The thermal history of cosmic gas in the dark ages remains largely unknown. It is important to quantify the impact of relevant physics on the IGM temperature between z = 10 and z ∼ 30, in order to interpret recent and oncoming observations, including results reported by EDGES. We revisit the gas heating due to structure formation shocks in this era, using a set of fixed grid cosmological hydrodynamical simulations performed by three different codes. In all our simulations, the cosmic gas is predicted to be in multiphase state since z > 30. The gas surrounding high density peaks gradually develops a relation more sharp than T ∝ ρ2/3, approximately T ∝ ρ2, from z = 30 to z = 11, might be due to shock heating. Meanwhile, the gas in void region tends to have a large local Mach number, and their thermal state varies significantly from code to code. In the redshift range 11 − 20, the mass fraction of gas shock heated above the CMB temperature in our simulations is larger than previous semi-analytical results by a factor of 2 to 8. At z = 15, the fraction varies from ∼ 19% to 52% among different codes. Between z = 11 and z = 20, the gas temperature \(\langle 1/T_K\rangle_M^{-1}\) is predicted to be ∼ 10 − 20 K by two codes, much higher than the adiabatic cooling model and some previous works. However, in our simulations performed by RAMSES, \(\langle 1/T_K\rangle_M^{-1}\) is predicted to be even below the temperature required to explain result of the EDGES. Given the fact that different codes give different predictions, currently, it seems a challenge to make solid prediction on the temperature of gas at z ∼ 17 in simulations.

Keywords cosmology: theory — dark ages, reionization, first stars — galaxies: high-redshift — intergalactic medium

https://doi.org/10.1088/1674–4527/20/6/95