The Impact of the WHIM on the IGM Thermal State Determined from the Low-z Lyman α Forest

Abstract

At z ≲ 1, shock heating caused by large-scale velocity flows and possibly violent feedback from galaxy formation, converts a significant fraction of the cool gas (T ∼ 104 K) in the intergalactic medium (IGM) into warm–hot phase (WHIM) with T > 105 K, resulting in a significant deviation from the previously tight power-law IGM temperature–density relationship, T = T0(ρ/ ρ̄)γ-1. This study explores the impact of the WHIM on measurements of the low-z IGM thermal state, [T0, γ], based on the b–NH I distribution of the Ly α forest. Exploiting a machine learning-enabled simulation-based inference method trained on Nyx hydrodynamical simulations, we demonstrate that [T0, γ] can still be reliably measured from the b–NH I distribution at z = 0.1, notwithstanding the substantial WHIM in the IGM. To investigate the effects of different feedback, we apply this inference methodology to mock spectra derived from the IllustrisTNG and Illustris simulations at z = 0.1. The results suggest that the underlying [T0, γ] of both simulations can be recovered with biases as low as | log (T0/K)| ≲ 0.05 dex, | γ | ≲ 0.1, smaller than the precision of a typical measurement. Given the large differences in the volume-weighted WHIM fractions between the three simulations (Illustris 38 per cent, IllustrisTNG 10 per cent, and Nyx 4 per cent), we conclude that the b–NH I distribution is not sensitive to the WHIM under realistic conditions. Finally, we investigate the physical properties of the detectable Ly α absorbers, and discover that although their T and distributions remain mostly unaffected by feedback, they are correlated with the photoionization rate used in the simulation.