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 (/) ^ -1. This study explores the impact of the WHIM on measurements of the low-z IGM thermal state, T0, γ, based on the b–N ₇\, { ₈} 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–N ₇\, { ₈} 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 |Δlog (T0/K) | ≲ 0. 05 dex, |Δγ| ≲ 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–N ₇\, { ₈} 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 Δ distributions remain mostly unaffected by feedback, they are correlated with the photoionization rate used in the simulation.