Non-Equilibrium Ionisation in Photoionised Haloes: Implications for Shock Stability and Absorption-Line Signatures

Abstract We investigate the impact of non-equilibrium ionisation (NEI) and the metagalactic radiation-field on the thermal evolution, virial shock stability, and absorption signatures of gas surrounding galaxies. Using 1D, spherically symmetric hydrodynamical simulations with an extended version of the hydra code, we follow dark-matter growth, gas dynamics, time-dependent ionisation and cooling in the presence of the UV background. We explicitly track all ions of H, He, C, N, O, Ne, Mg, Si, S, and Fe in haloes of mass 1011–1013 M⊙ from z = 100 to z = 0. Without a UV background, NEI enhances post-shock cooling due to underionised gas, reducing pressure support and raising the minimum mass for stable shock formation. Including the UV background pre-ionises the intergalactic medium (IGM), suppressing NEI, and restoring the collisional equilibrium threshold. We find that the IGM temperatures deviate from thermal equilibrium due to adiabatic expansion and collapse, while ionisation remains close to equilibrium in the presence of a UV background, except in transient rapidly cooling regions where NEI occurs. We compute absorption columns of O 6, C 4, and H 1, showing that, in our spherical models, the photoionised gas outside the shocked halo may produce substantial warm-ion columns extending beyond Rvir, including O 6 column densities comparable to observed values. Our models indicate weak halo-mass dependence and extended distributions. We also find that z ≳ 3 haloes can produce C 4 (NC, IV ~ 1013 − 15 cm−2) and H 1 (NH, I ~ 1015 − 17 cm−2) columns out to ~10Rvir. Our results highlight the role of the UV background in regulating the thermal state and observable signatures of the gas surrounding galaxies. This emphasizes the importance of accounting for IGM contributions when interpreting circumgalactic absorption-line observations.3