Studying HI recombination within the inner kiloparsec: simulations of AGN jet-triggered shock–cloud interactions

Observations of young radio sources hosting relativistic jets reveal fast atomic and molecular outflows, with line widths implying outward velocities of 100-1000 km/s. Numerical simulations of relativistic jets show that their bow shocks strongly ionise and drive turbulence in the ISM; however, explaining the presence of such lines requires that the shocked ISM gas undergo rapid recombination. This is hard to reconcile with the physical conditions seen in previous simulations of powerful jets. We study the production of AGN-driven outflows in realistic multiphase media through hydrodynamical simulations. By zooming in on the interaction between a planar shock front and a cold atomic cloud within the inner kiloparsec, we ensure adequate spatial resolution for the cloud structure. We include a simple chemical network tracking protons, electrons and atomic HI for the first time to our knowledge in this kind of simulations. The parameter space we explore is motivated by both numerical simulations and analytical models of jet propagation (e.g. Perucho 2024), which link the local physical conditions to the properties of the underlying AGN jet that triggered the shock wave. We find that mid-to-high-power jets fully ionise the cloud, whereas low-power jets allow significant HI survival. A multiphase outflow emerges, with the velocities of the cold phase showing good agreement with observations. These values provide a proxy for jet power in systems where AGN feedback is relevant.