Understanding narrow line regions in Seyfert galaxies: Numerical simulation of jet-cloud interaction

Recent HST observations suggest that the NLR in Seyfert Galaxies can be the result of interaction between jet and external inhomogeneous medium; following this suggestion we perform numerical simulations considering the impact of a radiative jet on a dense cloud. We approach the problem adopting a hydrodynamical code, that allows us to study in detail the jet hydrodynamics, while we choose a more simplified treatment of radiative processes, in order to give a qualitatively good interpretation of the emission processes. Our three main purposes are: i) to reproduce in our simulations the physical conditions observed in the NLR of Seyfert Galaxies, ii) to obtain physical constraints of the jet parameters and iii) to study the jet capacity to photoionize the surrounding medium. We find that the jet-cloud interaction leads to clumps of matter with density, temperature and velocity that agree with observations. Conversely, the photoionizing flux radiated by the jet-induced shocks does not appear to be sufficient to account for the NLR line luminosity but it may produce local and transient effects on the NLR ionization balance. Finally, the observational requirements can be matched only if jets in Seyfert galaxies are relatively heavy, ρjet & 1 cm −3, and with velocities smaller than . 50, 000 km s−1, very different from their counterparts in radio-galaxies.