Jet-cloud interactions and the brightening of the narrow-line region in Seyfert galaxies

We study the kinematical and brightness evolution of emission-line clouds in the narrow-line region (NLR) of Seyfert galaxies during the passage of a jet. We derive a critical density above which a cloud remains radiative after compression by the jet cocoon. The critical density depends mainly on the cocoon pressure. Supercritical clouds increase in emission-line brightness, while subcritical clouds generally are highly overheated, reducing their luminosity below that of the intercloud medium. As a result of the pressure stratification in the bow shock of the jet, a cylindrical structure of nested shells develops around the jet. The most compact and brightest compressed clouds surround the cloud-free channel of the radio jet. To support our analytical model, we present a numerical simulation of a supersonic jet propagating into a clumpy NLR. The position-velocity diagram of the simulated H alpha emission shows total line widths of the order of 500 km s(-1) with large-scale variations in the radial velocities of the clouds due to the stratified pressure in the bow shock region of the jet. Most of the luminosity is concentrated in a few dense clouds surrounding the jet. These morphological and kinematic signatures are all found in the well-observed NLR of NGC 1068 and other Seyfert galaxies.