A catapult model for the narrow-line region in Seyferts and radio galaxies

The kinematics and stability of clouds falling radially into a supersonic wind is studied theoretically. A critical parameter, the ejection coefficient is found which separates those clouds which continue to gravitate inwards from those which are catapulted outwards by the ram. The clouds are partially broken up by fluid dynamic instabilities and the fragments expelled with enhanced velocities. The model is applied to the narrow-line region of Seyferts and radio galaxies. A quasi-steady picture is established for the wind-ambivalent medium interaction zone, and typical cloud size of 0.01 pc is suggested on the basis of model calculations. It is found that line emission from the ejected clouds is blueshifted if the ambivalent medium is dusty.