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

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Abstract
We study the kinematics and stability of clouds falling radially into a supersonic wind. We find a critical parameter, the ejection coefficient, which separates clouds which continue to gravitate inwards from those which are catapulted out by the ram pressure of the wind. This leads to a maximum size for ejected clouds. The clouds are partially broken up by fluid dynamic instabilities and the fragments expelled with enhanced velocities. We apply this model to the narrow-line region of Seyferts and radio galaxies. A quasi-steady picture may be established for the wind–ambient medium interaction zone. The wind is shocked and escapes through jets or bubbles; the ambient medium cools, forming the clouds which gravitate inwards. Excessive collisions with ejected clouds are avoided if the covering factor of infalling clouds is less than unity. This implies a typical cloud size of 0.01 pc. Line emission from ejected clouds is blueshifted if the ambient medium is dusty. Correlations between radio power, line width, line power and X-ray emission are expected. A non-steady, non-spherical model in which cloud collisions produce X-rays leads to further predictions.

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