On the viability of acceleration mechanisms for high-velocity clouds and quasar emission-line clouds

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Abstract
The acceleration of dense features in astrophysical systems to high velocities is discussed in the light of the Rayleigh-Taylor instability, and it is shown that, unless stabilized by viscosity, such features must be formed comoving relative to an ambient hot flux. This principle is then applied to specific mechanisms for the production of high-velocity emission- and absorption-line clouds in active galactic nuclei. It is argued that radiative acceleration and coherent acceleration by cosmic rays are unable, without disruption, to produce the large velocities necessary. Our criterion for stability is satisfied by models where clouds form via thermal instabilities in a supersonic wind or behind a single shock, although in this case the large velocities can only be attained at large radial distances where the shock speed is sufficiently high. It is further shown that multiple-shock acceleration, which produces high-velocity clouds at lower radii, is viable if the clouds are strongly turbulent so that there is the necessary viscous damping. The strong shocks inherent in this mechanism are a natural source for this turbulence, but the difficulty with which clouds can survive this acceleration suggests that not all active nuclei with high luminosities and strong outflow need exhibit strong broad emission-or intrinsic absorption-lines.

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