Stability of gas clouds near quasi-stellar objects

Gas clouds near quasi-stellar objects are coherently accelerated by the ultraviolet ionizing continuum in such a way that the gas density remains nearly uniform. Outward propagating sound waves, considered in the rest frame of the (accelerating) clouds, will grow in amplitude due to radiative forces unless damped by self-radiation. The instability growth rate is large only for gapproximately-greater-than0.326 kc02, where g is the acceleration, k=2π/lambda, and c0 is the isothermal sound speed in the cloud. Relatively few optically thin QSO clouds having napproximately-greater-than107 cm-3 will be unstable to this process, and even these may be stabilized by the pressure in Lα photons.In the Appendix we show that the diffuse radiation pressure in Lα and in other resonance lines is negligible compared with gas pressure for most QSO clouds of interest