We discuss two constraints on the existence and stability of quasar emission-line gas. The case of a constant pressure photoionized cloud in pressure equilibrium with a surrounding hot gas is considered. We show that the radiation pressure of the internally generated line emission makes a significant contribution to the total pressure in standard broad-line region models, and that models with either a higher ionization parameter or lower gas density are dominated by radiation pressure and hence unstable. Further, clouds with ionization parameters lower than the standard value cannot be supported by any hot gas which is heated only by the observed radiation field. The result is a strict limit on the range of parameters which permit the existence of the observed emission-line gas. These results offer a natural explanation for the similarities among emission-line spectra of active galactic nuclei.

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Published in The Astrophysical Journal Letters, v. 285, no. 1, p. L11-L14.

© 1984. The American Astronomical Society. All rights reserved.

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