Cloudy 94 and applications to quasar emission line regions

This review discusses the most recent developments of the plasma simulation code Cloudy and its application to the emission-line regions of quasars. The long-term goal is to develop the tools needed to determine the chemical composition of the emitting gas and the luminosity of the central engine for any emission line source. Emission lines and the underlying thermal continuum are formed in plasmas that are far from thermodynamic equilibrium. Their thermal and ionization states are the result of a balance of a vast set of microphysical processes. Once produced, radiation must propagate out of the (usually) optically thick source. No analytic solutions are possible, and recourse to numerical simulations is necessary. I am developing the large-scale plasma simulation code Cloudy as an investigative tool for this work, much as an observer might build a spectrometer. This review describes the current version of Cloudy, version 94. It describes improvements made since the release of the previous version, C90 (Ferland et al. 1998). The major recent application has been the development of the "Locally Optimally-Emitting Cloud" (LOC) model of AGN emission line regions (Baldwin et al. 1995). Powerful selection effects, introduced by the atomic physics and line formation process, permit individual lines to form most effeciently only near certain selected parameters. These selection effects, together with the presence of gas with a wide range of conditions, are enough to reproduce the spectrum of a typical quasar with little dependence on details. The spectrum actually carries little information to the identity of the emitters. I view this as a major step forward since it provides a method to handle accidental details at the source, so that we can concentrate on essential information such as the luminosity or chemical composition of the quasar.