Locally Optimally Emitting Clouds and the Origin of Quasar Emission Lines

The similarity of quasar line spectra has been taken as an indication thatthe emission line clouds have preferred parameters, suggesting that theenvironment is subject to a fine tuning process. We show here that the observedspectrum is a natural consequence of powerful selection effects. We computed alarge grid of photoionization models covering the widest possible range ofcloud gas density and distance from the central continuum source. For each lineonly a narrow range of density and distance from the continuum source resultsin maximum reprocessing efficiency, corresponding to ``locallyoptimally-emitting clouds'' (LOC). These parameters depend on the ionizationand excitation potentials of the line, and its thermalization density. The meanQSO line spectrum can be reproduced by simply adding together the full familyof clouds, with an appropriate covering fraction distribution. The observedquasar spectrum is a natural consequence of the ability of various clouds toreprocess the underlying continuum, and can arise in a chaotic environment withno preferred pressure, gas density, or ionization parameter.Comment: 9 pages including 1 ps figure. LaTeX format using aaspp4.st