Photoionization analyses of emission spectra of quasars

A reassessment is made of the information conveyed by emission-line intensities in the spectra of quasars, under the assumption that the line-emitting gas is photoionized by continuum radiation. The emphasis is on exploring dependences upon parameters, rather than on proposing one particular model which is ''sufficient'' for explaining the line intensities. An essential feature of the discussion is the inclusion of two components of gas, each characterized by its value of U1, the radiation-to-gas ratio. Actual quasars' emission regions probably have continuous distributions of values of U1, but the two-component approximation is appropriate to the amount of information which is actually observable at present or in the near future. The carbon abundance proposed in some earlier one-component analyses, of the order of n/sub C//n/sub H/approx. =10-4 or somewhat less, is about equal to the lower limit on n/sub C//n/sub H/ in a two-component model. There also seems to be an upper limit, of the order of n/sub C//n/sub H/approx. =10-3 or somewhat less; so the allowable range embraces ''solar'' abundances. Likely values of U1 are also suggested. Some of the gas must have rather low values of U1; while a higher-excitation component, which produces the O VI and N V lines, either has high values of U1, or is heated by some mechanism in addition to photoionization