Radio recombination lines from quasars. I - Level populations of hydrogenic ions in a strong, nonthermal radiation field

Observable radio recombination lines can be produced by ionized gas in the vicinity of radio quasars through stimulation by the radio source. These same stimulated processes will alter the level populations of the highly excited levels which produce the radio recombination lines. The populations of highly excited levels in hydrogenic ions have been calculated in the presence of a strong, nonthermal (power-law spectrum) radiation field. An extensive grid of models is given for a range of values of the radio intensity and spectral index, and of the electron temperature and density in the ionized gas. The effect of low-frequency cutoffs in the spectrum of the radio source is considered. It is shown that in the limit where the radiation field becomes extremely strong, very weak absorption lines are produced. Stimulated transitions will also broaden the radio recombination lines; the widths and profiles of recombination lines produced through stimulation in quasars are calculated. The line strengths are calculated for the grid of models, and a procedure for deriving the parameters of the models from optical and radio observations of a quasar is briefly outlined.