OBSERVATIONS OF H53-ALPHA-EMISSION FROM BRIGHT GALAXIES

We have searched for the hydrogen recombination lines H40-alpha and H53-alpha in three bright spiral galaxies, and detected H53-alpha in NGC 2146 and, possibly, in NGC 1068. The strength of the detected lines, when compared with radio continuum and far-infrared (FIR) data, implies that they cannot be produced solely by spontaneous emission but that about three-quarters, or more, of the line photons are stimulated by continuum radiation. Model H II regions are presented which satisfy constraints imposed by the large stimulated H53-alpha fraction, and by the radio continuum and line fluxes. These H II regions must be a small, dense (n(e) approximately 10(5) cm--3) and numerous. These results contrast with our suggestion that the H53-alpha radiation from M82 is dominated by spontaneous emission; a conclusion which is still supported by the observed H40-alpha/H53-alpha line ratio.