Radio Recombination Lines from Inner Galaxy Diffuse Gas. I. High-Sensitivity Observations: He +/H + and Carbon

Abstract

We have observed radio recombination lines (RRLs) at �1.4 GHz toward the Galactic interior at hundreds of positions in a separately published survey. In some cases we observe not only the Ha but also the Hgamma, Hbeta, Healpha, and Calpha RRLs. We use previously published theory to estimate the enhancement in line intensity from non-LTE excitation, which amounts to �30% for all of the alpha lines; however, the theory does not agree with our observed ratios of a to higher order line intensities. Intensity ratios of alpha lines of different elements should be unaffected by non-LTE excitation. The ratio (nHe+/nHe)(nH+/nH) ? 0.13 in the diffuse ionized medium of the Galactic interior. This is difficult to explain, because photons with energy greater than 24.6 eV must be systematically excluded from this gas. If O stars are the source of the ionizing photons, then this implies an upper mass limit on the initial mass function. However, with this upper mass limit, stars cannot produce enough ionizing photons to satisfy the total Galactic ionization requirement. One solution to this quandary is to invoke larger uncertainties in various processes than we estimate. Other solutions require an ionization source other than stars as currently understood, and we discuss two possibilities. We detect the C RRL toward several H II regions. The C RRLs are too strong to come from H ionized gas and must come instead from photodissociation regions. We compare the C RRL with the C+ 158 mum line toward W43 and derive relatively unambiguous values for physical parameters in the associated photodissociation region.