Forbidden S II H-alpha intensity ratios in faint, extended H II regions and the origin of the interstellar emission-line background

Abstract

The Wisconsin large-aperture Fabry-Perot spectrometer was used to measure forbidden S II 6716/H-alpha intensity ratios in very faint (emission measures as low as 5 cm to the -6th pc), extended (diameters as large as 260 pc) H II regions surrounding O and early B stars to investigate whether such regions can be the source of the interstellar emission-line background. H II regions with very dilute ionizing radiation fields were found to have higher forbidden S II/H-alpha intensity ratios than more compact regions. However, the forbidden S II/H-alpha ratios in the dilute H II regions are not as high as the ratios found in the interstellar emission-line background. The data sugggest that, if the interstellar background is produced by photoionized H II regions, then the regions have emission measures EM less than 1 cm to the -6th pc and are ionized by a Lyman continuum flux F less than 6 x 10 to the 5th photons/cm s. This places important constraints on the origin and nature of the warm, ionized gas in the diffuse interstellar medium.