A search for the He I lambda 5876 recombination line from the diffuse interstellar medium

The He I lambda 5876/hydrogen alpha line intensity ratio in the warm ionized component of the interstellar medium is significantly less than that observed in traditional H II regions surrounding O stars. The observations, carried out in two directions at low Galactic latitude, reveal that the helium is primarily neutral, n(He II)/n(Hetotal approximately less than 0.27, within warm, low-density regions in which the hydrogen is primarily ionized. This implies that the spectrum of the diffuse interstellar radiation field that ionizes the hydrogen is softer than that from the Galactic O star population in the solar neighborhood. Specifically, the ratio of helium ionizing to hydrogen ionizing photons Q(He⁰)/Q(H⁰) approximately less than 0.030, which corresponds to a spectral type of O8 or later and, if representative of the interstellar background, challenges the O star ionization models.