Measurements of He I λ5876 Recombination Line Radiation from the Diffuse, Warm Ionized Medium in Irregular Galaxies

We present longslit optical spectroscopy of three high surface brightness Magellanic irregular galaxies. This paper draws attention to our detection of He I λ5876 line emission from the ionized gas outside the HII regions, or the warm ionized phase of the interstellar medium. We measure a mean reddening-corrected intensity ratio of He I λ5876 / Hα ≈ 0.041 independent of spatial location. This ratio is much higher than that measured in the diffuse, warm ionized interstellar medium of the Milky Way (Reynolds & Tufte 1995). The high value of He I λ5876 / Hα implies the helium ionization fraction is approximately equal to the hydrogen ionization fraction in the diffuse ionized gas (DIG). If the DIG is powered by young stars, then stars hotter than 40,000 K must contribute to the Lyman continuum radiation reaching the DIG. Since optical and ultraviolet spectra confirm the presence of such massive stars in these galaxies, stellar photoionization remains the most likely power source. The contrast with the low helium ionization in the Galactic DIG, however, is intriguing and provides strong evidence that the physical state of the DIG, not just its presence, varies among galaxies. Subject headings: ISM: structure – galaxies: individual NGC 1569, NGC 4214, NGC 4449 – galaxies: irregular – galaxies: ISM Observations reported here have been obtained in part with the Multiple Mirror Telescope, a joint facility of the University of Arizona and the Smithsonian Institution. Steward Observatory, University of Arizona, Tucson, AZ 85721 Currently, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 Hubble Fellow