Diffuse Ionized Gas in Spiral Galaxies: Probing Lyman Continuum Photon Leakage From H II Regions?

Abstract

As part of a large study to map the distribution of star formation across galactic disks, we have obtained deep Hα images of the nearby Sculptor Group spirals NGC 247 and NGC 7793. These images are of sufficiently high quality that they allow identification and analysis of diffuse Hα emission at surface brightness levels ranging from those of extremely low density H II regions down to those of the local Galactic disk diffuse emission. This paper presents a study of the large scale distribution and global energetics of diffuse ionized gas (DIG) in these galaxies and investigates the association between DIG and discrete H II regions. The radial and azimuthal intensity distributions of the DIG reveal that it is highly correlated with bright H II regions over both small and large scales. The observed Hα luminosities of the DIG are considerable and account for 30%-50% of the total Hα emission of each galaxy. The integrated minimum Lyman continuum (Lyc) power required to sustain the DIG in each galaxy is enormous and can only easily be met by the ionizing output from massive star formation; the mechanical energy from supernovae and stellar winds falls short of that required by at least a factor of two. Our results support the hypothesis that the DIG is photoionized by Lyc photons which leak out of traditional H II regions, and suggest that the local H I column density plays a role in regulating the amount of leakage which can occur. This interpretation has profound implications for the derivation of global and radial star-formation rates based on Hα emission-line fluxes, leading to qualitatively and quantitatively different behaviors depending on whether the Hα emission from the DIG is included or not. The contribution of the diffuse Hα component to the total Hα emission; i.e., the diffuse fraction, in these galaxies is similar to values found in other galaxies with differing Hubble types and star-formation rates. The constancy of the diffuse fraction is rather unexpected and implies that the overall fraction of photons which can leak out of H II regions and ionize the ISM over large scales is relatively invariant from one galaxy to another.