Diffuse Ionized Gas in Nearby Spiral Galaxies

Sensitive Hα images have been obtained for nine nearby spiral galaxies typically permitting detection of DIG down to a surface brightness of 1-2 × 10-17 ergs s-1 cm -2 arcsecond-2, or 5-10 pc cm -6 in emission measure. For four of these galaxies, SII images were obtained to permit analysis of the spectral properties of the DIG on galactic scales. Both the intensity and the diffuse nature of the emission were incorporated into an automated masking technique to distinguish the DIG and HII regions in all galaxies studied. For this reason, selection effects were minimized when comparing DIG properties between galaxies. Additionally, optical spectra of DIG regions with various surface brightness levels in M31 permitted detailed analysis of the ionization state for these regions. From these spectra, Hα, Hβ, Hγ, Hλ, NII (λ6583), SII (λ6717 + λ6731), OIII (λ5007), and OII (λ3727) line intensities were measured and provided a test to typical ionization models. Diffuse ionized gas is present in all galaxies observed, occurring as smooth, diffuse emission in distant galaxies, but resolving into clumps and filaments in nearby galaxies. It is responsible for roughly 35%-50% of the total Hα emission from each galaxy. The SII (λ6717 + λ6731)/Hα ratio observed in DIG is elevated compared to values in HII regions. Typical values for DIG are 0.40-0.60, while HII region values are 0.20- 0.25. The line ratio increases steadily from values around 0.2 to values of 0.7 as the surface brightness decreases from the brightest HII regions to the faintest DIG. This smooth decrease is a natural consequence of photo-ionization models with increasingly more dilute radiation fields ionizing the fainter regions. In M31, spectroscopic observations suggested that photoionization models in which dilute radiation fields are responsible for the ionization of DIG were the most successful at reproducing the observed optical line ratios.