The origin of the ionization of the diffuse interstellar medium in spiral galaxies I. Photometric measurements

We present a complete study of the diffuse ionized gas (DIG) in a sample of six spiral galaxies (NGC 157, NGC 3359, NGC 3631, NGC 6764, NGC 6951, NGC 7479) using very high quality H alpha images which allow analysis down to v ery low surface brightness (between 0.3 and 2 pc cm(-6)). Separation of the diffuse H alpha emission from that of the H II regions was performed using the most reliable method: subtracting from the integrated H alpha flux of a complete galaxy the contribution from its fully catalogued population of H II regions. The integrated luminosity of the DIG is considerable (similar to 10(40)-10(42) erg s(-1)) and is a high fraction of the total H alpha em ission of each galaxy. Lower and upper limits to the DIG emission in H alph a were derived. The lower limits vary from 25% to 50% and the upper limits from 45% to 70%, in our observed sample. Previous studies of a very small number of objects, showed that there is a spatial correlation between the DIG and the H II regions in spiral galaxies suggesting that the DIG is photoionized by Lyman continuum photons (Lyc) w hich leak from H II regions. Here we go further: we show that the correlati on of the DIG is stronger with the most luminous H II regions and we propos e a specific model for the ionization of the DIG: we show that the luminosi ty, in Lye photons, leaking from the most luminous H II regions is enough t o ionize the diffuse gas in a model where the H II regions with luminositie s greater than L-H alpha greater than or equal to 10(38.6) erg s(-1) are de nsity bounded (Beckman et al. 2000). This model predicts that a fraction of this flux escapes from the galaxy into the surrounding medium.