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The origin of the ionization of the diffuse interstellar medium in spiral galaxies
I. Photometric measurements
A. Zurita 1,
M. Rozas 1 and
J.E. Beckman 1,2
Received 4 April 2000 / Accepted 6 June 2000
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 images which allow analysis down to very low surface brightness (between 0.3 and 2 pc cm-6). Separation of the diffuse H emission from that of the H II regions was performed using the most reliable method: subtracting from the integrated H flux of a complete galaxy the contribution from its fully catalogued population of H II regions. The integrated luminosity of the DIG is considerable (1040-1042 erg s-1) and is a high fraction of the total H emission of each galaxy. Lower and upper limits to the DIG emission in H 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) which leak from H II regions. Here we go further: we show that the correlation of the DIG is stronger with the most luminous H II regions and we propose a specific model for the ionization of the DIG: we show that the luminosity, in Lyc photons, leaking from the most luminous H II regions is enough to ionize the diffuse gas in a model where the H II regions with luminosities greater than erg s-1 are density bounded (Beckman et al. 2000). This model predicts that a fraction of this flux escapes from the galaxy into the surrounding medium.
Key words: galaxies: general galaxies: ISM galaxies: spiral ISM: H ii regions
Send offprint requests to: A. Zurita
Online publication: December 5, 2000