Diffuse ionized gas in three sculptor group galaxies

We present a study of the diffuse ionized gas (DIG) in three Sculptor group galaxies: NGC 55, NGC 253, and NGC 300. The study is based on narrow band imagery in H alpha+ [N II](6548+6583 Angstrom) and [S II] (6717+6731 Angstrom). We find that DIG contributes 33 to 58% of the total Ha luminosity in these galaxies, or 30 to 54% after correcting for scattered light. We find that NGC 300 has a higher fractional DIG luminosity than the other galaxies in our sample, but it is not clear whether this is a significant difference or an effect of the high inclination of the other galaxies. The diffuse emission, averaged over the optical extent of the disk, has a face-on emission measure of 5 to 10 pc cm(-6). The DIG is concentrated near H II regions, although significant emission is seen at large distances from H II regions, up to 0.5 to 1 kpc. The [S II]/(H alpha + [N II]) ratio is enhanced in the DIG, typically around 0.3 to 0.5, compared to 0.2 for the H II regions in these galaxies. These properties are similar to those measured for the DIG in the Milky Way and in other nearby spirals. The line ratios, large-scale distribution, and energy requirement suggest that photoionization is the dominant ionization mechanism. (C) 1996 American Astronomical Society.