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 + [ NII ] ( 6548+6583Å ) and [ SII ] ( 6717+6731Å ) .
We find that DIG contributes 33 to 58 % of the total H \alpha 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 HII regions , although significant emission is seen at large distances from HII regions , up to 0.5 to 1 kpc .
The [ SII ] / ( H \alpha + [ NII ] ) ratio is enhanced in the DIG , typically around 0.3 to 0.5 , compared to 0.2 for the HII 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 .