The interstellar medium of M 31: III. Narrow-band imagery in H-alpha and S II .

We present deep CCD imagery of the NE half of M31 in Hα and [SII] emission lines. Nineteen fields covering the major spiral arms were observed. The images have a resolution of 2 - 3" or about 6 - 10pc. In this paper we describe the reduction and initial analysis of the data. In addition to discrete nebulae, the frames show widespread diffuse ionized gas with enhanced [SII]/Hα brightness ratios, similar to the Warm Ionized Medium in our Galaxy. We have constructed a catalog of 967 discrete nebulae with absolute fluxes accurate to better than 10%, together with grey scale images and finding charts. Two-thirds of the objects were detected in [SII] emission. This is the largest sample of objects in a single galaxy with accurate [SII] and Hα fluxes known to date. Most of the sources are HII regions but the catalog also contains supernova remnants and Planetary nebulae. We discuss the distribution of [SII]/Hα flux ratios for the objects and the differential Hα luminosity function. The latter shows a slope of - 0.95 for objects brighter than 10^37^ erg s^-1^, in good agreement with a previous determination. The luminosity function flattens below 10^37^ erg s^-1^; this correspond to the limit below which single rather than multiple-star ionization can account for the observed flux luminosities. The turnover is in agreement with predictions by Kennicutt et al.. A large fraction of the 163 unresolved sources in our survey are probably planetary nebulae. The number of these sources, their luminosities, and their spatial distribution are all consistent with this interpretation.