Planetary nebulae in face-on spiral galaxies. I. Planetary nebula photometry and distances

As the first step to determine disk mass-to-light ratios for normal spiral galaxies, we present the results of an imaging survey for planetary nebulae (PNe) in six nearby, face-on systems: IC 342, M74 (NGC 628), M83 (NGC 5236), M94 ( NGC 4736), NGC 5068, and NGC 6946. Using Blanco/Mosaic II and WIYN/OPTIC, we identify 165, 153, 241, 150, 19, and 71 PN candidates, respectively, and use the planetary nebula luminosity function (PNLF) to obtain distances. For M74 and NGC 5068, our distances of 8.6 +/- 0.3 and 5.4(-0.4)(+0.2) Mpc are the first reliable estimates to these objects; for IC 342 (3.5 +/- 0.3 Mpc), M83 (4.8 +/- 0.1 Mpc), M94 (4.4(-0.2)(+0.1) Mpc), and NGC 6946 (6.1 +/- 0.6 Mpc) our values agree well with those in the literature. In the larger systems, we find no evidence for any systematic change in the PNLF with galactic position, although we do see minor field-to-field variations in the luminosity function. In most cases, these changes do not affect the measurement of distance, but in one case the fluctuations result in a similar to 0.2 mag shift in the location of the PNLF cutoff. We discuss the possible causes of these small-scale changes, including internal extinction in the host galaxies and age/metallicity changes in the underlying stellar population.