The planetary nebula distance to M101

We report the results of an [O III] λ5007 survey for planetary nebulae (PNs) in the giant Sc galaxy M101 (NGC 5457). By comparing on-band/off-band [O III] λ5007 images with images taken in Ha and the R-band filter, we identify 65 PN candidates in the interarm and outer regions of the galaxy. From these data and the empirical planetary nebula luminosity function (PNLF), we derive a distance to the galaxy of 7.7 ± 0.5 Mpc, in excellent agreement with the distance of 7.4 ± 0.7 Mpc measured using Hubble Space Telescope observations of Cepheids (Kelson and coworkers). This observation demonstrates that the PNLF technique can be successfully applied to late-type galaxies, and provides an important overlap between the Population I and Population II distance scales. It also places a strong limit on the possible variation of the PNLF with population age.