Planetary Nebulae as Standard Candles. III. The Distance to M81

We present the results of a survey for planetary nebulae in the nearby Sb galaxy M81 (NGC 3031). Using the Kitt Peak 4 m telescope and a TI CCD, we identify 185 planetary nebula candidates. Photometry of the nebulae extends nearly 2.5 mag below the brightest candidate, although the sample is statistically homogeneous and complete for only 88 nebulae representing the top 1.2 mag of the luminosity function. We follow the methods outlined by Ciardullo et al. in 1989 for using the distinctive shape of the planetary nebula luminosity function to derive extragalactic distances. After correcting for a foreground extinction of E(B-V) = 0.10, we derive the distance to M81 to be 3.50 +/- 0.40 Mpc. This distance compares very well with values derived using traditional methods such as Cepheids (3.30 Mpc), red supergiants (3.2 Mpc), and the infrared Tully-Fisher relation (3.73 Mpc). M81 provides an excellent test case for comparing the distances derived from planetary nebulae with estimates using other techniques. It is a relatively nearby galaxy so that other stellar indicators can still be used. Furthermore it has an intermediate Hubble type, thereby allowing the use of late-type indicators (brightest stars, Cepheids, Tully-Fisher) while still permitting the unambiguous identification of numerous planetary nebulae. Based on the excellent agreement among the various distance techniques and the high degree of precision offered by the method, we conclude that planetary nebulae are as good as, or better than, other standard candles for deriving distances to galaxies beyond 10 Mpc.