Planetary nebulae as standard candles .10. Tests in the Coma I region

We present the results of an [O III] lambda 5007 survey for planetary nebulae (PNs) in three galaxies of the Coma I group: NGC 4278 (Hubble type E1), NGC 4494 (E1), and NGC 4565 (edge-on Sb). Using the planetary nebula luminosity function (PNLF), we derive distances to NGC 4494 (12.8 +/- 0.9 Mpc), NGC 4565 (10.5(-1.0)(+0.8) Mpc), and NGC 4278 (10.2(-1.0)(+0.7) Mpc). The larger distance for NGC 4494 is significant beyond the 99% confidence level when the common systematic errors in all three distances are removed. This agrees with the results of the globular cluster luminosity function and surface brightness fluctuation methods, both of which place NGC 4565 in front of NGC 4494. The large separation is also consistent with the results of Virgocentric flow models, which predict triple-valued solutions to the Hubble flow in that direction. Our planetary nebula survey of the small elliptical NGC 4278 also reveals two [O III] lambda 5007 sources more luminous than the nominal limit of the PNLF. Both objects can be excluded a priori from the list of PN candidates: one is quite bright in H alpha, the other is marginally resolved. Nevertheless, the existence of these objects in an otherwise normal elliptical galaxy poses a potential problem for the PNLF technique. We discuss the possible origins of objects brighter than the PNLF cutoff and consider one way in which their existence might be incorporated into PNLF distance measurements.