Extragalactic Distances from Planetary Nebulae

Abstract

The [O III] 5007 planetary nebula luminosity function (PNLF) occupiesan important place on the extragalactic distance ladder. Since itis the only method that is applicable to all the large galaxies ofthe Local Supercluster, it is uniquely useful for cross-checkingresults and linking the Population I and Population II distance scales.We review the physics underlying the method, demonstrate its precision,and illustrate its value by comparing its distances to distancesobtained from Cepheids and the Surface Brightness Fluctuation (SBF)method. We use the Cepheid and PNLF distances to 13 galaxies to showthat the metallicity dependence of the PNLF cutoff is in excellentagreement with that predicted from theory, and that no additionalsystematic corrections are needed for either method. However, whenwe compare the Cepheid-calibrated PNLF distance scale with the Cepheid-calibratedSBF distance scale, we find a significant offset: although the relativedistances of both methods are in excellent agreement, the PNLF methodproduces results that are systematically shorter by . We trace thisdiscrepancy back to the calibration galaxies and show how a smallamount of internal reddening can lead to a very large systematicerror. Finally, we demonstrate how the PNLF and Cepheid distancesto NGC 4258 argue for a short distance to the Large Magellanic Cloud,and a Hubble Constant that is larger than that derived by the HSTKey Project.