Extending the PNLF technique to spiral galaxies requires extra care due to 3 factors. First, potential confusion exists between PN and HII regions. Second, spiral arms contain obvious dust lanes that could reduce observed luminosities. Third, a young population of stars must exist in spiral arms. Nevertheless, the advantage of working in spirals is tremendous because their Cepheids provide the most accepted reference distances to test the PNLF method for systematic errors. Also, spirals are where zero-point calibrations are most believed.
To identify PN in spiral galaxies, we complement the 5007 on-band/off-band imaging technique with on-band/R-band images at H. We define the following criteria for PN candidates in spirals. A PN candidate must:
These criteria have been applied recently to observations of M51 (NGC 5194), M96 (NGC 3368), and M101 (NGC 5457) by Feldmeier, Ciardullo, & Jacoby (1997). M96 and M101 distances exist already from HST Cepheid surveys.
Figure 1 illustrates the excellent agreement between the PNLF (adjusted to the modern M31 distance and extinction) and Cepheid distance scales. The PNLF distances are based solely on M31 as the zero-point calibrator, yet the mean offset for the additional 6 Cepheid galaxies differs by only 1%. The random scatter of 8% about the ridge line is consistent with, or smaller than, the combined errors of the Cepheid and PNLF distances. Note that some uncertainties are common to both methods (e.g. foreground extinction) which slightly reduce the apparent uncertainty in the combined distance error.
Figure 1. A comparison between PNLF and Cepheid distances. Solid circles represent direct galaxy comparisons; the solid triangle is the calibrator galaxy, M31, which, by definition, falls exactly on the dashed 1:1 line. The points labeled N1023, Virgo, and Fornax represent comparisons between elliptical (PNLF) and spiral (Cepheid) galaxies within the same cluster. The lower panel clarifies the level of disagreement by plotting the relative differences in distances. Only M96 deviates by more than 1; at this time, it is unclear if the disagreement is significant, and if so, whether the PNLF or Cepheid distance is discrepant.
Figure 1 also includes comparisons between PNLF distances to elliptical galaxies and Cepheid distances for different galaxies in the same group. These indirect comparison points provide further support, but do not confirm the PNLF distances to the Virgo, Fornax, Leo, and NGC 1023 groups.
The lack of any evidence for systematic errors, either among the direct or indirect comparisons, strongly implies that any residual systematic errors for zero-points, population effects (age and metallicity), internal extinction, and methodology (adopted PNLF shape, sizes of PNLF samples) must be smaller than ~ 8%. Alternatively, one can insist that a conspiracy exists among these parameters such that both Cepheid and PNLF distances have errors that correlate.