The planetary nebula Abell 39


How Accurate is the PNLF Method?

It is impossible to know exactly how accurate a distance technique is because we have no perfect reference galaxies with exactly known distances to provide a measure of the true error. We can, though, compare one technique against another. Probably, the most widely accepted comparison method is Cepheid variables.

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-sigma; at this time, it is unclear if the disagreement is significant, and if so, whether the PNLF or Cepheid distance is discrepant.

The RMS deviation from the 1:1 line is 8%, but much of this scatter is caused by the most discrepant point, M96. Tanvir (priv. comm.) reports that with a much larger sample of Cepheids (48 instead of 7) in M96, his distance to M96 will drop slightly and the errors will grow slightly. Revised numbers are not available at this time.

If we omit M96 from the sample, the RMS deviation drops to 5%. Whether we include M96 or not, the net zero-point shift remains within 1% of the M31 zero-point.

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