The Effects of Metallicity on the Cepheid P-L Relation and the Consequences for the Extragalactic Distance Scale

Using published HST Cepheid data from 25 galaxies, we have found a correlation between the P-L dispersion and host galaxy metallicity which is significant at the ~3sigma level in the V band. This result supports the suggestion that the metallicity dependence of the Cepheid P-L relation may be stronger than expected. The high observed dispersions in the HST Cepheid P-L relations have the further consequence that the bias due to incompleteness in the P-L relation at faint magnitudes is more significant than previously thought. In the HST sample, galaxies at higher distance tend to have higher metallicity. This means that when a full metallicity/incompleteness correction is made, a scale error in the published Cepheid distances is seen in the sense that the published distance moduli are increasingly underestimates at larger distances. This results in the average distance modulus to the four galaxies in the Virgo cluster core increasing from (m-M)_0=31.2+-0.19 to (m-M)_0=31.8+-0.17 with similar increases for the Fornax and Ursa Major clusters. For the 18 HST galaxies with good Tully-Fisher distances and (m-M)_0>29.5 the Cepheid-TF distance modulus average residual increases from 0.44+-0.09 mag to 0.82+-0.1mag indicating a significant scale error in TF distances and resulting in the previous Pierce & Tully TF estimate of H_0=85 reducing to H_0=58+-7. Finally, for the 8 HST galaxies with SNIa, the metallicity corrected Cepheid distances now imply a metallicity dependence of SNIa peak luminosity in the sense that metal-poor hosts have lower luminosity SNIa. Thus SNIa Hubble diagram estimates of both H_0 and q_0 may also require significant metallicity corrections.