The dispersion in the Cepheid period-luminosity relation and the consequences for the extragalactic distance scale

Using published Hubble Space Telescope (HST) Cepheid data from 25 galaxies, we have found a correlation between the dispersion in the Cepheid period-luminosity (P-L) relation and host galaxy metallicity, which is significant at the 3 level in the V band. In the I band the correlation is less significant, although the tighter intrinsic dispersion of the P-L relation in I may make it harder to detect such a correlation in the HST sample. One possibility is that low metallicity galaxies have smaller metallicity gradients than high metallicity galaxies; if the Cepheid P-L relation has a significant dependence on metallicity then this might explain the higher P-L dispersion in the higher metallicity galaxies. A second possibility is that the increased P-L dispersion is driven by metallicity dispersion but now due to a relation between metallicity and Cepheid colour rather than luminosity. A third possibility is that the increased P-L dispersion is caused by an increase in the width of the instability strip with metallicity.