Systematics in the Cepheid and TRGB Distance Scales: Metallicity Sensitivity of the Wesenheit Leavitt Law

Kavli Affiliate: Wendy L. Freedman

| First 5 Authors: Barry F. Madore, Wendy L. Freedman, , ,

| Summary:

Using an updated and significantly augmented sample of Cepheid and TRGB
distances to 28 nearby spiral and irregular galaxies, covering a wide range of
metallicities, we have searched for evidence of a correlation of the zero-point
of the Cepheid Period-Luminosity relation with HII region (gas-phase)
metallicities. Our analysis, for the 21 galaxies closer than 12.5 Mpc, results
in the following conclusions: (1) The zero points of the Cepheid and TRGB
distance scales are in remarkably good agreement, with the mean offset in the
zero points of the most nearby distance-selected sample being close to zero,
Delta mod_o(Cepheid – TRGB) = -0.026 +- 0.015 mag (for an I-band TRGB zero
point of M_I = -4.05 mag); however, for the more distant sample, there is a
larger offset between the two distance scales, amounting to -0.073 +/- 0.057
mag. (2) The individual differences, about that mean, have a measured scatter
of +/- 0.068~mag. (3) We find no statistically significant evidence for a
metallicity dependence in the Cepheid distance scale using the reddening-free
W(V,VI) period-luminosity relation:
Delta mod_o (Cepheid – TRGB) = -0.022 (+/- 0.015) times ([O/H]-8.50) – 0.003
(+/- 0.007)

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