| Popis: |
Using an updated and significantly augmented sample of Cepheid and tip of the red giant branch (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 H ii 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, Δ μ _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 〈Δ μ _o 〉 (Cepheids—TRGB) = −0.026 ± 0.015 mag, for an I -band TRGB zero-point of M _I = −4.05 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: Δ μ _o ( Cepheid − TRGB ) = − 0.022( ± 0.015) × ([O/H] − 8.50) − 0.003(±0.007). |
