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

TL;DR

This study compares Cepheid and TRGB distance scales across galaxies with varying metallicities, finding good agreement and no significant metallicity dependence in the Cepheid period-luminosity relation.

Contribution

It provides updated, augmented distance measurements and investigates metallicity effects on the Cepheid Wesenheit Law, with implications for cosmic distance calibration.

Findings

Cepheid and TRGB distance scales agree within ~0.03 mag for nearby galaxies.

No significant metallicity dependence detected in the Wesenheit period-luminosity relation.

Larger offsets observed in more distant galaxy samples.

Abstract

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)