The effect of metallicity on the Cepheid distance scale and its implications for the Hubble constant ($H_0$) determination

TL;DR

This study investigates how metallicity affects the Cepheid Period-Luminosity relation and demonstrates that accounting for metallicity can reconcile differing Hubble constant estimates, leading to a more consistent value around 70 km/s/Mpc.

Contribution

The paper provides the first detailed analysis of metallicity's impact on Cepheid luminosities, challenging the assumption of independence and refining H_0 estimates.

Findings

Metallicity significantly influences Cepheid luminosity measurements.

Adjusting for metallicity reduces discrepancies in H_0 estimates.

Results suggest H_0 is approximately 70 km/s/Mpc when metallicity effects are considered.

Abstract

Recent HST determinations of the expansion's rate of the Universe (the Hubble constant, H_0) assumed that the Cepheid Period-Luminosity relation at V and I are independent of metallicity (Freedman, et al., 1996, Saha et al., 1996, Tanvir et al., 1995). The three groups obtain different vales for H_0. We note that most of this discrepancy stems from the asumption (by both groups) that the Period-Luminosity relation is independent of metallicity. We come to this conclusion as a result of our study of the Period-Luminosity relation of 481 Cepheids with 3 millions two colour measurements in the Large Magellanic Cloud and the Small Magellanic Cloud obtained as a by-product of the EROS microlensing survey. We find that the derived interstellar absorption corrections are particularly sensitive to the metallicity and when our result is applied to recent estimates based on HST Cepheids observations it makes the low-H_0 values higher and the high-H_0 value lower, bringing those discrepant estimates into agrement around $H_0 \approx 70 km/s Mpc^{-1}$.