Cepheid metallicity and Hubble constant

Abstract

This study combines data from a set of seven Cepheid-calibrated galaxies in Virgo, Leo I and Fornax and four supernovae with known metallicities, including metallicity corrections to the Cepheid distances, in order to determine a consistent mean value for the Hubble constant. We derive Virgo and Coma distances over different paths, starting from the metallicity corrected Cepheid distances of Virgo and Leo I galaxies, and we relate them to the Virgo and the Coma recession velocities, paying special attention to thereby introduced correlations. We also determine the velocity of the Fornax cluster and relate this velocity to its distance. In addition we use the standard constant maximum brightness SN Ia method, as well as results from multicolor light curve shape (MLCS) analysis of SNe Ia, to derive the Hubble constant. Using the requirement of statistical consistency, our data set determines the value of the metallicity coefficient (the change in magnitude per factor of ten in metal abundance) to be 0.31 (+0.15/-0.14) in good agreement with other known determinations. We also use consistency as a criterion to select between mutually exclusive data sets, such as the two known, but conflicting, Virgo recession velocities (in favor of the lower value), as well as between two different analyses of (partly) the same supernovae (in favor of MLCS analysis). When combining data we pay attention to the statistically correct handling of common errors. Our result is a Hubble constant with the value H_0 = 68 +- 5 km/s/Mpc.