2001ApJ...548..564W

We present a measurement of the Hubble constant based on Cepheid distances to 27 galaxies within 20 Mpc. We take the Cepheid data from published measurements by the Hubble Telescope Key Project on the Distance Scale (H₀KP). We calibrate the Cepheid period-luminosity (PL) relation with data from over 700 Cepheids in the LMC obtained by the OGLE collaboration; we assume an LMC distance modulus of 18.50 mag (d_LMC=50.1 kpc). Using this PL calibration, we obtain new distances to the H₀KP galaxies. We correct the redshifts of these galaxies for peculiar velocities using two distinct velocity field models: the phenomenological model of Tonry et al. and a model based on the IRAS density field and linear gravitational instability theory. We combine the Cepheid distances with the corrected redshifts for the 27 galaxies to derive H₀, the Hubble constant. The results are H₀=85±5 km.s^–1.Mpc^–1 (random error) at 95% confidence when the IRAS model is used, and 92±5 km.s^–1.Mpc^–1 when the phenomenological model is used. The IRAS model is a better fit to the data, and the Hubble constant it returns is more reliable. Systematic error stems mainly from LMC distance uncertainty, which is not directly addressed by this paper. Our value of H₀ is significantly larger than that quoted by the H₀KP, H₀=71±6 km.s^–1.Mpc^–1. Cepheid recalibration explains ∼30% of this difference, and velocity field analysis accounts for ∼70%. We discuss in detail possible reasons for this discrepancy and future studies needed to resolve it.