A Determination of the Hubble Constant from Cepheid Distances and a Model of the Local Peculiar Velocity Field

We present a measurement of the Hubble Constant based on Cepheid distances to 27 galaxies within 20 Mpc. The Cepheid data are taken from published measurements by the Hubble Telescope Key Project on the Distance Scale (H0KP). We calibrate the Cepheid Period-Luminosity (PL) relation using data for over 700 Cepheids in the LMC obtained by the OGLE collaboration; an LMC distance modulus of 18.50 ($d_{{\\rm LMC}}=50.1$ kpc) is assumed. Using this PL calibration we obtain new distances to the H0KP galaxies. The redshifts of these galaxies are corrected for peculiar velocities using two distinct velocity field models: the phenomenological model of Tonry et al (2000a), and a model based on the IRAS density field and linear gravitational instability theory. The Cepheid distances and corrected redshifts are combined in order to derive H_0. The results are H_0 = 85 +/- 5 km/s/Mpc (random error) at 95% confidence when the IRAS model is used, and 92 +/- 5 km/s/Mpc for the phenomenological model. The IRAS model is a better fit to the data and the Hubble constant it returns is more reliable. Our value of H_0 is signifcantly larger than that quoted by the H0KP, 71 +/- 6 km/s/Mpc. We discuss in detail possible reasons for this discrepancy and future lines of study need to resolve it.