The<i>Hubble Space Telescope</i>Key Project on the Extragalactic Distance Scale. XXV. A Recalibration of Cepheid Distances to Type Ia Supernovae and the Value of the Hubble Constant

Abstract

Cepheid-based distances to seven Type Ia supernovae (SNe)-host galaxies have\nbeen derived using the standard HST Key Project on the Extragalactic Distance\nScale pipeline. For the first time, this allows for a transparent comparison of\ndata accumulated as part of three different HST projects, the Key Project, the\nSandage et al. Type Ia SNe program, and the Tanvir et al. Leo I Group study.\nRe-analyzing the Tanvir et al. galaxy and six Sandage et al. galaxies we find a\nmean (weighted) offset in true distance moduli of 0.12+/-0.07 mag -- i.e., 6%\nin linear distance -- in the sense of reducing the distance scale, or\nincreasing H0. Adopting the reddening-corrected Hubble relations of Suntzeff et\nal. (1999), tied to a zero point based upon SNe~1990N, 1981B, 1998bu, 1989B,\n1972E and 1960F and the photometric calibration of Hill et al. (1998), leads to\na Hubble constant of H0=68+/-2(random)+/-5(systematic) km/s/Mpc. Adopting the\nKennicutt et al. (1998) Cepheid period-luminosity-metallicity dependency\ndecreases the inferred H0 by 4%. The H0 result from Type Ia SNe is now in good\nagreement, to within their respective uncertainties, with that from the\nTully-Fisher and surface brightness fluctuation relations.\n