TheHubble Space TelescopeKey Project on the Extragalactic Distance Scale. XXV. A Recalibration of Cepheid Distances to Type Ia Supernovae and the Value of the Hubble Constant

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Abstract
Cepheid-based distances to seven Type Ia supernovae (SNe) host galaxies have been derived using the standard Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale pipeline. For the first time, this allows for a transparent comparison of data accumulated as part of three different HST projects: the Key Project, the Sandage et al. Type Ia supernovae (SNe) program, and the Tanvir et al. Leo I Group study. Reanalyzing the Tanvir et al. galaxy and six Sandage et al. galaxies, we find a mean (weighted) offset in true distance moduli of 0.12 ± 0.07 mag; i.e., 6% in linear distance, in the sense of reducing the distance scale or increasing H0. Adopting the reddening-corrected Hubble relations of Suntzeff et al. tied to a zero point based on SNe 1990N, 1981B, 1998bu, 1989B, 1972E, and 1960F and the photometric calibration of Hill et al. leads to a Hubble constant of H0 = 68 ± 2(random) ± 5(systematic) km s-1 Mpc-1. Adopting the Kennicutt et al. Cepheid period-luminosity-metallicity dependency decreases the inferred H0 by 4%. The H0 result from Type Ia SNe is now in good agreement, to within their respective uncertainties, with that from the Tully-Fisher and surface brightness fluctuation relations.
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