Cepheid calibrations from the hubble space telescope of the luminosity of two recent type Ia supernovae and a redetermination of the hubble constant

We report observations of two nearby Type la supernovae (SNe Ia) for which observations of Cepheid variables in the host galaxies have been obtained with the Hubble Space Telescope: SN 1994ae in NGC 3370 and SN 1998aq in NGC 3982. For NCG 3370, we used the Advanced Camera for Surveys to observe 64 Cepheids that yield a distance of 29 Mpc, the farthest direct measurement of Cepheids. We have measured emission lines from H II regions in both host galaxies that provide metallicity-dependent corrections to their period-luminosity relations. These two SNe la double the sample of "ideal" luminosity calibrators: objects with well-observed and well-calibrated light curves of typical shape and with low reddening. By comparing them to all similarly well-measured SNe la in the Hubble flow, we find that H₀ = 73 ± 4 (statistical) ± 5 (systematic) km s^-1 Mpc^-1. A detailed analysis demonstrates that most of the past disagreement over the value of H₀ as determined from SNe la is abated by the replacement of past, problematic data by more accurate and precise, modern data.