A new Cepheid distance to the maser-host galaxy NGC 4258 and its implications for the Hubble constant

We present initial results from a time series BVI survey of two fields in NGC 4258 using the HSTACS. This galaxy was selected because of its accurate maser-based distance, which is anticipated to have a total uncertainty of similar to 3%. The goal of the HST observations is to provide an absolute calibration of the Cepheid distance scale and to measure its dependence on chemical abundance (the so-called metallicity effect). We carried out observations of two fields at different galactocentric distances with a mean abundance difference of 0.5 dex. We discovered a total of 281 Cepheids with periods ranging from 4 to 45 days (the duration of our observing window). We determine a Cepheid distance modulus for NGC 4258 (relative to the LMC) of Delta mu(0) = 10.88 +/- 0.04 (random) +/- 0.05 (systematic) mag. Given the published maser distance to the galaxy, this implies mu(0)(LMC) = 18.41 +/- 0.10(r) +/- 0.13(s) mag or D(LMC) = 48.1 +/- 2.3(gamma) +/- 2.9(s) kpc. We measure a metallicity effect of gamma = -0.29 +/- 0.09(r) +/- 0.05(s) mag dex(-1). We see no evidence for a variation in the slope of the period-luminosity relation as a function of abundance. We estimate a Hubble constant of H-0 = 74 +/- 3(r) +/- 6(s) km s(-1) Mpc(-1) using a recent sample of four well-observed Type Ia SNe and our new calibration of the Cepheid distance scale. It may soon be possible to measure the value of H-0 with a total uncertainty of 5%, with consequent improvement in the determination of the equation of state of dark energy.