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 HST ACS. This galaxy was selected because of its accurate maser-based distance, which is anticipated to have a total uncertainty of 63%. 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 mu sub(0) = 10.88 c 0.04 (random) c 0.05 (systematic) mag. Given the published maser distance to the galaxy, this implies k sub(0)(LMC) = 18.41 c 0.10 sub(r) c 0.13 sub(s) mag or D(LMC) = 48.1 c 2.3 sub(r) c 2.9 sub(s) kpc. We measure a metallicity effect of g = -0.29 c 0.09 sub(r) c 0.05 sub(s) mag dex super(-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 sub(0) = 74 c 3 sub(r) c 6 sub(s) km s super(-1) Mpc super(-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 sub(0) with a total uncertainty of 5%, with consequent improvement in the determination of the equation of state of dark energy.