2006ApJ...652.1133M

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 ∼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 Δµ₀=10.88±0.04 (random)±0.05 (systematic) mag. Given the published maser distance to the galaxy, this implies µ₀(LMC)=18.41±0.10_r±0.13_s mag or D(LMC)=48.1±2.3_r±2.9_skpc. We measure a metallicity effect of γ=-0.29±0.09_r±0.05_smag/dex. 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 H₀=74±3_r±6_skm/s/Mpc 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 H₀with a total uncertainty of 5%, with consequent improvement in the determination of the equation of state of dark energy.