A new Cepheid distance to the maser-host galaxy NGC 4258 and its implications for the Hubble constant
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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 0 = 10.88 0.04 (random) 0.05 (systematic) mag. Given the published maser distance to the galaxy, this implies 0(LMC) = 18.41 0.10r 0.13s mag or D(LMC) = 48.1 2.3r 2.9s kpc. We measure a metallicity effect of = -0.29 0.09r 0.05s 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 H0 = 74 3r 6s 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 H0 with a total uncertainty of 5%, with consequent improvement in the determination of the equation of state of dark energy. 2006. The American Astronomical Society. All rights reserved.