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 ~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 Δμ 0 = 10.88 ± 0.04 (random) ± 0.05 (systematic) mag. Given the published maser distance to the galaxy, this implies μ 0 (LMC) = 18.41 ± 0.10 r ± 0.13 s mag or D (LMC) = 48.1 ± 2.3 r ± 2.9 s kpc. We measure a metallicity effect of γ = -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.