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 NGC4258 using the Advanced Camera for Surveys onboard the Hubble Space Telescope.This galaxy was selected because of its accurate maser-based distance, which isanticipated to have a total uncertainty of ~3%. The goal of the HSTobservations is to provide an absolute calibration of the Cepheid DistanceScale and to measure its dependence on chemical abundance (the so-called"metallicity effect"). We carried out observations of two fields at different galactocentricdistances with a mean abundance difference of 0.5 dex. We discovered a total of281 Cepheids with periods ranging from 4 to 45 days (the duration of ourobserving window). We determine a Cepheid distance modulus for NGC 4258(relative to the LMC) of 10.88 +- 0.04 (random) +- 0.05 (systematic) mag. Giventhe published maser distance to the galaxy, this implies \mu (LMC)=18.41 +-0.10 (r) +- 0.13 (s) mag or D(LMC)= 48.1 +- 2.3 (r) +- 2.9 (s) kpc. We measurea metallicity effect of \gamma=-0.29 +- 0.09 (r) +- 0.05 (s) mag/dex. We see noevidence for a variation in the slope of the Period-Luminosity relation as afunction of abundance. We estimate a Hubble Constant of H_0= 74 +- 3 (r) +- 6 (s) km/s Mpc using arecent sample of 4 well-observed type Ia SNe and our new calibration of theCepheid Distance Scale. It may soon be possible to measure the value of H_0with a total uncertainty of 5%, with consequent improvement in thedetermination of the equation of state of dark energy.