A Direct Stellar Metallicity Determination in the Disk of the Maser Galaxy NGC4258

Abstract: We present the first direct determination of a stellar metallicity in the spiral galaxy NGC4258 (D=7.6 Mpc) based on the quantitative analysis of a low-resolution (~5 AE) Keck LRIS spectrum of a blue supergiant star located in its disk. A determination of stellar metallicity in this galaxy is important for the absolute calibration of the Cepheid Period-Luminosity relation as an anchor for the extragalactic distance scale and for a better characterization of its dependence as a function of abundance. We find a value 0.2 dex lower than solar metallicity at a galactocentric distance of 8.7 kpc, in agreement with recent HII region studies using the weak forbidden auroral oxygen line at 4363 AE. We determine the effective stellar temperature, gravity, luminosity and line-of-sight extinction of the blue supergiant being studied. We show that it fits well on the flux-weighted gravity--luminosity relation (FGLR), strengthening the potential of this method as a new extragalactic distance indicator.