2024ApJ...977..217K

A quantitative spectroscopic study of blue supergiant stars in the Hubble constant anchor galaxy NGC 4258 is presented. The non-LTE analysis of Keck I telescope LRIS spectra yields a central logarithmic metallicity (in units of the solar value) of [Z] = -0.05 ± 0.05 and a very shallow gradient of -(0.09 ± 0.11) r/r₂₅ with respect to galactocentric distance in units of the isophotal radius. Good agreement with the mass-metallicity relationship of star-forming galaxies based on stellar absorption line studies is found. A comparison with H II region oxygen abundances obtained from the analysis of strong emission lines shows reasonable agreement when the M. Pettini & B. E. J. Pagel calibration is used, while the D. Zaritsky et al. calibration yields values that are 0.2-0.3 dex larger. These results allow us to put the metallicity calibration of the Cepheid period-luminosity relation in this anchor galaxy on a purely stellar basis. Interstellar reddening and extinction are determined using Hubble Space Telescope and JWST photometry. Based on extinction-corrected magnitudes, combined with the stellar effective temperatures and gravities we determine, we use the flux-weighted gravity-luminosity relationship to estimate an independent spectroscopic distance. We obtain a distance modulus m - M = 29.38 ± 0.12 mag, in agreement with the geometrical distance derived from the analysis of the water maser orbits in the galaxy's central circumnuclear disk.