Blue supergiants in the Pinwheel Galaxy M101: comparison with H II region chemical abundances, spectroscopic distance and an independent determination of the Hubble constant

Abstract

We present a quantitative spectroscopic study of 13 blue supergiant stars in the Pinwheel Galaxy M101, based on data obtained with the Low Resolution Imaging Spectrometer available at the Keck I telescope. The average stellar metallicity decreases from ~1.9 Zsun near the center of the galaxy to ~0.3 Zsun at the optical outskirts. The galactocentric radial metallicity gradient is statistically consistent with previous studies of the gas-phase oxygen abundance from H II regions using the direct method. The H II region-based Cepheid metallicities used by Riess et al. in their determination of the Hubble constant H_0 are in substantial agreement with our measurements. The direct method gas-phase metallicities of the 18 star-forming galaxies we have analyzed so far, when adjusted upward for a mean ~0.15 dex oxygen dust depletion factor, are in good agreement with those we infer from the supergiants, over a factor of 50 in metallicity. From the same data, we derive an expression for the metal-dependent depletion of oxygen in photoionized nebulae. Utilizing the flux-weighted gravity - luminosity relationship (FGLR) of blue supergiants, we measure a distance to M101, D=6.5 +\- 0.2 Mpc (m-M = 29.06 +\- 0.08), which is within 1 sigma from determinations based on the tip of the red giant branch and Cepheids. With M101 as a nearby SN Ia host and using the observed standardized B-band magnitude of the supernova, our FGLR distance yields an independent value H_0 = 72.5 +\- 4.6 km/s/Mpc.