Quantitative Spectroscopy of Blue Supergiant Stars in the Disk of M81: Metallicity, Metallicity Gradient, and Distance

The quantitative spectral analysis of low-resolution (∼5 Å) Keck LRIS spectra of blue supergiants in the disk of the giant spiral galaxy M81 is used to determine stellar effective temperatures, gravities, metallicities, luminosities, interstellar reddening, and a new distance using the flux-weighted gravity–luminosity relationship. Substantial reddening and extinction are found with E ( B − V ) ranging between 0.13 and 0.38 mag and an average value of 0.26 mag. The distance modulus obtained after individual reddening corrections is 27.7 ± 0.1 mag. The result is discussed with regard to recently measured tip of the red giant branch and Cepheid distances. The metallicities (based on elements such as iron, titanium, magnesium) are supersolar (≈0.2 dex) in the inner disk ( R ≲ 5 kpc) and slightly subsolar (≈ − 0.05 dex) in the outer disk ( R ≳ 10 kpc) with a shallow metallicity gradient of 0.034 dex kpc −1 . The comparison with published oxygen abundances of planetary nebulae and metallicities determined through fits of Hubble Space Telescope color–magnitude diagrams indicates a late metal enrichment and a flattening of the abundance gradient over the last 5 Gyr. This might be the result of gas infall from metal-rich satellite galaxies. Combining these M81 metallicities with published blue supergiant abundance studies in the Local Group and the Sculptor Group, a galaxy mass–metallicity relationship based solely on stellar spectroscopic studies is presented and compared with recent studies of Sloan Digital Sky Survey star-forming galaxies.