Quantitative spectroscopy of 24 A supergiants in the sculptor galaxy NGC 300: Flux-weighted gravity-luminosity relationship, metallicity, and metallicity gradient

A quantitative spectral analysis of 24 A supergiants in the Sculptor Group spiral galaxy NGC 300 at a distance of 1.9 Mpc is presented. A new method is introduced to analyze low-resolution(similar to 5 angstrom) spectra, which yields metallicities accurate to 0.2 dex including the uncertainties arising from the errors in T-eff ( 5%) and log g ( 0.2 dex). For the first time the stellar metallicity gradient based on elements such as titanium and iron in a galaxy beyond the Local Group is investigated. Solar metallicity is measured in the center and 0.3 solar in the outskirts and a logarithmic gradient of -0.08 dex kpc(-1). An average reddening of E(B-V) similar to 0.12 mag is obtained, however, with a large variation from 0.07 to 0.24 mag. We also determine stellar radii, luminosities, and masses and discuss the evolutionary status. Finally, the observed relationship between absolute bolometric magnitudes M-bol and flux-weighted gravities gF = g/T-eff(4) is investigated. At high temperatures the strength of the Balmer lines depends solely on the flux- weighted gravity, which allows a precise direct determination of log gF with an accuracy of 0.05-0.1 dex. We find a tight relationship between Mbol and log gF in agreement with stellar evolution theory. Combining these new results with previous work on Local Group galaxies, we obtain a new flux- weighted gravity-luminosity relationship (FGLR), which is very well defined and appears to be an excellent alternative tool to determine distances to galaxies.