The Metallicity and Distance of NGC 2403 from Blue Supergiants

Abstract

We present the first quantitative spectral analysis of blue supergiant stars in the nearby galaxy NGC 2403. Out of a sample of 47 targets observed with the LRIS spectrograph at the Keck I telescope we have extracted 16 B- and A-type supergiants for which we have data of sufficient quality to carry out a comparison with model spectra of evolved massive stars and infer the stellar parameters. The radial metallicity gradient of NGC 2403 that we derive has a slope of −0.14(±0.05) dex r e − 1 , and is in accordance with the analysis of H ii region oxygen abundances. We present evidence that the stellar metallicities that we obtain in extragalactic systems in general agree with the nebular abundances based on the analysis of the auroral lines, over more than 1 order of magnitude in metallicity. Adopting the known relation between stellar parameters and intrinsic luminosity we find a distance modulus μ = 27.38 ± 0.08 mag. While this can be brought into agreement with Cepheid-based determinations, it is 0.14 mag short of the value measured from the tip of the red giant branch. We update the mass–metallicity relation secured from chemical abundance studies of stars in resolved star-forming galaxies.