The metallicity and distance of NGC 2403 from blue supergiants

TL;DR

This study analyzes blue supergiant stars in NGC 2403 to determine its metallicity gradient and distance, finding consistency with nebular abundances and refining the galaxy's mass-metallicity relation.

Contribution

First quantitative spectral analysis of blue supergiants in NGC 2403, deriving metallicity gradient and distance, and comparing stellar and nebular metallicities.

Findings

Metallicity gradient slope of -0.14 dex/r_e consistent with H II region data.

Stellar metallicities agree with nebular abundances over a wide metallicity range.

Distance modulus of 27.38 mag aligns with some methods but differs from red giant branch measurements.

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, 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 one order of magnitude in metallicity. Adopting the known relation between stellar parameters and intrinsic luminosity we find a distance modulus m-M = 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.