Quantitative Spectroscopy of 24 A Supergiants in the Sculptor Galaxy NGC 300: Flux-weighted Gravity-Luminosity Relationship, Metallicity, and Metallicity Gradient

Abstract

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 (~5 Å) 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) ~ 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 g_F = g/T⁴_eff 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 g_F with an accuracy of 0.05-0.1 dex. We find a tight relationship between M_bol and log g_F 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.

Based on VLT observations for ESO Large Program 171.D-0004.