A spectroscopic study of blue supergiant stars in Local Group spiral galaxies: Andromeda and Triangulum

TL;DR

This study uses spectroscopic data from LAMOST and Keck to analyze blue supergiant stars in M 31 and M 33, measuring metallicities, gradients, and distances, and revealing metallicity asymmetries and distance measurement consistencies.

Contribution

It provides new measurements of metallicity gradients and distances in M 31 and M 33 using blue supergiant stars and the FGLR method, including metallicity distribution asymmetries.

Findings

Metallicity gradients of -0.37 and -0.36 dex/R_25 for M 31 and M 33.

Metallicity center values of 0.30 and 0.11 solar units.

Distances of 24.51 and 24.93 mag for M 31 and M 33, respectively.

Abstract

Low-resolution LAMOST and Keck spectra of blue supergiant stars distributed over the disks of the Local Group spiral galaxies M 31 and M 33 are analyzed to determine stellar effective temperatures, gravities, metallicities, and reddening. Logarithmic metallicities at the center of the galaxies (in solar units) of $0.30\pm0.09$ and $0.11\pm0.04$ and metallicity gradients of $-0.37\pm0.13$ dex/$R_{25}$ and $-0.36\pm0.16$ dex/$R_{25}$ are measured for M 31 and M 33, respectively. For M 33 the 2-dimensional distribution of metallicity indicates a deviation from azimutal symmetry with an off-centre peak. The flux-weighted gravity-luminosity relationship of blue supergiant stars is used to determine a distance modulus of 24.51$\pm$0.13 mag for M 31 and 24.93$\pm$0.07 mag for M 33. For M 31 the flux-weighted gravity--luminosity relationship (FGLR) distance agrees well with other methods. For M 33 the FGLR-based distance is larger than the distances from Cepheids studies but it is in good agreement with work on eclipsing binaries, planetary nebulae , long-period variables, and the tip of the red giant branch.