Quantitative Spectroscopy of Blue Supergiant Stars in the Disk of M81: Metallicity, Metallicity Gradient and Distance

TL;DR

This study uses spectral analysis of blue supergiants in M81 to determine stellar parameters, metallicity distribution, and galaxy distance, revealing a shallow metallicity gradient and insights into galaxy evolution.

Contribution

It provides the first detailed spectroscopic metallicity gradient and distance measurement for M81 using blue supergiants, and compares these with other galaxy metallicity relations.

Findings

Distance modulus of 27.7 mag with individual reddening corrections.

Metallicity gradient of 0.034 dex/kpc indicating shallow variation.

Evidence of late metal enrichment and gradient flattening over 5 Gyrs.

Abstract

The quantitative spectral analysis of low resolution Keck LRIS spectra of blue supergiants in the disk of the giant spiral galaxy M81 is used to determine stellar effective temperatures, gravities, metallicities, luminosites, interstellar reddening and a new distance using the Flux-weighted Gravity--Luminosity Relationship (FGLR). Substantial reddening and extinction is found with E(B-V) ranging between 0.13 to 0.38 mag and an average value of 0.26 mag. The distance modulus obtained after individual reddening corrections is 27.7+/-0.1 mag. The result is discussed with regard to recently measured TRGB and Cepheid distances. The metallicities (based on elements such as iron, titanium, magnesium) are supersolar (~0.2 dex) in the inner disk (R<=5kpc) and slightly subsolar (~ -0.05 dex) in the outer disk (R>10 kpc) with a shallow metallicity gradient of 0.034 dex/kpc. The comparison with published oxygen abundances of planetary nebulae and metallicities determined through fits of HST color-magnitude diagrams indicates a late metal enrichment and a flattening of the abundance gradient over the last 5 Gyrs. This might be the result of gas infall from metal rich satellite galaxies. Combining these M81 metallicities with published blue supergiant abundance studies in the Local Group and the Sculptor Group a galaxy mass metallicity-relationship based solely on stellar spectroscopic studies is presented and compared with recent studies of SDSS star forming galaxies.