A spectroscopic study of blue supergiant stars in the Sculptor galaxy NGC 55: chemical evolution and distance

TL;DR

This study analyzes blue supergiant stars in NGC 55 to determine their metallicity distribution, chemical evolution, and distance, revealing a metallicity gradient, recent gas accretion, and in-situ star formation outside the disk.

Contribution

It provides the first detailed spectroscopic analysis of blue supergiants in NGC 55, including metallicity gradient measurement and a new distance estimate using the flux-weighted gravity-luminosity relationship.

Findings

Detected a metallicity gradient of -0.22 dex/R_25.

Reproduced metallicity distribution with a chemical evolution model.

Estimated galaxy distance modulus as 26.85 mag.

Abstract

Low resolution (4.5 to 5 Angstroem) spectra of 58 blue supergiant stars distributed over the disk of the Magellanic spiral galaxy NGC 55 in the Sculptor group are analyzed by means of non-LTE techniques to determine stellar temperatures, gravities and metallicities (from iron peak and alpha-elements). A metallicity gradient of -0.22 +/- 0.06$ dex/R_25 is detected. The central metallicity on a logarithmic scale relative to the Sun is [Z] = -0.37 +\- 0.03. A chemical evolution model using the observed distribution of stellar and interstellar medium gas mass column densities reproduces the observed metallicity distribution well and reveals a recent history of strong galactic mass accretion and wind outflows with accretion and mass-loss rates of the order of the star formation rate. There is an indication of spatial inhomogeneity in metallicity. In addition, the relatively high central metallicity of the disk confirms that two extra-planar metal poor HII regions detected in previous work 1.13 to 2.22 kpc above the galactic plane are ionized by massive stars formed in-situ outside the disk. For a sub-sample of supergiants, for which Hubble Space Telescope photometry is available, the flux-weighted gravity--luminosity relationship is used to determine a distance modulus of 26.85 +\- 0.10 mag.