Distances to Galaxies from the Brightest Stars in the Universe

TL;DR

This paper discusses how Blue Supergiants can be used as precise distance indicators for galaxies, leveraging spectral analysis and the FGLR method, with implications for understanding galaxy properties and calibrating other distance measures.

Contribution

It introduces the application of the FGLR method to determine extragalactic distances using BSGs and compares metallicity gradients with HII region studies.

Findings

BSGs are effective for distance measurement unaffected by extinction and metallicity.

Distances derived from FGLR are consistent with other methods.

Future telescopes and Gaia will enhance calibration accuracy.

Abstract

Blue Supergiants (BSGs) are the brightest stars in the universe at visual light with absolute magnitudes up to Mv=-10 mag. They are ideal stellar objects for the determination of extragalactic distances, in particular, because the perennial uncertainties troubling most of the other stellar distance indicators, interstellar extinction and metallicity, do not affect them. The quantitative spectral analysis of low resolution spectra of individual BSGs provides accurate stellar parameters and chemical composition, which are then used to determine accurate reddening and extinction from photometry for each individual object. Accurate distances can be determined from stellar gravities and effective temperatures using the "Flux Weighted Gravity - Luminosity Relationship (FGLR)". Most recent results of the quantitative spectral analysis of BSGs in galaxies within and beyond the Local Group based on medium and low resolution spectra obtained with the ESO VLT and the Keck telescopes on Mauna Kea are presented and distances obtained with the FGLR-method are discussed together with the effects of patchy extinction and abundance gradients in galaxies. BSG metallicities and metallicity gradients are compared with results from strong-line HII region studies and the consequences for the empirical calibration of the metallicity dependence of the Cepheid period-luminosity relationship are pointed out. The perspectives of future work are discussed, the use of the giant ground-based telescopes of the next generation such as the TMT on Mauna Kea and the E-ELT and the tremendous value of the GAIA mission to allow for the ultimate calibration of the FGLR using galactic BSGs.