The Hubble constant anchor galaxy NGC 4258: metallicity and distance from blue supergiants

TL;DR

This study uses blue supergiant stars' spectroscopy in NGC 4258 to determine metallicity and distance, providing stellar-based calibration for the galaxy's properties and confirming the distance with independent methods.

Contribution

It offers a stellar spectroscopic analysis of NGC 4258 to calibrate metallicity and distance, aligning stellar and nebular metallicity scales and validating the galaxy's distance via the FGLR method.

Findings

Metallicity of [Z] = -0.05±0.05 with a shallow gradient

Distance modulus of 29.38±0.12 mag consistent with maser measurements

Reasonable agreement between stellar and nebular metallicity calibrations

Abstract

A quantitative spectroscopic study of blue supergiant stars in the Hubble constant anchor galaxy NGC 4258 is presented. The non-LTE analysis of Keck I telescope LRIS spectra yields a central logarithmic metallicity (in units of the solar value) of [Z] = -0.05\pm0.05 and a very shallow gradient of -(0.09\pm0.11)r/r25 with respect to galactocentric distance in units of the isophotal radius. Good agreement with the mass-metallicity relationship of star forming galaxies based on stellar absorption line studies is found. A comparison with HII region oxygen abundances obtained from the analysis of strong emission lines shows reasonable agreement when the Pettini & Pagel (2004) calibration is used, while the Zaritsky et al. (1994) calibration yields values that are 0.2 to 0.3 dex larger. These results allow to put the metallicity calibration of the Cepheid Period--Luminosity relation in this anchor galaxy on a purely stellar basis. Interstellar reddening and extinction are determined using HST and JWST photometry. Based on extinction-corrected magnitudes, combined with the stellar effective temperatures and gravities we determine, we use the Flux-weighted Gravity--Luminosity Relationship (FGLR) to estimate an independent spectroscopic distance. We obtain a distance modulus m-M = 29.38\pm0.12 mag, in agreement with the geometrical distance derived from the analysis of the water maser orbits in the galaxy's central circumnuclear disk.