Revisiting the abundance gradient in the maser host galaxy NGC 4258

TL;DR

This study measures the oxygen abundance gradient in NGC 4258 using spectroscopic data, revealing a very shallow gradient that challenges previous assumptions about metallicity effects on Cepheid distance measurements.

Contribution

It provides new spectroscopic measurements and analysis of the oxygen abundance gradient in NGC 4258, highlighting potential systematic biases in Cepheid-based distance estimates.

Findings

Shallow oxygen abundance gradient of -0.012 +/- 0.002 dex/kpc.

Inner field metallicity comparable to the LMC.

Potential biases in Cepheid distance measurements due to metallicity effects.

Abstract

New spectroscopic observations of 36 HII regions in NGC 4258 obtained with the Gemini telescope are combined with existing data from the literature to measure the radial oxygen abundance gradient in this galaxy. The [OIII]4363 auroral line was detected in four of the outermost targets (17 to 22 kpc from the galaxy center), allowing a determination of the electron temperature Te of the ionized gas. From the use of different calibrations of the R23 abundance indicator an oxygen abundance gradient of approximately -0.012 +/- 0.002 dex/kpc is derived. Such a shallow gradient, combined with the difference in the distance moduli measured from the Cepheid Period-Luminosity relation by Macri et al. between two distinct fields in NGC 4258, would yield an unrealistically strong effect of metallicity on the Cepheid distances. This strengthens the suggestion that systematic biases might affect the Cepheid distance of the outer field. Evidence for a similar effect in the differential study of M33 by Scowcroft et al. is presented. A revision of the transformation between strong-line and Te-based abundances in Cepheid-host galaxies is discussed. In the Te abundance scale, the oxygen abundance of the inner field of NGC 4258 is found to be comparable with the LMC value.