The Abundance Scatter in M33 from HII Regions: Is There Any Evidence for Azimuthal Metallicity Variations?

TL;DR

This study measures the metallicity distribution in M33's H II regions, finding minimal azimuthal variation and highlighting the importance of accurate abundance indicators like [O III]4363 over R23.

Contribution

It provides the first detailed analysis of metallicity scatter in M33 using auroral lines, confirming low azimuthal variation and cautioning against R23 underestimation in certain conditions.

Findings

Oxygen abundance scatter is about 0.06 dex, smaller than previous reports.

No significant azimuthal metallicity variations detected in M33.

R23 indicator underestimates oxygen abundance in regions with hard ionizing radiation.

Abstract

Optical spectra of 25 H II regions in the inner two kpc of the M33 disk have been obtained with the GMOS spectrograph at the Gemini North telescope. The oxygen abundance gradient measured from the detection of the [O III]4363 auroral line displays a scatter of approximately 0.06 dex, a much smaller value than recently reported by Rosolowsky & Simon in this galaxy. The analysis of the abundances for a large sample of H II regions derived from the R23 strong-line indicator confirms that the scatter is small over the full disk of M33, consistent with the measuring uncertainties, and comparable to what is observed in other spiral galaxies. No evidence is therefore found for significant azimuthal variations in the present-day metallicity of the interstellar medium in this galaxy on spatial scales from ~100 pc to a few kpc. A considerable fraction of M33 H II regions with auroral line detections show spectral features revealing sources of hard ionizing radiation (such as He II emission and large [Ne III], [O III] line fluxes). Since R23 is shown to severely underestimate the oxygen abundances in such cases, care must be taken in chemical abundance studies of extragalactic H II regions based on this strong-line indicator.