The abundance scatter in M33 from HII regions: Is there any evidence for azimuthal metallicity variations?

Abstract

Optical spectra of 25 H II regions in the inner 2kpc of the M33 disk have been obtained with the Gemini Multi-Object Spectrograph at the Gemini North telescope. The oxygen abundance gradient measured from the detection of the [O III]λ4363 auroral line displays a scatter of approximately 0.06dex, 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 R 23 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 100pc to a fewkpc. 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 R 23 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. © 2011. The American Astronomical Society. All rights reserved.