The metallicity gradient of M 33: chemical abundances of HII regions

TL;DR

This study measures chemical abundances in 72 HII regions of M 33 to determine its metallicity gradient, revealing a two-slope pattern with steeper gradient in the center and flatter in the outer regions.

Contribution

It provides new spectroscopic data for 72 HII regions in M 33 and characterizes the galaxy's detailed metallicity gradient with a two-slope model, improving understanding of its chemical structure.

Findings

Oxygen abundance gradient of -0.054 dex/kpc in 14 HII regions.

Overall metallicity gradient shows a two-slope pattern: -0.19 dex/kpc inside 3 kpc, -0.038 dex/kpc outside.

Confirmed the presence of a radial metallicity gradient in M 33.

Abstract

We present spectroscopic observations of a sample of 72 emission-line objects, including mainly HII regions, in the spiral galaxy M 33. Spectra were obtained with the multi-object, wide field spectrograph AF2/WYFFOS at the 4.2m WHT telescope. Line intensities, extinction, and electron density were determined for the whole sample of objects. The aim of the present work was to derive chemical and physical parameters of a set of HII regions, and from them the metallicity gradient. Electron temperatures and chemical abundances were derived for the 14 HII regions where both [OII] and [OIII] emission line fluxes were measured, including the electron temperature sensitive emission line [OIII] 436.3 nm and in a few cases [NII] 575.5 nm. The ionization correction factor (ICF) method was used to derive the total chemical abundances. The presence of abundance gradients was inferred from the radial behaviour of several emission-line ratios, and accurately measured from chemical abundances directly derived in 14 HII regions. The oxygen abundances of our HII regions, located in the radial region from ~2 to ~7.2 kpc, gave an oxygen gradient -0.054+/-0.011 dex/kpc The overall oxygen gradient for M 33 obtained using ours and previous oxygen determinations in a large number of HII regions with direct electron temperature determination as well as abundance in young stars presented a two slope shape: -0.19 dex/kpc for the central regions (R<3kpc), and -0.038dex/kpc for the outer regions (R>=3kpc).