The metallicity gradient of M 33: chemical abundances of H ii regions ^*,**

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: laura@arcetri.astro.it
² Instituto de Astrofísica de Andalucía (CSIC) Apartado de Correos 3004, 18080 Granada, Spain
³ Instituto de Astrofísica de Canarias, c. vía Láctea s/n, 38200 La Laguna, Tenerife, Canarias, Spain
⁴ Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa Cruz de La Palma, Canarias, Spain

Received: 9 March 2007
Accepted: 10 May 2007

Abstract

Context. We present spectroscopic observations of a sample of 72 emission-line objects, including mainly H ii regions, in the spiral galaxy M 33. Spectra were obtained with the multi-object, wide field spectrograph AF2/WYFFOS at the 4.2 m WHT telescope. Line intensities, extinction, and electron density were determined for the whole sample of objects.

Aims.The aim of the present work was to derive chemical and physical parameters of a set of H ii regions, and from them the metallicity gradient.

Methods.Electron temperatures and chemical abundances were derived for the 14 H ii regions where both [O ii] and [O iii] emission line fluxes were measured, including the electron temperature sensitive emission line [O iii] 436.3 nm and in a few cases [N ii] 575.5 nm. The ionization correction factor (ICF) method was used to derive the total chemical abundances.

Results.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 H ii regions. The oxygen abundances of our H ii regions, located in the radial region from ~2 to ~7.2 kpc, gave an oxygen gradient -0.054±0.011 dex kpc^-1

Conclusions.The overall oxygen gradient for M 33 obtained using ours and previous oxygen determinations in a large number of H ii regions with direct electron temperature determination as well as abundance in young stars presented a two slope shape: -0.19 dex kpc^-1 for the central regions ( kpc), and -0.038 dex kpc^-1 for the outer regions (R ≥ 3 kpc).