2010A&A...512A..63M

Nearby galaxies are ideal places to study metallicity gradients in detail and their time evolution. We analyse the spatial distribution of metals in M 33 using a new sample and the literature data on H II regions, and constrain a model of galactic chemical evolution with H II region and planetary nebula (PN) abundances. We consider chemical abundances of a new sample of H II regions complemented with previous data sets. We compared H II region and PN abundances obtained with a common set of observations taken at MMT. With an updated theoretical model, we followed the time evolution of the baryonic components and chemical abundances in the disk of M 33, assuming that the galaxy is accreting gas from an external reservoir. From the sample of H II regions, we find that i) the 2D metallicity distribution has an off-centre peak located in the southern arm; ii) the oxygen abundance gradients in the northern and southern sectors, as well as in the nearest and farthest sides, are identical within the uncertainties, with slopes around -0.03-4dex/kpc; iii) bright giant H II regions have a steeper abundance gradient than the other H II regions; iv) H II regions and PNe have O/H gradients very close within the errors; v) our updated evolutionary model is able to reproduce the new observational constraints, as well as the metallicity gradient and its evolution. Supported by a uniform sample of nebular spectroscopic observations, we conclude that i) the metallicity distribution in M 33 is very complex, showing a central depression in metallicity probably due to observational bias; ii) the metallicity gradient in the disk of M 33 has a slope of -0.037±0.009dex/kpc in the whole radial range up to ∼8kpc, and -0.044±0.009dex/kpc excluding the central kpc; iii) there is little evolution in the slope with time from the epoch of PN progenitor formation to the present.