Metal production in M 33: space and time variations^*

¹ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: [laura;edvige;galli]@arcetri.astro.it
² National Optical Astronomy Observatories, Tucson, AZ 85719, USA e-mail: lstanghellini@noao.edu
³ Universidad Autónoma de Madrid, Departamento de Física Teórica C-XI, 28049 Madrid, Spain e-mail: eva.villaver@uam.es

Received: 28 October 2009
Accepted: 18 December 2009

Abstract

Context. Nearby galaxies are ideal places to study metallicity gradients in detail and their time evolution.

Aims. 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.

Methods. 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.

Results. 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 -0.03-4 dex kpc^-1; 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.

Conclusions. 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 ± 0.009 dex kpc^-1 in the whole radial range up to ~8 kpc, and -0.044 ± 0.009 dex kpc^-1 excluding the central kpc; iii) there is little evolution in the slope with time from the epoch of PN progenitor formation to the present.