The building up of the disk galaxy M 33 and the evolution of the metallicity gradient^*

INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy e-mail: laura@arcetri.astro.it

Received: 1 February 2007
Accepted: 17 April 2007

Abstract

Context.The evolution of radial gradients of metallicity in disk galaxies and its relation to disk formation are not well understood. Theoretical models of galactic chemical evolution make contrasting predictions about the time evolution of metallicity gradients.

Aims.To test chemical evolution models and trace the star formation and accretion history of low luminosity disk galaxies we focus on the Local Group galaxy M 33.

Methods.We analyze O/H and S/H abundances in planetary nebulae, Hii regions, and young stars, together with known [Fe/H] abundances in the old stellar population of M 33. With a theoretical model, we follow the time evolution of gas (diffuse and condensed in clouds), stars, and chemical abundances in the disk of M 33, assuming that the galaxy is accreting gas from an external reservoir.

Results.Our model is able to reproduce the available observational constraints on the distribution of gas and stars in M 33 and to predict the time evolution of several chemical abundances. In particular, we find that a model characterized by a continuous infall of gas on the disk, at a rate of   yr^-1, almost constant with time, can also account for the relatively high rate of star formation and for the shallow chemical gradients.

Conclusions.Supported by a large sample of high resolution observations for this nearby galaxy, we conclude that the metallicity in the disk of M 33 has increased with time at all radii, with a continuous flattening of the gradient over the last ~8 Gyr.