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Astron. Astrophys. 355, 929-948 (2000)

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1. Introduction

The observed chemical and spectro-photometric properties of galaxies are one of the main sources of information for our understanding of galaxy formation and evolution. The corresponding theoretical modelling involves star formation (SF) as a basic ingredient. Unfortunately, this process is rather poorly known on the large scales relevant to galaxy evolution. Portinari & Chiosi (1999, hereinafter PC99) analysed the effects of adopting different SF laws in a chemical model for the Galactic Disc, a system which we can study in great detail. In this paper we address another phenomenon which can bear interesting effects on the chemical evolution of galaxies: radial gas flows. A few papers in literature (Sect. 2) demonstrate that radial gas flows influence chemical models for the Disc, especially in their predictions on the metallicity gradient. It is therefore interesting to discuss the radial profile of the Disc with models including also radial flows, in addition to various options for the SF law. In particular, radial flows can help to overcome some difficulties that "static" models find in reproducing, at the same time, the metallicity gradient and the radial gas profile of the Disc (PC99).

We develop a chemical model with radial gas flows as a multi-dimensional extension of the model of Portinari et al. (1998, hereinafter PCB98). The model is described in Sect. 3 and in the appendices. In Sect. 4 we discuss the general qualitative effects of superposing radial flows upon a chemical model. In Sect. 5 we present models for the Galactic Disc with radial gas flows and different SF laws, showing that radial flows provide an alternative or additional dynamical effect to the "inside-out" formation scenario to explain the metallicity gradient. Sect. 6 is dedicated to qualitative simulations of the dynamical effects of the Galactic Bar upon the gas distribution, with the aim to reproduce the molecular ring around 4 kpc, which static models cannot account for (PC99). Sect. 7 contains a final summary and conclusions.

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© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000