We thank Joachim Köppen for his advice on numerical modelling of radial gas flows, Yuen K. Ng and Antonella Vallenari for useful discussions about Galactic structure, and our referee, Mike Edmunds, whose detailed comments much improved the presentation of our paper. L.P. acknowledges kind hospitality from the Nordita Institute in Copenhagen, from the Observatory of Helsinki and from Sissa/Isas in Trieste. This study has been financed by the Italian MURST through a PhD grant and the contract "Formazione ed evoluzione delle galassie", n. 9802192401.
© European Southern Observatory (ESO) 2000
Online publication: March 21, 2000