THE DETERMINATION OF THE STAR-FORMATION RATE IN GALAXIES

A spectrophotometric model able to compute the integrated spectrum of a galaxy, including the contribution both of the stellar populations a nd of the ionized interstellar gas of the HII regions powered by young hot stars, has been used to study several spectral features and photo metric quantities in order to derive calibrations of the star formatio n history of late type galaxies. Attention has been paid to analyze th e emission of the Balmer lines and the [OII]lambda 3727 line to test t heir attitude at providing estimates of the present star formation rat e in galaxies. Other features, like D-4000 and the equivalent width of the H-delta line, influenced by the presence of intermediate age star s, have been considered. Several ways of estimating the star formation rates in normal galaxies are discussed and some considerations concer ning the applicability of the models are presented. Criteria have been also studied for ascertaining the presence of a burst, current or end ed not long ago, Bursts usually hinder the determination of the past s tar formation rate.