Astron. Astrophys. 324, 490-504 (1997)

1. Introduction

The star formation history is a fundamental quantity in the study of the galaxy populations and their evolution. It is becoming evident that this evolution can be influenced by the interactions with the intergalactic medium or with other galaxies, giving rise to episodes of intense star formation (bursts). Accordingly the elaboration of methods for evaluating the star formation rate (SFR) is one of the tasks of the research in this field.

Clearly the needed information arises from the integrated spectra and especially from spectral features contributed by stars or by the ionized interstellar gas. In this work we present an analysis of some of such features which are able to disclose the presence of stars of a given age and therefore can yield estimates of the SFR in different epochs of the galaxy life. To this aim we have developed a spectrophotometric model which takes into account both the stellar and the nebular contribution. The model predictions have been compared with a sample of observational data and from this comparison suitable quantities for the evaluation of the SFR have been singled out. Moreover criteria for ascertaining the occurrence of bursts, both current or recently concluded, have been derived.

The observational material consists of the sample of spectra of local galaxies of Kennicutt (1992a) including both normal galaxies, that is galaxies of the standard morphological sequence, and starburst galaxies. With such spectra, available in digital records from the Astronomical Data Center, Kennicutt's analysis has been extended by deriving other spectral features besides those computed by him. The comparison with the spectrophotometric model has allowed: a) to derive calibrations of the SFR in terms of several quantities, especially the intensities of the emission lines, b) to single out relations which are valid independently of the history of star formation of the galaxy, c) to find out quantities which can descriminate between normal and starburst galaxies.

The comparison with the spectra of normal galaxies allows to test the capability of the model to reproduce a large set of spectral features for all the galactic types. The model thus tested can be used generally in the study of distant galaxies in the frame of the problems of galaxy evolution.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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