Dissecting the star-formation history of starburst galaxies: the case of NGC7673

We have collected archival data on NGC7673 to constrain the star-formation history that produced the young star clusters and the field stellar population in this galaxy during the last 2 Gyr. We have considered the sample of 50 star clusters detected by HST/WFPC2 in the UV, V and I bands and estimated their age, intrinsic reddening, and mass via comparison of their colours with STARBURST99 models. We have found two prominent epochs of cluster formation occurred about 20 Myr and 2 Myr ago, with somewhat minor events between 3 Myr and 6 Myr ago. The star clusters are characterised by an intrinsic reddening E(B-V) < 0.4 mag and a mass lower than 2e+06 solar masses. Out of the 50 star clusters, we have selected 31 located within the boundaries of the IUE large slit that was employed to obtain the spectrum of NGC7673 between 1150 Ang. and 3350 Ang. For each cluster, we have built a synthetic spectrum corresponding to the age, mass and intrinsic reddening derived from the cluster colours, properly redshifted to NGC7673. The spectra have then been added together in a final, clusters integrated spectrum. This and the IUE and FUSE spectra of NGC7673 have allowed us to describe the star-formation history of the unresolved stars in the field as either exponentially decaying or multi-burst. In the first case, we have derived an e-folding time of 700 (900) Myr and an initial star-formation rate of 16 (13) solar masses per year when the Fitzpatrick's (Calzetti's) extinction law is used. In the case of a multi-burst star-formation history, the field population turns out to be composed by a young (< 40 Myr) component 3 (2) times brighter than the star clusters, and a component as old as 850 (450) Myr, about 200 (100) times more massive than the star clusters together.