OBSERVATIONS OF THE HUBBLE DEEP FIELD WITH THE INFRARED SPACE-OBSERVATORY .5. SPECTRAL ENERGY-DISTRIBUTIONS, STARBURST MODELS AND STAR-FORMATION HISTORY

We have modelled the spectral energy distributions of the 13 Hubble De ep Field (HDF) galaxies reliably detected by the Infrared Space Observ atory (ISO). For two galaxies the emission detected by ISO is consiste nt with being starlight or the infrared 'cirrus' in the galaxies. For the remaining II galaxies there is a clear midinfrared excess, which w e interpret as emission from dust associated with a strong starburst. 10 of these galaxies are spirals or interacting pairs, while the remai ning one is an elliptical with a prominent nucleus and broad emission lines. We give a new discussion of how the star formation rate can be deduced from the far-infrared luminosity, and derive star formation ra tes for these galaxies of 8-1000 phi M. yr(-1), where phi takes accoun t of the uncertainty in the initial mass function, The HDF galaxies de tected by ISO are clearly forming stars at, a prodigious rate compared with nearby normal galaxies, We discuss the implications of our detec tions for the history of star and heavy element formation in the Unive rse, Although uncertainties in the calibration, reliability of source detection, associations and starburst models remain, it is clear that dust plays an important role in star formation out to redshift 1 at le ast.