  We have modelled the spectral energy distributions of the 13 HDF galaxies
reliably detected by ISO. For 2 galaxies the emission detected by ISO is
consistent with being starlight or the infrared 'cirrus' in the galaxies. For
the remaining 11 galaxies there is a clear mid-infrared excess, which we
interpret as emission from dust associated with a strong starburst. 10 of these
galaxies are spirals or interacting pairs, while the remaining 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 rates for these galaxies
of 8-1000 $\phi M_{\sun}$ per yr, where $\phi$ takes account of the uncertainty
in the initial mass function. The HDF galaxies detected by ISO are clearly
forming stars at a prodigious rate compared with nearby normal galaxies. We
discuss the implications of our detections for the history of star and heavy
element formation in the universe. 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 least.
