We analyze a morphologically-selected complete sample of 52 late-type (spiral
and irregular) galaxies in the Hubble Deep Field North with total K-magnitudes
brighter than K=20.47. This sample exploits in particular the ultimate imaging
quality achieved by HST in this field, allowing us to clearly disentangle the
galaxy morphologies, based on accurate profiles of the surface brightness
distributions. Our purpose was to investigate systematic differences between
the two classes, as for colours, redshift distributions and ages of the
dominant stellar populations. This sample appears to miss significantly
galaxies above z=1.4 (in a similar way as an early-type galaxy sample
previously studied by us), a fact which may be explained as a global decline of
the underlying mass function for galaxies at these high redshifts. Differences
between early and late-types are apparent -particularly in the colour
distributions and the evolutionary star-formation (SF) rates per unit volume-,
although the complication in spectro-photometric modelling introduced by
dust-extinction in the gas-rich systems prevents us to reach conclusive results
on the single sources. However, we find that an integrated quantity like the
comoving star-formation rate density (SFR(t)) as a function of redshift is much
less affected by these uncertainties: by combining this with the previously
studied early-type galaxy sample, we find a shallower dependence of SFR(t) on z
between z=0.2 and z=1.5 than found by Lilly et al. (1995).Comment: 16 pages, 13 figures - Version accepted for publication in A&
