The shape of Model B resembles the SFR history expected for the
``monolithic collapse'' model of galaxy evolution, although expectations
for hierarchical galaxy formation models are not ruled out
([32],
see discussion in [31];
see also the contribution of G. Kauffmann to these Proceedings). Thus the
monolithic-versus-hierarchical dilemma is still unsolved by our
current knowledge of the SFR history. Values of the SFR density at
higher redshifts (z 
5) will be
able place more definite
constraints on the galaxy evolution scenario.
The final question we want to ask is what fraction of the total SFR
density the Lyman-break galaxies represent at each given redshift. And
how much of the SF is so deeply buried in dust that its accounting is
missing. The obscuration curve discussed in
Section 3 is technically
valid only for UV-bright star-forming galaxies; it cannot, obviously,
correct for objects which are missing from the sample because they are
too dusty. On the one hand, Model B is only slightly in excess of the
obscuration-corrected SFR density calculated from the z > 2 galaxies
(by a negligible amount within the observational uncertainties), and
the SFR history of Model B is perfectly sufficient to reproduce the
observed CIB. It appears that the fraction of SF missed by considering
the Lyman-break galaxies only is relatively small. On the other hand,
a number of considerations invite to take this as a preliminary
statement. We know that at low redshift a fraction of the star
formation is deeply buried in dust, and is obscured even at IR
wavelengths. The same could happen at high redshift, and the SCUBA
sources seem to suggest that large dust contents are not impossible in
high-z galaxies. The angular density of the SCUBA sources is about
1/2-1 of that of the z ~ 3 galaxies, and are spread over a
(possibly) much larger redshift range than the Lyman-break galaxies,
namely over ~ 5-10x larger cosmological volumes. The SCUBA
sources are then ~ 5-20% of the Lyman-break galaxies by number
density, but are forming stars with
SFR 
300-500
M
yr-1. Thus the SCUBA galaxies
could still add 25-100% to
the SFR density of the
obscuration-corrected Lyman-break galaxies, although an assessment of
the AGN contribution is still missing.
Because of their characteristics, the two populations, the UV-bright Lyman-break galaxies and the FIR-bright SCUBA sources, are likely to be complementary, rather than overlapping. At the level of current knowledge, it appears that about 50-80% of the SF in the early phases of the Universe is accounted for by the obscuration-corrected Lyman-break galaxies; the remaining 20-50% of the SF may be contained in FIR-bright sources. However, more investigation of the nature, luminosity distribution and redshift placement of the SCUBA sources is needed before these figures can be taken at face value.
ACKNOWLEDGEMENTS
I am indebted with C. Steidel, M. Giavalisco, and M. Dickinson for making their most recent results on the Lyman-break galaxies available to me prior to publication, for discussions, and for a critical reading of the manuscript. I would like to thank the Organizing Committee for inviting me to this stimulating meeting and for financially support my stay at the Ringberg Castle.