Recent progress in our understanding of faint galaxy data made possible
by the combination of Hubble Space Telescope (HST) deep imaging and
ground-based spectroscopy has dramatically increased our knowledge of the
evolution of the stellar birthrate in optically-selected galaxies from the
present-epoch up to z 
4
[12],
[31],
[21].
The explosion in the quantity of information available on the high-redshift
universe at optical wavelengths has been complemented by the measurement
of the far-IR/sub-mm background by DIRBE and FIRAS onboard the COBE
satellite
[9],
[15],
[27],
by the detection of distant ultraluminous sub-mm sources with the SCUBA camera
[28],
[9],
and by theoretical progress made in understanding how intergalactic gas follows
the dynamics dictated by dark matter halos until radiative, hydrodynamic, and
star formation processes take over
[2],
[30],
[24].
The IR data have revealed the ``optically-hidden'' side of galaxy
formation, and
shown that a significant fraction of the energy released by stellar
nucleosynthesis is re-emitted as thermal radiation by dust
[8],
[18].
The underlying goal of all these efforts is to understand the
growth of cosmic structures,
the internal properties of galaxies and their evolution, and ultimately
to map the star formation history of the universe from the end of
the cosmic `dark age' to the present epoch.
In this talk I will focus on the galaxy number-apparent magnitude relation and its first moment, the integrated galaxy contribution to the extragalactic background light (EBL). The logarithmic slope of the differential galaxy counts is a remarkably simple cosmological probe of the history of stellar birth in galaxies, as it must drop below 0.4 to yield a finite value for the EBL. The recently released Hubble Deep Field-South (HDF-S) images, together with other existing HST and ground-based observations, provide a unique dataset to estimate the spectrum and amplitude of the optical EBL from discrete sources. Together with the far-IR/sub-mm background, the optical EBL is an indicator of the total luminosity of the universe, as the cumulative emission from young and evolved galactic systems, as well as from active galactic nuclei (AGNs), is recorded in this background. As such it provides, for a given initial mass function, a quantitative estimate of the baryonic mass that has been processed by stars throughout cosmic history.
Unless otherwise stated, an Einstein-de Sitter (EdS) cosmology (
M = 1,
= 0) with
H0 = 100 h km s-1 Mpc-1
will be adopted in this talk.
All magnitudes will be given in the AB system. The work presented
here has been done in collaboration with L. Pozzetti.