There is little doubt that the last few years have been very exciting times in
galaxy formation and evolution studies. The remarkable 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 permitted to shed new light on the evolution of
the stellar birthrate in the universe, to identify the epoch 1
z
2 where most of
the optical extragalactic background light was produced, and to set important
contraints on galaxy evolution scenarios. The
explosion in the quantity of information available on the high-redshift
universe at optical wavelengths has been complemented by the detection of
the far-IR/sub-mm background by DIRBE and FIRAS onboard the COBE
satellite, and by theoretical progress
made in understanding how cosmic structure forms from initial density
fluctuations
[41].
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. The underlying
goal of all these efforts is to understand the growth of cosmic structures,
the internal properties of galaxies and their evolution,
the mechanisms that shaped Hubble's morphological sequence,
and ultimately to map
the transition from the cosmic `dark age' to a ionized
universe populated with luminous sources. While one of the important
questions recently emerged is the nature (starbursts or active galactic
nuclei?) and redshift distribution of the ultraluminous sub-mm sources
discovered by SCUBA, of perhaps equal interest is the
possible existence of a large population of faint galaxies still undetected
at high redshifts, as the color-selected ground-based and
Hubble Deep Field (HDF) samples include only the brightest and
bluest star-forming objects. In any hierarchical clustering (`bottom-up')
scenario (the
cold dark matter model being the best studied example), subgalactic
structures are the first non-linearities to form. High-z
dwarf galaxies and/or mini-quasars (i.e. an early generation of stars and
accreting black holes in dark matter halos with circular velocities
vc ~
50 km s-1) may then be one of the main source of UV photons and
heavy elements at early epochs.
In this talk I will focus on some of the open issues and controversies
surrounding our present understanding of the history of the conversion of
cold gas into stars within galaxies, and of the evolution
of luminous sources in the universe. An Einstein-deSitter (EdS) universe
(
M = 1,
= 0) with h =
H0 / 100 km s-1 Mpc-1 = 0.5 will be
adopted in the following.