To appear in the proceedings of the 9th Annual
Astrophysics Conference in Maryland, "After the Dark Ages: When
Galaxies were Young", edited by S. S. Holt and E. P. Smith.
(astro-ph/9901237)
FAINT GALAXIES, EXTRAGALACTIC BACKGROUND LIGHT, AND THE REIONIZATION
OF THE UNIVERSE
Piero Madau
Space Telescope Science
Institute, Baltimore, USA
ABSTRACT. I review recent observational and theoretical progress
in our understanding
of the cosmic evolution of luminous sources. Largely due to a
combination of deep HST imaging, Keck spectroscopy, and
COBE far-IR background measurements,
new constraints have emerged on the emission history of the galaxy
population as
a whole. Barring large systematic effects, the global ultraviolet, optical,
near- and far-IR photometric properties of galaxies as a function of cosmic
time cannot be reproduced by a
simple stellar evolution model defined by a constant (comoving)
star-formation density
and a universal (Salpeter) IMF, and require instead a substantial
increase in the
stellar birthrate with lookback time.
While the bulk of the stars present today appears to have formed relatively
recently, the
existence of a decline in the star-formation density above z 
2
remains uncertain.
The history of the transition from the cosmic ``dark age'' to a ionized
universe populated with luminous sources can constrain the star formation
activity at high redshifts.
If stellar sources are responsible for photoionizing the
intergalactic medium at z 5, the rate of star formation at this
epoch must be comparable or greater than the one inferred from optical
observations of galaxies at z 3. A population of dusty, Type II AGNs
at z 
2 could make
a significant contribution
to the FIR background if the accretion efficiency is ~ 10%.
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