Faint galaxies, extragalactic background light, and the reionization of the Universe

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 of order 10%.