Starlight in the Universe

There has been remarkable progress recently in both observational and theoretical studies of galaxy formation and evolution. Largely due to a combination of deep Hubble Space Telescope (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. The global ultraviolet, optical, near- and far-IR photometric properties of the universe 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) initial mass function, 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 study of the transition from cosmic `dark age' to an ionized universe populated with luminous sources can shed new light on the star formation activity at high redshifts, and promises answers to some fundamental questions on the formation of cosmic structures. If stellar sources are responsible for photoionizing the intergalatic 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 quasars at z ≲ 2 could make a significant contribution to the extragalactic background light if dust-obscured accretion onto supermassive black holes is an efficient process.