STScI Preprint #1323

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 ~ 10%.

Status:

Appeared in: the proceedings of the 9th Annual October Astrophysics Conference in Maryland, After the Dark Ages: When Galaxies Were Young

Affiliations:

Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA