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 approx 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 approx 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 approx 3. A population of dusty, Type II AGNs at z ltapprox 2 could make a significant contribution to the FIR background if the accretion efficiency is ~ 10%.


Table of Contents

INTRODUCTION

OPTICAL/FIR BACKGROUND

COSMIC STAR FORMATION

STELLAR BARYON BUDGET

TWO SIMPLE MODELS

TYPE II AGNS

SOURCES OF IONIZING RADIATION

REIONIZATION OF THE IGM

REFERENCES

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