To appear in Physica Scripta,
Proceedings of the Nobel Symposium, Particle Physics and the
Universe (Enkoping, Sweden, August 20-25, 1998).
For a PDF version of the article, click
here.
STARLIGHT IN THE UNIVERSE
Piero Madau
Space Telescope Science
Institute, Baltimore, MD 21218
ABSTRACT. 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 the 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
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 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.
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