Next Contents Previous


Recent studies of the volume-averaged history of stellar birth are pointing to an era of intense star formation at z approx 1-1.5. The optical datasets imply that a fraction close to 65% of the present-day stars was produced at z >1, and only 25% at z >2. About half of the stars observed today would be more than 9 Gyr old, and only 10% would be younger than 5 Gyr. (6) There is no ``single epoch of galaxy formation'': rather, it appears that galaxy formation is a gradual process. Numerous uncertainties remain, however, particularly the role played by dust in obscuring star-forming objects. Our first glimpse of the history of galaxies to z ~ 4 leads to the exciting question of what happened before. Substantial sources of ultraviolet photons must have been present at z gtapprox 5 to keep the universe ionized, perhaps low-luminosity quasars [23] or a first generation of stars in dark matter halos with virial temperature Tvir ~ 104-105 K [42], [22]. Early star formation provides a possible explanation for the widespread existence of heavy elements in the Lyalpha forest [9], while reionization by QSOs may produce a detectable signal in the radio extragalactic background at meter wavelengths [36]. A detailed exploration of such territories must await projected facilities like the Next Generation Space Telescope and the Square Kilometer Radio Telescope.


Support for this work was provided by NASA through ATP grant NAG5-4236.

6 Unlike the measured number densities of objects and rates of star formation, the integrated stellar mass density does not depend on the assumed cosmological model.