Lyman Break Galaxies and Reionization of the Universe

We present the star formation histories, luminosities, colors, mass to light ratios, and halo masses of "galaxies" formed in a simulation of cosmological reionization. We compare these galaxies with Lyman Break Galaxies observed at high redshift. While the simulation is severely limited by the small box size, the simulated galaxies do appear as fainter cousins of the observed ones. They have similar colors, and both simulated and observed galaxies can be fitted with the same luminosity function. This is significant to the process of reionization since in the simulation the stars alone are responsible for reionization at a redshift consistent with that of the Gunn-Peterson trough seen in the absorption spectra of QSO's. The brightest galaxies at z = 4 are indeed quite young in accord with observational studies. But these brightest galaxies are free riders - they contributed only about 25% to the reionization of the universe at z > 6. Instead, the bulk of the work was done by dimmer galaxies, those that fall within the 28 < I < 30 magnitude range at z = 4. These dimmer galaxies are not necessarily less massive than the brightest ones.