Reionization of the Universe and the Early Production of Metals

Abstract

We simulate a plausible cosmological model in considerable physical and numerical detail through the successive phases of reheating (at 10<z<20), formation of Pop III stars at z=15 (due to molecular hydrogen cooling), with subsequent reionization at z=7. We assume an efficiency of high mass star formation appropriate to leave the universe, after it becomes transparent, with an ionizing background J_21=0.4 (at z=4), near (and perhaps slightly below) the observed value. Since the same stars produce the ionizing radiation and the first generation of heavy elements, a mean metallicity of Z/Z_\sun=1/200 is produced in this early phase, but there is a large variation about this mean, with the high density regions having Z/Z_\sun=1/30 and low density regions essentially no metals. Reionization, when it occurs, is very rapid, which will leave a signature which may be detectable by very large area meter-wavelength radio instruments. Also, the background UV radiation field will show a sharp drop from 1Ryd to 4Ryd due to absorption edges. The simulated volume is too small to form L_* galaxies, but the smaller objects which are found in the simulation obey the Faber-Jackson relation. In order to explore theoretically this domain of "the end of the dark ages" quantitatively, numerical simulations must have a mass resolution of the order of 10^{4.5} M_\sun} in baryons, high spatial resolution (1 kpc) to resolve strong clumping, and allow for detailed and accurate treatment of both the radiation field and atomic/molecular physics.