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) and 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 J₂₁ ≈ 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_☉ ~ 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_☉ ≈ 1/30 and the low density regions (or the Lyα forest with N_{H I} 10^13.5 cm²) having essentially no metals. When it occurs, reionization is very rapid (phase change-like), which will leave a signature that may be detectable by very large area meter-wavelength radio instruments. Also, the background UV radiation field will show a sharp drop of ~10^-3 from 1 to 4 ryd because of absorption edges. The simulated volume is too small to form L_* galaxies, but the smaller objects that 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_☉ in baryons, have 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.