The host haloes of OI absorbers in the reionization epoch

We use a radiation hydrodynamic simulation of the hydrogen reionization epoch to study OI absorbers at z ~ 6. The intergalactic medium (IGM) is reionized before it is enriched; hence, OI absorption originates within dark matter haloes. The predicted abundance of OI absorbers is in reasonable agreement with observations. Atz = 10, ≈70 per cent of sightlines through atomically cooled haloes encounter a visible (N_OI > 10¹⁴cm^-2) column. Reionization ionizes and removes gas from haloes less massive than 10^8.4M_⊙, but 20 per cent of sightlines through more massive haloes encounter visible columns even at z = 5. The mass scale of absorber host haloes is 10-100 times smaller than the haloes of Lyman-break galaxies and Lyman α emitters, hence absorption probes the dominant ionizing sources more directly. OI absorbers have neutral hydrogen columns of 10¹⁹-10²¹ cm^-2, suggesting a close resemblance between objects selected in OI and HI absorption. Finally, the absorption in the foreground of the z = 7.085 quasar ULAS J1120+0641 cannot originate in a dark matter halo because halo gas at the observed HI column density is enriched enough to violate the upper limits on the OI column. By contrast, gas at less than one-third the cosmic mean density satisfies the constraints. Hence, the foreground absorption likely originates in the IGM.