The host haloes of O I absorbers in the reionization epoch

We use a radiation hydrodynamic simulation of the hydrogen reionization epoch to study O I absorbers at z ˜ 6. The intergalactic medium (IGM) is reionized before it is enriched; hence, O I absorption originates within dark matter haloes. The predicted abundance of O I absorbers is in reasonable agreement with observations. At z = 10, ≈70 per cent of sightlines through atomically cooled haloes encounter a visible (NOI > 1014cm-2) column. Reionization ionizes and removes gas from haloes less massive than 108.4 M⊙, 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. O I absorbers have neutral hydrogen columns of 1019-1021 cm-2, suggesting a close resemblance between objects selected in O I and H I 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 H I column density is enriched enough to violate the upper limits on the O I 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.