Probing the metallicity and ionization state of the circumgalactic
  medium at z~6 and beyond with OI absorption
  Low ionization metal absorption due to OI has been identified as an important
probe of the physical state of the inter-/circumgalactic medium at the tail-end
of reionization. We use here high-resolution hydrodynamic simulations to
interpret the incidence rate of OI absorbers at z~6 as observed by Becker et
al. (2011). We infer weak OI absorbers (EW > 0.1 A) to have typical HI column
densities in the range of sub-DLAs, densities of 80 times the mean baryonic
density and metallicities of about 1/500 th solar. This is similar to the
metallicity inferred at similar overdensities at z~3, suggesting that the metal
enrichment of the circumgalactic medium around low-mass galaxies has already
progressed considerably by z~6. The apparently rapid evolution of the incidence
rates for OI absorption over the redshift range 5<z<6 mirrors that of
self-shielded Lyman-Limit systems at lower redshift and is mainly due to the
rapid decrease of the meta-galactic photo-ionization rate at z>5. We predict
the incidence rate of OI absorbers to continue to rise rapidly with increasing
redshift as the IGM becomes more neutral. If the distribution of metals extends
to lower density regions, OI absorbers will allow the metal enrichment of the
increasingly neutral filamentary structures of the cosmic web to be probed.
