Misc,
Probing the metallicity and ionization state of the circumgalactic medium at z~6 and beyond with OI absorption
(2013)cite arxiv:1310.5859Comment: 13 pages, 9 figures, 1 table.
Abstract
Low ionization metal absorption due to OI has been identified as an important
probe 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. For plausible assumptions about the
photo-ionization rate due to the meta-galactic UV background and the
self-shielding of optically thick absorbers, our modelling reproduces the
equivalent width distribution and incidence rate of the observations by Becker
et al. (2011) with a model where the weak OI absorbers 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 CGM has already progressed considerably by z~6. The
apparently rapid evolution of the incidence rates for OI absorption over 5<z<6
mirrors that of LLSs at lower redshift and is mainly due to the rapid decrease
of the meta-galactic photo-ionization rate at z>5. Assuming the same
metallicity-density relation inferred at z~6, we predict the incidence rate of
OI absorbers at z=7-8 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.
