A "minihalo" model for the Lyman limit absorption systems at high redshift

We propose that a large fraction of QSO Lyman limit absorption systems (LLSs) observed at high redshift (z greater than or similar to 3) originate from gas trapped in small objects, such as minihalos, that form prior to reionization. In the absence of a strong UV flux, the gas is predominantly neutral and may form clouds with H I column density N(H I) greater than or similar to 10(18) cm(-2). Owing to their high densities and high H I column densities, these clouds an not destroyed by the onset of the UV background at a later time. Thus, if not disrupted by other processes, such as mergers into larger systems or "blow away" by supernovae, they will produce LLSs. We show that the observed number density of LLSs at high redshifts can be well reproduced by the survived "minihalos" in hierarchical clustering models such as the standard cold dark matter model. The number density of LLSs in such a population increases with z even beyond the redshifts accessible to current observations and dies off quickly at z less than or similar to 2. This population is distinct from other populations because the absorbing systems have small velocity widths and a close to primordial chemical composition. The existence of such a population requires that the reionization of the universe occurs late, at z less than or similar to 20.