2007ApJ...656..666O

We present the results of a survey for super Lyman limit systems (SLLSs; defined to be absorbers with 19.0/cm2≤logN_HI_≤20.3/cm2) from a large sample of high-resolution spectra acquired using the Keck and Magellan telescopes. Specifically, we present 47 new SLLSs from 113 QSO sight lines. We focus on the neutral hydrogen frequency distribution f_HI_(N,X) of the SLLS and its moments and compare these results with the Lyα forest and the damped Lyα systems (DLAs; absorbers with logN_HI_≥20.3/cm2). We find that the f_HI_(N,X) of the SLLSs can be reasonably described with a power law of index α=-1.43^+0.15_–0.16 or -1.19^+0.20_–0.21depending on whether we set the lower N_HI_ bound for the analysis at 10^19.0 or 10^19.3/cm2, respectively. The results indicate a flattening in the slope of f_HI_(N,X) between the SLLSs and DLAs. We find little evidence for redshift evolution in the shape of f_HI_(N,X) for the SLLSs over the redshift range of the sample 1.68<z<4.47 and only tentative evidence for evolution in the zeroth moment of f_HI_(N,X), the line density ℓ_SLLS(X). We introduce the observable distribution function O(N,X) and its moment, which elucidates comparisons of H I absorbers from the Lyα forest through to the DLA. We find that a simple three-parameter function can fit O(N,X) over the range 17.0/cm2≤logN_HI_≤22.0/cm2. We use these results to predict that f_HI_(N,X) must show two additional inflections below the SLLS regime to match the observed f_HI_(N,X) distribution of the Lyα forest. Finally, we demonstrate that SLLSs contribute a minor fraction (~15%) of the universe's hydrogen atoms and, therefore, an even smaller fraction of the mass in predominantly neutral gas.