The Keck plus Magellan survey for Lyman limit absorption. I. The frequency distribution of super Lyman limit systems

We present the results of a survey for super Lyman limit systems (SLLSs; defined to be absorbers with 19.0 cm(-2) <= log N(HI) <= 20.3 cm(-2)) 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 Ly alpha forest and the damped Ly alpha systems (DLAs; absorbers with log N(HI) >= 20.3 cm(-2)). We find that the f(HI)(N; X) of the SLLSs can be reasonably described with a power law of index alpha = -1.43(-0.16)(+0: 15) or -1.19(-0.21)(+0: 20) depending on whether we set the lower NH i bound for the analysis at 1019.0 or 1019.3 cm(-2), 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 zerothmoment 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 HI absorbers from he Ly alpha forest through to theDLA. We find that a simple threeparameter function can fit O(N; X) over the range 17.0 cm(-2) <= log N(HI) < 22: 0 cm(-2). 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 Ly alpha forest. Finally, we demonstrate that SLLSs contribute a minor fraction (approximate to 15%) of the universe's hydrogen atoms and, therefore, an even smaller fraction of the mass in predominantly neutral gas.