THE KECK + MAGELLAN SURVEY FOR LYMAN LIMIT ABSORPTION. I. THE FREQUENCY DISTRIBUTION OF SUPER LYMAN LIMIT SYSTEMS

摘要

We present the results of a survey for super Lyman limit systems (SLLSs; defined to be absorbers with 19.0 cm~(-2) ≤ N_(H_I) ≤ 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_(H_i)(N,X) of the SLLS and its moments and compare these results with the Lya forest and the damped Lyα systems (DLAs; absorbers with log N_(H_i) ≥ 20.3 cm~(-2)). We find that the f_(H_i)(N, X) of the SLLSs can be reasonably described with a power law of index a = -1.43_(-0.16)~(+0.15) or-1.19_(-0.21)~(+0.20) depending on whether we set the lower N_(H_i) bound for the analysis at 10~(19.0) or 10~(19.3) cm~(-2), respectively. The results indicate a flattening in the slope of f_(H_i)(N, X) between the SLLSs and DLAs. We find little evidence for redshift evolution in the shape f_(H_i)(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_(H_i)(N, X), the line density e_(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 cm~(-2) < log N_(H_i) ≤ 22.0 cm~(-2). We use these results to predict that f_(H_i)(N, X) must show two additional inflections below the SLLS regime to match the observed f_(H_i)(N, X) distribution of the Lya 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.