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2010ApJ...718..392P - Astrophys. J., 718, 392-416 (2010/July-3)

A definitive survey for Lyman limit systems at z ∼ 3.5 with the Sloan Digital Sky Survey.

PROCHASKA J.X., O'MEARA J.M. and WORSECK G.

Abstract (from CDS):

We perform a semi-automated survey for τ912≥ 2 Lyman limit systems (LLSs) in quasar spectra from the Sloan Digital Sky Survey, Data Release 7. From a starting sample of 2473 quasars with zem= 3.6-5.0, we analyze 429 spectra meeting strict selection criteria for a total redshift path Δz = 93.8 and identify 190 intervening systems at zLLS≥ 3.3. The incidence of τ912≥ 2 LLSs per unit redshift, ℓτ≥2(z), is well described by a single power law at these redshifts: ℓ_τ≥2(z)= C_LLS [(1+z)/(1+z*)]γ_LLS, with z*≡ 3.7, CLLS= 1.9±0.2, and γLLS= 5.1±1.5 (68% c.l.). These values are systematically lower than previous estimates (especially at z < 4) but are consistent with recent measurements of the mean free path to ionizing radiation. Extrapolations of this power law to z = 0 are inconsistent with previous estimations of ℓ(z) at z < 1 and may indicate a break at z ≈ 2, similar to that observed for the Lyα forest. Our results also indicate that the systems giving rise to LLS absorption decrease by ≈ 50% in comoving number density and/or physical size from z = 4 to 3.3, perhaps due to an enhanced extragalactic ultraviolet background. The observations place an integral constraint on the H I frequency distribution f(N_ H I_, X) and indicate that the power-law slope β≡dlnf(N_H i,X)/dlnN_H i is likely shallower than β = -1 at {N_H i~10^{18} cm^-2}. Including other constraints on f(N_ H I_, X) from the literature, we infer that β is steeper than β = -1.7 at {N_H i~10^{15} cm^-2}, implying at least two inflections in f(N_ H I_, X). We also perform a survey for proximate LLSs (PLLSs) and find that ℓPLLS(z) is systematically lower (≈ 25%) than intervening systems. Finally, we estimate that systematic effects impose an uncertainty of 10%-20% in the ℓ(z) measurements; these effects may limit the precision of all future surveys.

Journal keyword(s): diffuse radiation - intergalactic medium

VizieR on-line data: <Available at CDS (J/ApJ/718/392): table1.dat table2.dat table3.dat table6.dat table8.dat>