The physical properties of z > 2 Lyman limit systems: New constraints for feedback and accretion models

We study the physical properties of a homogeneous sample of 157 optically thick absorption line systems at redshifts ~1.8-4.4, selected from a high-dispersion spectroscopic survey of Lyman limit systems (LLSs). By means of multiple ionization models and Bayesian techniques, we derive the posterior probability distribution functions for the density, metallicity, temperature and dust content of the absorbing gas.We find that z > 2 LLSs are highly ionized with ionization parameters between -3 ≲ logU ≲ -2, depending on the HI column density. LLSs are characterized by low temperatures (T < 5 × 104K) and reside in dust-poor environments. Between z ~ 2.5-3.5, ~80 per cent of the LLSs have physical densities between nH ~ 10-3.5-10-2 cm-3 for the assumed UV background, but we caution that a degeneracy between the ionization parameter and the intensity of the radiation field prevents robust inference on the density and sizes of LLSs. Conversely, metallicity estimates are less sensitive to the assumptions behind ionization corrections. LLSs at z > 2 are characterized by a broad unimodal distribution over > 4 orders of magnitude, with a peak at log Z/Z⊙ ~-2. LLSs are metal poor, significantly less enriched than DLAs, with ~70 per cent of the metallicity PDF below log Z/Z⊙ = -1.5. The median metallicity of super LLSs with logNHI = 19 rapidly evolves with redshift, with a 10-fold increase between z ~ 2.1-3.6 (~1.5 Gyr). Based on this sample, we find that LLSs at z = 2.5-3.5 account for ~15 per cent of all the metals produced by UV-selected galaxies. The implications for theories of cold gas accretion and metal ejection from galaxies are also discussed.