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

TL;DR

This study characterizes the physical properties of high-redshift Lyman limit systems, revealing their ionization, metallicity, and dust content, and discusses implications for galaxy evolution and gas accretion models.

Contribution

It provides new constraints on the physical conditions of z>2 LLSs using a large homogeneous sample and Bayesian ionization modeling, highlighting their low metallicity and evolution.

Findings

LLSs are highly ionized with low temperatures.

Most LLSs are metal-poor, with metallicity below 1/30th solar.

Median metallicity increases tenfold from z~3.6 to z~2.1.

Abstract

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 ionisation 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 ionised with ionisation parameters between -3<log U<-2, depending on the HI column density. LLSs are characterised by low temperatures (T<5x10^4 K) and reside in dust-poor environments. Between z~2.5-3.5, ~80% of the LLSs have physical densities between n(H)~10^-3.5-10^-2 cm^-3 for the assumed UV background, but we caution that a degeneracy between the ionisation 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 ionisation corrections. LLSs at z>2 are characterised by a broad unimodal distribution over >4 orders of magnitude, with a peak at log Z/Zsun~-2. LLSs are metal poor, significantly less enriched than DLAs, with ~70% of the metallicity PDF below log Z/Zsun<-1.5. The median metallicity of super LLSs with log N(HI)>19 rapidly evolves with redshift, with a ten-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% 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.