Gas Accretion via Lyman Limit Systems

TL;DR

This paper reviews evidence from cosmological simulations and observations that low-metallicity Lyman limit systems indicate ongoing cold gas accretion onto galaxies, fueling star formation across cosmic time.

Contribution

It synthesizes recent empirical findings on the metallicity of pLLSs and LLSs, highlighting their role as indicators of cold gas accretion in galaxy evolution.

Findings

Presence of metal-poor gas at all redshifts in pLLSs and LLSs

Low-metallicity gas located within 100-200 kpc of galaxies

Significant mass of cold, dense, low-metallicity gas in the CGM

Abstract

In cosmological simulations, a large fraction of the partial Lyman limit systems (pLLSs; 16<log N(HI)<17.2) and LLSs (17.2log N(HI)<19) probes large-scale flows in and out of galaxies through their circumgalactic medium (CGM). The overall low metallicity of the cold gaseous streams feeding galaxies seen in these simulations is the key to differentiating them from metal rich gas that is either outflowing or being recycled. In recent years, several groups have empirically determined an entirely new wealth of information on the pLLSs and LLSs over a wide range of redshifts. A major focus of the recent research has been to empirically determine the metallicity distribution of the gas probed by pLLSs and LLSs in sizable and representative samples at both low (z<1) and high (z>2) redshifts. Here I discuss unambiguous evidence for metal-poor gas at all z probed by the pLLSs and LLSs. At z<1, all the pLLSs and LLSs so far studied are located in the CGM of galaxies with projected distances <100-200 kpc. Regardless of the exact origin of the low-metallicity pLLSs/LLSs, there is a significant mass of cool, dense, low-metallicity gas in the CGM that may be available as fuel for continuing star formation in galaxies over cosmic time. As such, the metal-poor pLLSs and LLSs are currently among the best observational evidence of cold, metal-poor gas accretion onto galaxies.