Gas Accretion via Lyman Limit Systems

In cosmological simulations, a large fraction of the partial Lyman limit systems (pLLSs; \(16\lesssim \log N_{\text{HI}} 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 \(\lesssim\) 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.