A Carbon-enhanced Lyman Limit System: Signature of the First Generation of Stars?

Abstract

We present the study of a Lyman limit system (LLS) at z abs  = 1.5441 toward quasar J134122.50+185213.9 observed with VLT X-shooter. This is a very peculiar system with strong C  i absorption seen associated with a neutral hydrogen column density of log N (H i ) (cm −2 ) = 18.10, too small to shield the gas from any external UV flux. The low-ionization absorption lines exhibit a simple kinematic structure consistent with a single component. Using CLOUDY models to correct for ionization, we find that the ionization parameter of the gas is in the range −4.5 < log U < –4.2 and the gas density –1.5 < log  n (H) (cm −3 ) +2.2 at [Fe/H] ∼–1.6. Such a metal abundance pattern is reminiscent of carbon-enhanced metal-poor stars detected in the Galaxy halo. Metal enrichment by the first generation of supernovae provide a plausible explanation for the inferred abundance pattern in this system.