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

TL;DR

This study analyzes a unique Lyman limit system with high carbon enhancement at redshift 1.5441, suggesting it may be a signature of the first generation of stars and their supernovae enrichment.

Contribution

It reports the discovery of a carbon-enhanced Lyman limit system with unusual abundance patterns, linking it to early stellar populations and first-generation supernovae.

Findings

High carbon overabundance relative to iron ([C/Fe] > +2.2)

Metallicity similar to metal-poor stars in the Galaxy halo

Possible signature of first-generation star enrichment

Abstract

We present the study of a Lyman limit system (LLS) at $z_{\rm abs}$ = 1.5441 towards 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$^{-3}$) $<$ $-$1.2. The models suggest that carbon is overabundant relative to iron, [C/Fe] $>$ +2.2 at [Fe/H] $\sim$ $-$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 provides a plausible explanation for the inferred abundance pattern in this system.