2020MNRAS.494.1411W

Using science verification observations obtained with ESPRESSO at the Very Large Telescope (VLT) in 4UT mode, we report the first bound on the carbon isotope ratio ¹²C/¹³C of a quiescent, near-pristine damped Ly α (DLA) system at z = 2.34. We infer a limit log₁₀ ¹²C/¹³C > +0.37 (2σ). We use the abundance pattern of this DLA, combined with a stochastic chemical enrichment model, to infer the properties of the enriching stars, finding the total gas mass of this system to be log₁₀(M_ gas_/ M_☉)=6.3^+1.4_–0.9 and the total stellar mass to be log₁₀(M_*/M_☉) = 4.8 ± 1.3. The current observations disfavour enrichment by metal-poor asymptotic giant branch (AGB) stars with masses < 2.4 M_☉, limiting the epoch at which this DLA formed most of its enriching stars. Our modelling suggests that this DLA formed very few stars until ≳1 Gyr after the cosmic reionization of hydrogen and, despite its very low metallicity (∼1/1000 of solar), this DLA appears to have formed most of its stars in the past few hundred Myr. Combining the inferred star formation history with evidence that some of the most metal-poor DLAs display an elevated [C/O] ratio at redshift z <= 3, we suggest that very metal-poor DLAs may have been affected by reionization quenching. Finally, given the simplicity and quiescence of the absorption features associated with the DLA studied here, we use these ESPRESSO data to place a bound on the possible variability of the fine-structure constant, Δα/α = (-1.2 ± 1.1) x 10^–5.