2019MNRAS.483.2736R

We report the discovery and analysis of only the third Lyman-limit system in which a high-quality resolution, echelle spectrum reveals no metal absorption lines, implying a metallicity <=1/10 000 solar. Our HIRES spectrum of the background quasar, PSS1723+2243, provides a neutral hydrogen column density range for LLS1723 of N_ H I_=10^17.9–18.3 cm^–2 at redshift z_abs ≃ 4.391. The lower bound on this range and the lack of detectable absorption from the strongest low-ionization metal lines are combined in photoionization models to infer a robust, conservative upper limit on the metallicity: log (Z/Z_☉) < -4.14 at 95 per cent confidence. Such a low metallicity raises the question of LLS1723's origin and enrichment history. Previous simulations of the circumgalactic medium imply that LLS1723 is a natural candidate for a cold gas stream accreting towards a galaxy. Alternatively, LLS1723 may represent a high-density portion of the intergalactic medium containing either pristine gas - unpolluted by stellar debris for 1.4 Gyr after the big bang - or the remnants of low-energy supernovae from (likely low-mass) Population III stars. Evidence for the circumgalactic scenario could be obtained by mapping the environment around LLS1723 with optical integral-field spectroscopy. The intergalactic possibilities highlight the need for - and opportunity to test - simulations of the frequency with which such high-density, very low metallicity systems arise in the intergalactic medium.