Exploring the origins of a new, apparently metal-free gas cloud at z = 4.4

Abstract

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 NHI=1017.9-18.3 cm−2 at redshift zabs ≈ 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.