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

Lyman limit systems (LLSs) are promising probes of pristine or near-pristine environments, being possibly metal-free, or polluted only by the ejecta from PopIII stars. As such, they may prove an important probe of the PopIII star population at redshifts ≥ 15. Such LLSs will have a very-low metallicity, ≤ 1/1000th of the solar value at least, and show weak metal absorption lines against a background quasar's continuum in the near-pristine case. In P. Frédéric Robert et al. 2018 (MNRAS, submitted), I report the discovery and analysis of only the third LLS, LLS1723, in which a high-quality, echelle-resolution spectrum reveals no metal absorption lines, implying a metallicity ≤ 1/10000 solar. 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. These 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. Observationally, mapping the galaxy distribution around such systems may assist in better understanding their origin. New optical integral field spectrographs, such as KCWI, and MUSE are already demonstrating this to be a promising approach (e.g. Fumagalli et al. 2016).