Exploring the origins of a new, apparently metal-free gas cloud at z =
  4.4
  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 $\lesssim$1/10000 solar. Our HIRES spectrum of
the background quasar, PSS1723$+$2243, provides a neutral hydrogen column
density range for LLS1723 of $N_\textrm{HI}=10^{\text{17.9--18.3}}$ cm$^{-2}$
at redshift $z_\textrm{abs}\approx4.391$. The lower bound on this range, and
the lack of detectable absorption from the strongest low-ionisation metal
lines, are combined in photoionisation models to infer a robust, conservative
upper limit on the metallicity: $\log(Z/Z_\odot)<-4.14$ at 95% 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.
