Possible Population III Remnants at Redshift 3.5
  The first stars, known as Population III (PopIII), produced the first heavy
elements, thereby enriching their surrounding pristine gas. Previous detections
of metals in intergalactic gas clouds, however, find a heavy element enrichment
larger than $1/1000$ times that of the solar environment, higher than expected
for PopIII remnants. In this letter we report the discovery of a Lyman limit
system (LLS) at $z=3.53$ with the lowest metallicity seen in gas with
discernable metals, $10^{-3.41\pm0.26}$ times the solar value, at a level
expected for PopIII remnants. We make the first relative abundance measurement
in such low metallicity gas: the carbon-to-silicon ratio is $10^{-0.26\pm0.17}$
times the solar value. This is consistent with models of gas enrichment by a
PopIII star formation event early in the Universe, but also consistent with
later, Population II enrichment. The metals in all three components comprising
the LLS, which has a velocity width of 400 km s$^{-1}$, are offset in velocity
by $\sim+6$ km s$^{-1}$ from the bulk of the hydrogen, suggesting the LLS was
enriched by a single event. Relative abundance measurements in this
near-pristine regime open a new avenue for testing models of early gas
enrichment and metal mixing.
