Possible Population III Remnants at Redshift 3.5

TL;DR

This paper reports the discovery of a highly metal-poor gas cloud at redshift 3.53 with properties consistent with being a remnant of Population III stars, providing new insights into early universe metal enrichment.

Contribution

It presents the first measurement of relative abundances in such low metallicity gas, supporting models of early Population III star enrichment.

Findings

Metallicity of the gas is $10^{-3.41}$ times solar.

Carbon-to-silicon ratio is $10^{-0.26}$ times solar.

Velocity offset suggests enrichment by a single event.

Abstract

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.