Discovery of three new near-pristine absorption clouds at $z=2.6$-4.4

TL;DR

This study reports the discovery of three new near-pristine Lyman Limit Systems at high redshifts, providing insights into early universe metal enrichment and the potential remnants of Population III supernovae.

Contribution

It presents the first targeted survey successfully identifying near-pristine LLSs at high redshift, expanding the sample for studying primordial metal abundances.

Findings

Discovered three new near-pristine LLSs at z=2.6-4.4.

Measured metallicities around 1/1000 solar in these systems.

Demonstrated the feasibility of future surveys to detect PopIII signatures.

Abstract

We report the discovery of three new "near-pristine" Lyman Limit Systems (LLSs), with metallicities ~1/1000 solar, at redshifts 2.6, 3.8 and 4.0, with a targeted survey at the Keck Observatory. High resolution echelle spectra of eight candidates yielded precise column densities of hydrogen and weak, but clearly detected, metal lines in seven LLSs; we previously reported the one remaining, apparently metal-free LLS, to have metallicity <1/10000 solar. Robust photoionisation modelling provides metallicities [Si/H] = -3.05 to -2.94, with 0.26 dex uncertainties (95% confidence) for three LLSs, and [Si/H] >~ -2.5 for the remaining four. Previous simulations suggest that near-pristine LLSs could be the remnants of PopIII supernovae, so comparing their detailed metal abundances with nucleosynthetic yields from supernovae models is an important goal. Unfortunately, at most two different metals were detected in each new system, despite their neutral hydrogen column densities (10^{19.2-19.4} cm^{-2}) being two orders of magnitude larger than the two previous, serendipitously discovered near-pristine LLSs. Nevertheless, the success of this first targeted survey for near-pristine systems demonstrates the prospect that a much larger, future survey could identify clear observational signatures of PopIII stars. With a well-understood selection function, such a survey would also yield the number density of near-pristine absorbers which, via comparison to future simulations, could reveal the origin(s) of these rare systems.