A survey of extremely metal-poor gas at cosmic noon: evidence of elevated [O/Fe]

TL;DR

This study analyzes the chemical abundances of extremely metal-poor gas clouds at cosmic noon (redshift 2-4), revealing elevated [O/Fe] ratios and insights into early stellar populations and enrichment processes.

Contribution

It provides high-precision abundance measurements of eight metal-poor gas clouds, demonstrating elevated [O/Fe] and proposing a model of enrichment by two massive progenitors with specific properties.

Findings

EMP DLAs have mean [O/Fe]=+0.50 with low scatter.

VMP DLAs show [O/Fe]=+0.40 with minimal scatter.

Evidence suggests redshift evolution of [C/O] ratios.

Abstract

We aim to study the high-precision chemical abundances of metal-poor gas clouds at cosmic noon (2<z<4) and investigate the associated enrichment histories. We analyse the abundances of four newly discovered metal-poor gas clouds utilising observations conducted with Keck/HIRES and VLT/UVES. These systems are classified as very metal-poor (VMP), with [Fe/H]<-2.57, and one system qualifies as an extremely metal-poor (EMP) Damped Lyman-alpha (DLA) system with [Fe/H]=-3.13+/-0.06. In combination with new high-resolution data of two previously known EMP DLAs and 2 systems reported in the literature, we conduct a comprehensive analysis of eight of the most metal-poor gas clouds currently known. We focus on high-precision abundance measurements using the elements: C, N, O, Al, Si, and Fe. Our findings indicate increasing evidence of elevated [O/Fe] abundances when [Fe/H]<-3. EMP DLAs are well-modelled with a mean value of [O/Fe]=+0.50 +/- 0.04 and an intrinsic scatter of $\sigma_{int,[O/Fe]}=0.13^{+0.06}_{-0.04}$. While VMP DLAs are well-modelled with [O/Fe]=+0.40 +/- 0.02 and $\sigma_{int,[O/Fe]}$=0.06 +/- 0.02. We further find tentative evidence of a redshift evolution of [C/O] across these most metal-poor DLAs with lower redshift systems showing elevated [C/O] ratios. Using the measured abundances, combined with a stochastic chemical enrichment model, we investigate the properties of the stellar population responsible for enriching EMP gas at cosmic noon. We find that the chemistry of these systems is best explained via the enrichment of just two massive progenitors, N_*=2+/-1, that ended their lives as core collapse SNe with a typical explosion energy E_exp=(1.6 +/- 0.6)x10$^{51}$ erg. These progenitors formed obeying a Salpeter-like power-law IMF, where all stars of mass greater than M_max=32$^{+10}_{-4}$ M_sun collapse directly to black holes and do not contribute to the metal enrichment.