Potential signature of Population III pair-instability supernova ejecta in the BLR gas of the most distant quasar at z = 7.54

TL;DR

This study investigates the chemical signatures in a high-redshift quasar's broad-line region, suggesting evidence of Population III pair-instability supernovae enriching the early universe, with implications for understanding the first stars.

Contribution

It introduces a novel flux-to-abundance conversion method and proposes PISN as the source of unusual iron enrichment in a distant quasar, advancing Pop III star research.

Findings

The quasar's BLR shows extremely high iron abundance and low Mg/Fe ratio.

The observed abundance pattern cannot be explained by standard chemical evolution models.

PISN from massive Pop III stars likely contributed to early universe metal enrichment.

Abstract

The search for Population III (Pop III) stars has fascinated and eluded astrophysicists for decades. One promising place for capturing evidence of their presence must be high-redshift objects; signatures should be recorded in their characteristic chemical abundances. We deduce the Fe and Mg abundances of the broad-line region (BLR) from the intensities of ultraviolet Mg II and Fe II emission lines in the near-infrared spectrum of UKIDSS Large Area Survey (ULAS) J1342+0928 at $z = 7.54$, by advancing our novel flux-to-abundance conversion method developed for $z\sim 1$ quasars. We find that the BLR of this quasar is extremely enriched, by a factor of 20 relative to the solar Fe abundance, together with a very low Mg/Fe abundance ratio: $[\mathrm{Fe/H}]=+1.36\pm0.19$ and $[\mathrm{Mg/Fe}]=-1.11\pm0.12$, only 700 million years after the Big Bang. We conclude that such an unusual abundance feature cannot be explained by the standard view of chemical evolution that considers only the contributions from canonical supernovae. While there remains uncertainty in the high-mass end of the Pop III IMF, here we propose that the larger amount of iron in ULAS J1342+0928 was supplied by a pair-instability supernova (PISN) caused by the explosion of a massive Pop III star in the high-mass end of the possible range of 150-300 $M_\odot$. Chemical-evolution models based on initial PISN enrichment well explain the trend in [Fe/Mg]-$z$ all the way from $z < 3$ to $z = 7.54$. We predict that stars with very low [Mg/Fe] at all metallicities are hidden in the Galaxy, and they will be efficiently discovered by ongoing new-generation photometric surveys.