Pair Instability Supernova Origin of the High-[Fe/Mg] and High-[Si/Mg] Nature of the Quasar ULAS J1342+0928 at Redshift z = 7.54

The discovery of ULAS J1342+0928, a metal-enriched quasar ([Fe/H]∼1) at redshift z = 7.54 with an extremely high iron-to-magnesium abundance ratio ([Fe/Mg]∼1), has sparked a new insight into the origin of heavy elements in the early Universe. An earlier study by Yoshii et al. has shown that standard chemical evolution models of the quasar host galaxy involving Type Ia supernovae (SNe Ia) and core-collapse supernovae cannot reproduce the high-[Fe/Mg] nature of the broad-line region (BLR) gas of this quasar. Rather, the [Fe/Mg] value estimated in their study would be the result of heavy-element enrichment from a Population III (Pop III) pair-instability supernova (PISN). To investigate their result, we have run a series of photoionization models of the BLR gas enriched by such a PISN and have developed a highly updated version of their original method to convert heavy-element emission-line intensities to their abundances in the BLR gas. Our analysis reveals, for the first time, that the observed equivalent widths of Mg II, Si IV, and Fe II emission lines in ULAS J1342+0928 indicate abundance trends of [Mg/H] = 0.10 ± 0.09 (random) ± 0.3 (systematic), [Si/Mg] = 0.93 ± 0.02 (random) ± 0.2 (systematic), and [Fe/Mg] = 0.87 ± 0.06 (random) ± 0.2 (systematic). Particularly, the values of [Si/Mg] and [Fe/Mg] are best explained by a Pop III PISN with a progenitor mass of 240–280 M ⊙ , in support of Yoshii et al.’s result. If future observations of ULAS J1342+0928 confirm the significant odd–even effect of [Al/Mg] ≲ −1, as predicted by nucleosynthesis calculations for Pop III PISN, this would provide unambiguous evidence for the existence of a Pop III PISN at z ≳ 7 and shed light on the first Pop III stars in the Universe.