Fe Abundances of Early Galaxies at $z=9-12$ Derived with Deep JWST Spectra

Abstract: We derive Fe-abundance ratios of 6 galaxies at $z=9-12$ with $-22<M_{\mathrm{UV}}<-19$ whose JWST/NIRSpec spectra achieve very high signal-to-noise ratios, $\mathrm{SNR}=40-230$, at the rest-frame UV wavelength. We fit stellar synthesis model spectra to these JWST spectra, carefully masking out nebular emission, interstellar absorption, and non-iron stellar absorption lines, and obtain Fe-abundance ratios of $\mathrm{[Fe/H]}=-2-0$ for 4 galaxies and upper limits of $\mathrm{[Fe/H]}\sim-1$ for 2 galaxies. We compare these [Fe/H] values with the oxygen abundances of these galaxies ($7.2<12+\log{\mathrm{(O/H)}}<7.9$) in the same manner as previous studies of low-$z$ galaxies, and derive oxygen-to-iron abundance ratios [O/Fe]. We find that majority of (4 out of 6) galaxies are consistent with iron-poor abundance ratios ($\mathrm{[O/Fe]}\gtrsim0$) while that 2 out of 6 galaxies, GS-z11-0 and GN-z11, show Fe enhancements ($\mathrm{[O/Fe]}<0$), especially GS-z11-0 ($z=11.12$) with a Fe enhancement ($\mathrm{[O/Fe]}=-0.91_{-0.42}^{+0.35}$) beyond the solar-abundance ratio at $\sim3\sigma$. Because, unlike GS-z11-0, GN-z11 ($z=10.60$) may be an AGN, we constrain [O/Fe] via FeII emission under the assumption of AGN and confirm that the Fe enhancement is consistent even in the case of AGN. While [O/Fe] values of the majority of the galaxies are explained by the chemical enrichment of core-collapse supernovae (CCSNe), the Fe enhancements of GS-z11-0 and GN-z11 are puzzling. We develop chemical evolution models, and find that the iron enhancements against oxygen in GS-z11-0 and GN-z11 can be explained by 1) pair-instability supernovae/bright hypernovae with little contribution of CCSNe or 2) Type-Ia supernovae with short delay time ($\sim30-50$ Myr) with a top-light initial mass function.