2025ApJ...994...65N

We derive Fe abundance ratios of seven galaxies at z = 9-12 with -22 < M_UV < -19 whose JWST/NIRSpec spectra achieve very high signal-to-noise ratios, S/N = 60-320, at rest-frame UV wavelength. We fit stellar population synthesis model spectra to these JWST spectra, masking out nebular emission lines, and obtain Fe abundance ratios of [Fe/H] = -1-0 dex for five galaxies and upper limits of [Fe/H] ∼ -2-0 dex for two galaxies. We compare these [Fe/H] values with the oxygen abundances of these galaxies ($7.4\lt 12+{\mathrm{log}}({\rm{O}}/{\rm{H}})\lt 8.4$) in the same manner as previous studies of z ∼ 2-6 galaxies, and derive oxygen-to-iron abundance ratios [O/Fe]. We find that two out of seven galaxies, GS-z11-0 and GN-z11, show Fe enhancements relative to O ([O/Fe] < 0 dex), especially GS-z11-0 (z = 11.12) with an Fe enhancement ($[{\rm{O}}/{\rm{Fe}}]=-0.6{8}_{-0.55}^{+0.37}$ dex) beyond the solar abundance ratio at ∼2σ. Because, unlike GS-z11-0, GN-z11 (z = 10.60) may host an active galactic nucleus (AGN), we constrain [O/Fe] via Fe II emission under the assumption of an AGN and confirm that the Fe enhancement is consistent even in this case. While the [O/Fe] values of most galaxies are comparable to those of core-collapse supernovae (CCSNe) yields, the Fe enhancements of GS-z11-0 and GN-z11 are puzzling. We develop chemical evolution models, and find that the Fe enhancements in GS-z11-0 and GN-z11 can be explained by either (1) pair-instability supernovae or bright hypernovae with little contribution from CCSNe, or (2) Type Ia supernovae with short delay times (∼30-50 Myr) and top-light initial mass functions.