Fe Abundances of Early Galaxies at <i>z</i> = 9–12 Derived with Deep JWST Spectra

Abstract We derive Fe abundance ratios of seven galaxies at z = 9–12 with −22 &lt; M UV &lt; −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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>7.4</mml:mn> <mml:mo>&lt;</mml:mo> <mml:mn>12</mml:mn> <mml:mo>+</mml:mo> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>&lt;</mml:mo> <mml:mn>8.4</mml:mn> </mml:math> ) 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] &lt; 0 dex), especially GS-z11-0 ( z = 11.12) with an Fe enhancement ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">[</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">Fe</mml:mi> <mml:mo stretchy="false">]</mml:mo> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>0.6</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.55</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.37</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> dex) beyond the solar abundance ratio at ∼2 σ . 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.