Fe Abundances of Early Galaxies at <i>z</i> = 9–12 Derived with Deep JWST Spectra
Minami Nakane, Masami Ouchi, Kimihiko Nakajima, Yoshiaki Ono, Yuichi Harikane, Yuki Isobe, K. Nomoto, Miho N. Ishigaki, Hiroto Yanagisawa, Daichi Kashino, Nozomu Tominaga, Koh Takahashi, Moka Nishigaki, Y. Takeda, Kuria Watanabe
Abstract
Abstract 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>7.4</mml:mn> <mml:mo><</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><</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] < 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.