REBELS–MOSFIRE: Weak C <scp>III</scp> ] Emission Is Typical among Extremely UV-bright, Massive Galaxies at <i>z</i>  ∼ 7

Abstract We present Keck/MOSFIRE H -band spectroscopic measurements covering the [C iii ] λ 1907, C iii ] λ 1909 doublet for a sample of eight z ∼ 7 spectroscopically confirmed star-forming galaxies drawn from the Reionization Era Bright Emission Line Survey (REBELS). This REBELS–MOSFIRE sample is notable for its bright median UV luminosity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">UV</mml:mi> </mml:mrow> <mml:mrow/> </mml:msubsup> <mml:mo>=</mml:mo> </mml:math> −22.5 AB) and large median stellar mass ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mfenced close=")" open="("> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> </mml:mrow> </mml:mfenced> <mml:mo>=</mml:mo> <mml:mn>9.2</mml:mn> </mml:math> ). Although three sources show tentative evidence of a C iii ] detection, we obtain no confident detections for any of the eight REBELS–MOSFIRE sources. The median [C iii ] λ 1907+C iii ] λ 1909 3 σ upper limit in equivalent width (EW) for the REBELS–MOSFIRE sample is 6.5 Å, and a stack of their H -band MOSFIRE spectra yields a nondetection with an associated 3 σ upper limit of 2.6 Å. These upper limits fall significantly below the C iii ] EW measured in a composite spectrum of representative z ∼ 7 star-forming galaxies, as well as those measured for notable early star-forming galaxies such as GN-z11, GHZ2, GS-z12, and RXCJ2248-ID. The lack of strong C iii ] emission can be understood within the context of the stellar populations of the REBELS galaxies, as well as the ionization conditions and gas-phase metallicity implied by rest-frame optical spectroscopic properties ([O iii ]+H β EWs, and [O iii ] λ 5007/[O ii ] λ 3727 and [Ne iii ] λ 3869/[O ii ] λ 3727 line ratios). The REBELS–MOSFIRE sample represents the higher-mass, higher-metallicity, lower-excitation tail of the z ∼ 7 galaxy population, whose ionizing properties must be fully characterized to constrain the role of star-forming galaxies during cosmic reionization.