2026ApJ...999...95E

We present Keck/MOSFIRE H-band spectroscopic measurements covering the [C III]λ1907, C III]λ1909 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 (${M}_{{\rm{UV}}}^{}=$-22.5 AB) and large median stellar mass (${\mathrm{log}}\left({M}_{* }/{M}_{\odot }\right)=9.2$). 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]λ1907+C III]λ1909 3σ 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 3σ 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]+Hβ EWs, and [O III]λ5007/[O II]λ3727 and [Ne III]λ3869/[O II]λ3727 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.