REBELS-MOSFIRE: Weak CIII] Emission is Typical Among Extremely UV-bright, Massive Galaxies at $z\sim7$

TL;DR

The paper analyzes rest-frame UV CIII] emission in eight $z\sim 7$ REBELS galaxies using Keck/MOSFIRE H-band spectroscopy, leveraging precise ALMA [CII] redshifts to target the CIII] doublet. Across the sample, no robust detections emerge, with a median 3$\sigma$ EW upper limit of about 6.5 Å and a stack limit of 2.6 Å, indicating weak CIII] emission in this massive, metal-rich population. JWST/NIRSpec rest-optical measurements for four targets reveal higher metallicities and lower excitation compared with typical $z\sim7$ stacks, predicting $\mathrm{EW(CIII]} )\lesssim 5$ Å and aligning with the MOSFIRE non-detections. The results imply that the strongest CIII] emitters at $z>6$ tend to be less massive and more metal-poor, underscoring the diversity of ISM conditions in early galaxies and informing how CIII] traces star formation and reionization. The study emphasizes the need to combine rest-UV and rest-optical diagnostics, and it highlights the value of JWST in revealing the ISM properties of high-redshift, metal-rich systems.

Abstract

We present Keck/MOSFIRE H-band spectroscopic measurements covering the [CIII]1907, CIII]1909 doublet for a sample of 8 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 (Muv=-22.5 AB) and large median stellar mass (log(Mstar/Msun)=9.2). Although three sources show tentative evidence of a CIII] detection, we obtain no confident detections for any of the 8 REBELS-MOSFIRE sources. The median [CIII]1907+CIII]1909 3-sigma upper limit in equivalent width (EW) for the REBELS-MOSFIRE sample is 6.5 AA, and a stack of their H-band MOSFIRE spectra yields a non-detection with an associated 3-sigma upper limit of 2.6 AA. These upper limits fall significantly below the CIII] 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 CIII] 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 ([OIII]+Hb EWs, and [OIII]5007/[OII]3727 and [NeIII]3869/[OII]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.

Figures (4)

• Figure 1: The distributions of (a) spectroscopic redshifts, (b) absolute UV magnitudes, (c) photometric [O iii]+H$\beta$ EWs, and (d) photometric stellar masses for the parent sample of REBELS galaxies with ALMA [C ii] detections (black) and the REBELS-MOSFIRE subset targeted for C iii] follow up (red).
• Figure 2: The 2D MOSFIRE H-band spectra around the C iii] doublet in our eight targeted REBELS-MOSFIRE $z\sim7$ galaxies. Black is positive flux. Yellow marks indicate the expected wavelength and spatial positions of the [C iii]$\lambda$1907 and C iii]$\lambda$1909 features, while the cyan marks indicate where we would expect negative signal given the $\pm$1.5$^{\prime\prime}$ A--B nodding dither pattern. We do not confidently detect any doublet feature. Tentative ($\approx$3.5$\sigma$) detections in REBELS-15, 36, and 39 are highlighted. The spectral regions with noticeably higher noise correspond to the wavelengths of known OH emission lines in the background sky spectrum, and reinforce the challenging nature of our ground-based C iii] measurements due to OH skyline contamination.
• Figure 3: Compilation of measurements of C iii] EW over a broad range of redshifts. The majority of the REBELS-MOSFIRE sample (5 out of 8) have C iii] EW upper limits (red markers) that fall below the average trend from RobertsBorsani2024 (black symbols). The dusty metal-rich $z=6.73$ galaxy GOODSN-100182 Shapley2025 also has a much lower C iii] EW compared to typical $z\sim7$ galaxies Shapley2025. These upper limits from REBELS-MOSFIRE and GOODSN-100182 highlight the diversity of C iii] EWs among massive, luminous $z\sim 7$ galaxies relative to early work Stark2017Hutchison2019. The typical C iii] EW of the REBELS-MOSFIRE galaxies appears similar to that of comparably massive ($M_\ast \sim 10^{9-10}\,M_{\odot}{}$) galaxies at $z\sim1-4$ compiled by Du2017, Shapley2003, and LeFevre2019.
• Figure 4: C iii] EW vs. rest-frame optical emission-line properties. Individual REBELS-MOSFIRE galaxies with NIRSpec rest-optical spectroscopy are plotted in red, while the median rest-frame optical properties of REBELS-IFU galaxies from Rowland2025 are indicated with dashed vertical lines. The massive, metal-rich $z=6.73$ galaxy GOODSN-100182 Shapley2025 is indicated with an orange square. Lower-redshift data compiled in Tang2021 and Tang2023_CEERS are indicated in grey ($z\sim 0$) and cyan ($z\sim 1-3$). The $z\sim 7$ stacked values from RobertsBorsani2024 are shown in black. Left: C iii] EW vs. [O iii]+H$\beta$ EW. Right: C iii] EW vs. O32. In this panel, the vertical dashed line represents an upper limit on the REBELS-IFU sample median O32 from Rowland2025 (see Section \ref{['sec:discussion']}).