REBELS-IFU: evidence for metal-rich massive galaxies at z ∼ 6−8

ABSTRACT Metallicity is a crucial tracer of galaxy evolution, providing insights into gas accretion, star formation, and feedback. At high redshift, these processes reveal how early galaxies assembled and enriched their interstellar medium. In this work, we present rest-frame optical spectroscopy of 12 massive ($\log (M_*/\mathrm{{\rm M}_{\odot }})>9$) galaxies at $z\sim 6$–8 from the Reionization Era Bright Emission Line Survey (REBELS) Atacama Large Millimetre/submillimetre Array (ALMA) large program, observed with the James Webb Space Telescope (JWST) NIRSpec integral field unit spectroscopy in the prism mode. These observations span emission lines from [O ii]$\lambda\lambda$3727,9 to [S ii]$\lambda\lambda$6716,31, providing key information on nebular dust attenuation, ionization states, and chemical abundances. We find lower O32 ratios (average $\sim 3.7$) and [O iii]$\lambda$5007 equivalent widths (median EW${_{[\mathrm{ O\,{{\small III}}]}}}\sim 480$ Å) than are generally found in existing large spectroscopic surveys at $z>6$, indicating less extreme ionizing conditions. Strong-line diagnostics suggest that these systems are some of the most metal-rich galaxies observed at $z>6$ (average $Z_{\mathrm{gas}}\sim 0.4 Z_{\odot }$), including sources with near-solar oxygen abundances, in line with their high stellar masses (average $\log {M_*/\mathrm{{\rm M}_{\odot }}}\sim 9.5$). Supplementing with literature sources at lower masses, we investigate the mass–metallicity and fundamental metallicity relations (MZR and FMR, respectively) over a 4 dex stellar mass range at $6< z< 8$. In contrast to recent studies of lower mass galaxies, we find no evidence for negative offsets to the $z=0$ FMR for the REBELS galaxies. This work demonstrates the existence of chemically enriched galaxies just $\sim 1$ Gyr after the big bang, and indicates that the MZR is already in place at these early times, in agreement with other recent $z>3$ studies.