A JWST/NIRCam study of key contributors to reionization: the star-forming and ionizing properties of UV-faint z 7-8 galaxies

Spitzer/Infrared Array Camera (IRAC) imaging has revealed that the brightest z ~ 7-8 galaxies often exhibit young ages and strong nebular line emission, hinting at high ionizing efficiency among early galaxies. However, IRAC's limited sensitivity has long hindered efforts to study the fainter, more numerous population often thought largely responsible for reionization. Here, we use Cosmic Evolution Early Release Science (CEERS) JWST/NIRCam data to characterize 116 ultraviolet (UV)-faint (median M_UV = -19.5) z ~ 6.5-8 galaxies. The spectral energy distributions are typically dominated by young (~10-50 Myr), low-mass (M_* ~ 10⁸ M_⊙) stellar populations, and we find no need for extremely high stellar masses (~10¹¹ M_⊙). Considering previous studies of UV-bright (M_UV ~ -22) z ~ 7-8 galaxies, we find evidence for a strong (5-10 times) increase in specific star formation rate (sSFR) toward lower luminosities (median sSFR = 103 Gyr^-1 in CEERS). The larger sSFRs imply a more dominant contribution from OB stars in the relatively numerous UV-faint population, perhaps suggesting that these galaxies are very efficient ionizing agents (median ξ_ion = 10^25.7 erg^-1 Hz). In spite of the much larger sSFRs, we find little increase in [O III] + H β equivalent widths towards fainter M_UV (median ≈780 $\mathrm{\mathring{A}}$). If confirmed, this may indicate that a substantial fraction of our CEERS galaxies possess extremely low metallicities (≲3 per cent Z_⊙) where [O III] emission is suppressed. Alternatively, high ionizing photon escape fractions or bursty star formation histories can also weaken the nebular lines in a subset of our sample. While the majority of galaxies in our sample are very blue (median β = -2.0), we identify a significant tail of very dusty galaxies (β ~ -1) at ≈0.5$L_\mathrm{UV}^\ast$ which may contribute significantly to the z ~ 7-8 star formation rate density.