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

Spitzer/Infrared Array Camera (IRAC) imaging has revealed that the brightest z similar to 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 similar to 6.5-8 galaxies. The spectral energy distributions are typically dominated by young (similar to 10-50 Myr), low-mass (M-* similar to 10(8) M-circle dot) stellar populations, and we find no need for extremely high stellar masses (similar to 10(11) M-circle dot). Considering previous studies of UV-bright (M-UV similar to -22) z similar to 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 xi(ion) = 10(25.7) erg(-1) Hz). In spite of the much larger sSFRs, we find little increase in [O III] + H beta equivalent widths towards fainter M-UV (median approximate to 780 angstrom). If confirmed, this may indicate that a substantial fraction of our CEERS galaxies possess extremely low metallicities (less than or similar to 3 per cent Z(circle dot)) 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 beta = -2.0), we identify a significant tail of very dusty galaxies (beta similar to -1) at approximate to 0.5L(UV)* which may contribute significantly to the z similar to 7-8 star formation rate density.

Automated summary

Using CEERS JWST/NIRCam data, 116 UV-faint galaxies at z ~ 6.5–8 were characterized and found to mainly consist of young, low-mass stellar populations without extremely high stellar masses. The galaxies have higher specific star formation rates (sSFRs), implying high ionizing efficiency, but there is no significant increase in [O III] + H beta equivalent widths as the UV luminosity decreases, indicating low metallicities or possible photon escape fractions and bursty star formation histories. Additionally, a subset of very dusty galaxies at ~0.5L(UV)* was identified.