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8029E89A-B89E-4074-A382-ACB580C2367B

# A JWST/NIRCam Study of Key Contributors to Reionization: The Star-forming and Ionizing Properties of UV-faint $z\sim 7-8$ Galaxies

Ryan Endsley, Daniel P. Stark, Lily Whitler, Michael W. Topping, Zuyi Chen, Adele Plat, John Chisholm, Stéphane Charlot
arXiv:2208.14999

Submitted on 31 August 2022

## Abstract

Spitzer/IRAC imaging has revealed that the brightest $z\sim 7-8$ 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 CEERS JWST/NIRCam data to characterize 118 UV-faint (median M${}_{UV}=-19.6$) $z\sim 6.5-8$ galaxies. We find that the SEDs are typically dominated by young ($\sim$10-50 Myr), low-mass () stellar populations, with no need for extreme masses ($\sim {10}^{11}{M}_{\odot }$) among our sample in contrast to recent findings in CEERS. Considering previous studies of UV-bright (M${}_{UV}\sim -22$) $z\sim 7-8$ galaxies, we find evidence for a strong (5-10$×$) increase in specific star formation rate toward lower luminosities (median sSFR=82 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 their much larger sSFRs, we find no significant increase in [OIII]$+$H$\beta$ EWs towards fainter M${}_{UV}$ (median $\approx$780 $\stackrel{˚}{A}$). If confirmed, this may indicate that a substantial fraction of our CEERS galaxies possess extremely low metallicities ($\lesssim$3% ${Z}_{\odot }$) where [OIII] 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 CEERS galaxies. While the majority of our objects are very blue (median $\beta =-2.0$), we identify a significant tail of very dusty galaxies ($\beta \sim -1$) at $\approx$0.5${L}_{UV}^{\ast }$ which may contribute significantly to the $z\sim 7-8$ star formation rate density.

## Preprint

Comment: 19 pages, 14 figures. Catalog of $z\sim6.5-8$ galaxies at https://www.ryan-endsley.com/ceers-z6p5to8-catalog

Subject: Astrophysics - Astrophysics of Galaxies