A JWST/NIRCam Study of Key Contributors to Reionization: The Star-forming and Ionizing Properties of UV-faint z78 Galaxies

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

Submitted on 31 August 2022


Spitzer/IRAC imaging has revealed that the brightest z78 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 MUV=19.6) z6.58 galaxies. We find that the SEDs are typically dominated by young (10-50 Myr), low-mass (M108 M) stellar populations, with no need for extreme masses (1011M) among our sample in contrast to recent findings in CEERS. Considering previous studies of UV-bright (MUV22) z78 galaxies, we find evidence for a strong (5-10×) increase in specific star formation rate toward lower luminosities (median sSFR=82 Gyr1 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 ξion=1025.7 erg1 Hz). In spite of their much larger sSFRs, we find no significant increase in [OIII]+Hβ EWs towards fainter MUV (median 780 A˚). If confirmed, this may indicate that a substantial fraction of our CEERS galaxies possess extremely low metallicities (3% Z) 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 β=2.0), we identify a significant tail of very dusty galaxies (β1) at 0.5LUV which may contribute significantly to the z78 star formation rate density.


Comment: 19 pages, 14 figures. Catalog of $z\sim6.5-8$ galaxies at

Subject: Astrophysics - Astrophysics of Galaxies