Abstract
Spitzer/IRAC imaging has revealed that the brightest $z\sim7-8$ 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$_UV=-19.6$) $z\sim6.5-8$
galaxies. We find that the SEDs are typically dominated by young ($\sim$10-50
Myr), low-mass ($M_\ast\sim10^8\ M_ødot$) stellar populations, with no need
for extreme masses ($\sim10^11 M_ødot$) among our sample in contrast to
recent findings in CEERS. Considering previous studies of UV-bright
(M$_UV\sim-22$) $z\sim7-8$ galaxies, we find evidence for a strong
(5-10$\times$) 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 $A$). If confirmed, this may
indicate that a substantial fraction of our CEERS galaxies possess extremely
low metallicities ($łesssim$3% $Z_ødot$) 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 $z\sim7-8$
star formation rate density.
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