Reionization is thought to be driven by faint star-forming galaxies, but
characterizing this population in detail has long remained very challenging.
Here we utilize deep nine-band NIRCam imaging from JADES to study the
star-forming and ionizing properties of 756 z∼6−9 galaxies, including
hundreds of very UV-faint objects (MUV>−18). The faintest
(m∼30) galaxies in our sample typically have stellar masses of
M∗∼(1−3)×107M⊙ and young light-weighted ages (∼50
Myr), though some show strong Balmer breaks implying much older ages (∼500
Myr). We find no evidence for extremely massive galaxies (>3×1010M⊙) in our sample. We infer a strong (factor >2) decline in the
typical [OIII]+Hβ EWs towards very faint z∼6−9 galaxies, yet a
weak UV luminosity dependence on the Hα EWs at z∼6. We demonstrate
that these EW trends can be explained if fainter galaxies have systematically
lower metallicities as well as more recently-declining star formation histories
relative to the most UV-luminous galaxies in our sample. Our data provide
evidence that the brightest galaxies are frequently experiencing a recent
strong upturn in SFR. We also discuss how the EW trends may be influenced by a
strong correlation between MUV and Lyman continuum escape fraction.
This alternative explanation has dramatically different implications for the
contribution of galaxies along the luminosity function to cosmic reionization,
highlighting the need for deep spectroscopic follow-up. Finally, we quantify
the photometric overdensities around two z>7 strong Lyα emitters in
the JADES footprint. One Lyα emitter lies close to a strong photometric
overdensity while the other shows no significant nearby overdensity, perhaps
implying that not all strong z>7 Lyα emitters reside in large ionized
bubbles.Comment: 29 pages, 16 figures. Submitted to MNRAS. Comments welcom