The star-forming and ionizing properties of dwarf z ~ 6-9 galaxies in JADES: insights on bursty star formation and ionized bubble growth

Reionization is thought to be driven by faint star-forming galaxies, but characterizing this population has long remained very challenging. Here, we utilize deep nine-band JADES (JWST Advanced Deep Extragalactic Survey)/NIRCam (Near-Infrared Camera) imaging to study the star-forming and ionizing properties of $756$ $z \sim 6-9$ galaxies, including hundreds of very ultraviolet (UV)-faint objects ($M_{\mathrm{UV}} > -18$ ). The faintest ($m \sim 30$ ) galaxies in our sample typically have stellar masses of $M_{\ast} \sim (1-3) \times 10^{7}\ M_{\odot}$ and young light-weighted ages ($\sim 50\ \mathrm{Myr}$ ), though some show strong Balmer breaks implying much older ages ($\sim 500\ \mathrm{Myr}$ ). We find no evidence for extremely massive galaxies ($> 3 \times 10^{10}\ M_{\odot}$ ) in our sample. We infer a strong (factor $> 2$ ) decline in the typical $\mathrm{[OIII]}+\mathrm{H}\beta$ equivalent widths (EWs) towards very faint $z \sim 6-9$ galaxies, yet a weak UV luminosity dependence on the $\mathrm{H}\alpha$ EWs at $z \sim 6$ . 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. Our data provide evidence that the brightest galaxies are frequently experiencing a recent strong upturn in star formation rate. We also discuss how the EW trends may be influenced by a strong correlation between $M_{\mathrm{UV}}$ and Lyman continuum escape fraction. This alternative explanation has dramatically different implications for the contribution of galaxies along the luminosity function to cosmic reionization. Finally, we quantify the photometric overdensities around two $z > 7$ strong $\mathrm{Ly}\alpha$ emitters. One $\mathrm{Ly}\alpha$ emitter lies close to a strong photometric overdensity, while the other shows no significant nearby overdensity, perhaps implying that not all strong $z > 7$ $\mathrm{Ly}\alpha$ emitters reside in large ionized bubbles.

Monthly Notices of the Royal Astronomical Society, Volume 533, Issue 1, pages 1111-1142