James Webb Space Telescope Feed Post

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

Paper abstract: 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~ 6-9 galaxies, including hundreds of very ultraviolet (UV)-faint objects (M_\mathrm{UV}\gt -18). The faintest (m~ 30) galaxies in our sample typically have stellar masses of M_\ast ~ (1-3)\times 10^7\ \mathrm{ M}_\odot 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 (\gt 3\times 10^{10}\ \mathrm{ M}_\odot) in our sample. We infer a strong (factor \gt 2) decline in the typical [O III]+H \beta equivalent widths (EWs) towards very faint z~ 6-9 galaxies, yet a weak UV luminosity dependence on the H \alpha 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. 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\,\gt\,7 strong Ly \alpha emitters. One 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\,\gt\, 7 Ly \alpha emitters reside in large ionized bubbles.