The Star-forming and Ionizing Properties of Dwarf z~6-9 Galaxies in JADES: Insights on Bursty Star Formation and Ionized Bubble Growth

TL;DR

This study uses deep NIRCam imaging to analyze the properties of 756 galaxies at redshifts 6-9, revealing insights into their star formation, metallicity, and role in cosmic reionization, especially focusing on faint, UV-bright, and Lyα-emitting galaxies.

Contribution

It provides the first comprehensive analysis of faint high-redshift galaxies' star-forming and ionizing properties using deep JWST data, highlighting bursty star formation and ionized bubble growth.

Findings

Faint galaxies have lower metallicities and declining star formation histories.

Bright galaxies often show recent strong star formation upturns.

Evidence of diverse environments around Lyα emitters, affecting reionization models.

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 NIRCam imaging from JADES to study the star-forming and ionizing properties of 756 $z\sim6-9$ galaxies, including hundreds of very UV-faint objects ($M_\mathrm{UV}>-18$). The faintest ($m\sim30$) galaxies in our sample typically have stellar masses of $M_\ast\sim(1-3)\times10^7$ $M_\odot$ and young light-weighted ages ($\sim$50 Myr), though some show strong Balmer breaks implying much older ages ($\sim$500 Myr). We find no evidence for extremely massive galaxies ($>3\times10^{10}$ $M_\odot$). We infer a strong (factor $>$2) decline in the typical [OIII]$+$H$\beta$ EWs towards very faint $z\sim6-9$ galaxies, yet a weak UV luminosity dependence on the H$\alpha$ EWs at $z\sim6$. 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 $M_\mathrm{UV}$ 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 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 $z>7$ Ly$\alpha$ emitters reside in large ionized bubbles.