A GLIMPSE into the UV Continuum Slopes of the Faintest Galaxies in the Epoch of Reionization

TL;DR

This study uses JWST data to measure UV slopes of faint high-redshift galaxies, revealing their diverse properties and estimating their contribution to cosmic reionization, especially focusing on galaxies with M$_{ m UV}$ > -16.

Contribution

First measurement of UV continuum slopes for 555 faint galaxies at z > 6 down to M$_{ m UV}$ ≈ -12.5, linking UV slopes to escape fractions and reionization contributions.

Findings

Faint galaxies show diverse properties, including dusty, old, and low-mass populations.

UV slope evolution with redshift is modest, with a flattening at M$_{ m UV}$ > -16.5.

Galaxies between M$_{ m UV}$ -18 and -14 contributed about 60% of reionization photons.

Abstract

As observations have yet to constrain the ionizing properties of the faintest (M$_{\rm UV}$ > -16) galaxies, their contribution to cosmic reionization remains unclear. The rest-frame ultraviolet (UV) continuum slope ($\beta$) is a powerful diagnostic of stellar populations and one of the few feasible indicators of the escape fraction of ionizing photons (f$_{\rm esc}$) for such faint galaxies at high-redshift. Leveraging ultra-deep JWST/NIRCam GLIMPSE imaging of strong lensing field Abell S1063, we estimate UV continuum slopes of 555 galaxies at z $>$ 6 with absolute magnitudes down to M$_{\rm UV}$ $\simeq -$12.5. We find a modest evolution of $\beta$ with redshift and a flattening in the $\beta$-M$_{\rm UV}$ relation such that galaxies fainter than M$_{\rm UV}$ $\sim -$16.5 no longer exhibit the bluest UV slopes. The 138 ultra-faint galaxies with M$_{\rm UV}$ $> -$16 are a diverse population encompassing dusty (30\%), old (15\%), and low-mass (50\%) galaxies. We apply the empirical $\beta$-f$_{\rm esc}$ relation from local Lyman continuum leakers, finding the mean f$_{\rm esc}$ peaks at $\sim 20\%$ at M$_{\rm UV}=-$16.5 and declines towards fainter galaxies, while remaining consistent with f$_{\rm esc}$ = 14\% within uncertainties, in agreement with recent radiative transfer simulations. Incorporating GLIMPSE constraints on the UV luminosity function, ionizing photon production efficiency, and escape fractions produces a reionization history consistent with independent observational constraints. Our results indicate galaxies with M$_{\rm UV}$ between $-18$ and $-14$ supplied $\sim 60\%$ of the ionizing photons to cosmic reionization, while the lower f$_{\rm esc}$ of fainter galaxies produces a natural cutoff in the ionizing photon production rate density.