Faintest galaxies in early universe defy expectations on star formation
New observations of ultra-dim galaxies from the universe's first billion years reveal they're far more diverse than theory predicted—many are old, dusty, or low-mass. This challenges how scientists estimate whether these galaxies could have powered cosmic reionization, a key event that reshaped the universe and affects our models of early star and galaxy formation.
Originaltitel: A GLIMPSE into the UV Continuum Slopes of the Faintest Galaxies in the Epoch of Reionization
<p>As observations have yet to constrain the ionizing properties of the faintest (<em>M</em><sub>UV</sub> ≳ −16) galaxies, their contribution to cosmic reionization remains unclear. The rest-frame ultraviolet (UV) continuum slope (<em>β</em>) is a powerful diagnostic of stellar populations and one of the few feasible indicators of the escape fraction of ionizing photons (<em>f</em><sub>esc</sub>) for such faint galaxies at high redshift. Leveraging ultradeep JWST/NIRCam GLIMPSE imaging of the strong lensing field Abell S1063, we estimate the UV continuum slopes of 553 galaxies at <em>z </em>> 6 with absolute magnitudes down to <em>M</em><sub>UV</sub> ≃ −12.5. We find a modest evolution of <em>β</em> with redshift and a flattening in the <em>β</em>–<em>M</em><sub>UV</sub> relation such that galaxies fainter than <em>M</em><sub>UV</sub> ∼ −16.5 no longer exhibit the bluest UV slopes. The 136 ultrafaint galaxies with <em>M</em><sub>UV</sub> > −16 are a diverse population encompassing dusty (30%), old (15%), and low-mass (50%) galaxies. We apply the empirical <em>β</em>–<em>f</em><sub>esc</sub> relation from local Lyman continuum leakers, finding the mean <em>f</em><sub>esc</sub> peaks at ∼20% at <em>M</em><sub>UV</sub> = −16.5 and declines towards fainter galaxies, while remaining consistent with <em>f</em><sub>esc</sub> = 14% within the 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 an <em>M</em><sub>UV</sub> between −18 and −14 supplied ∼60% of the ionizing photons to cosmic reionization, while the lower <em>f</em><sub>esc</sub> of fainter galaxies produces a natural cutoff in the ionizing photon production rate density.</p>