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

Abstract As observations have yet to constrain the ionizing properties of the faintest ( M UV ≳ −16) galaxies, their contribution to cosmic reionization remains unclear. The rest-frame ultraviolet (UV) continuum slope ( β ) is a powerful diagnostic of stellar populations and one of the few feasible indicators of the escape fraction of ionizing photons ( f esc ) 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 z > 6 with absolute magnitudes down to M UV ≃ −12.5. We find a modest evolution of β with redshift and a flattening in the β – M UV relation such that galaxies fainter than M UV ∼ −16.5 no longer exhibit the bluest UV slopes. The 136 ultrafaint galaxies with M UV > −16 are a diverse population encompassing dusty (30%), old (15%), and low-mass (50%) galaxies. We apply the empirical β – f esc relation from local Lyman continuum leakers, finding the mean f esc peaks at ∼20% at M UV = −16.5 and declines towards fainter galaxies, while remaining consistent with f esc = 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 M UV between −18 and −14 supplied ∼60% of the ionizing photons to cosmic reionization, while the lower f esc of fainter galaxies produces a natural cutoff in the ionizing photon production rate density.

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Journal
The Astrophysical Journal
Published
2026-04-20
DOI
https://doi.org/10.3847/1538-4357/ae5246
Primary Topic
Galaxies: Formation, Evolution, Phenomena
Type
article
Field-Weighted Citation Impact
0.00

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article

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

Seiji Fujimoto, Jed McKinney, Damien Korber, Arghyadeep Basu et al.
The Astrophysical Journal
Galaxies: Formation, Evolution, Phenomena
article

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

Seiji Fujimoto, Jed McKinney, Damien Korber, Arghyadeep Basu, Lukas J. Furtak, Jérémy Blaizot, Vasily Kokorev, Danielle A. Berg, Kristen B. W. McQuinn, Rohan P. Naidu, Angela Adamo, M. Dessauges‐Zavadsky, Tiger Yu-Yang Hsiao, Yoshihisa Asada, Emma Giovinazzo, D. Schaerer, Iryna Chemerynska, Hakim Atek, Pascal A. Oesch, Ryan Endsley, John Chisholm, J. Muñoz, Michelle Jecmen, Richard Pan, Harley Katz, A. Saldana-Lopez
article en

Abstract

Abstract As observations have yet to constrain the ionizing properties of the faintest ( M UV ≳ −16) galaxies, their contribution to cosmic reionization remains unclear. The rest-frame ultraviolet (UV) continuum slope ( β ) is a powerful diagnostic of stellar populations and one of the few feasible indicators of the escape fraction of ionizing photons ( f esc ) 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 z > 6 with absolute magnitudes down to M UV ≃ −12.5. We find a modest evolution of β with redshift and a flattening in the β – M UV relation such that galaxies fainter than M UV ∼ −16.5 no longer exhibit the bluest UV slopes. The 136 ultrafaint galaxies with M UV > −16 are a diverse population encompassing dusty (30%), old (15%), and low-mass (50%) galaxies. We apply the empirical β – f esc relation from local Lyman continuum leakers, finding the mean f esc peaks at ∼20% at M UV = −16.5 and declines towards fainter galaxies, while remaining consistent with f esc = 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 M UV between −18 and −14 supplied ∼60% of the ionizing photons to cosmic reionization, while the lower f esc of fainter galaxies produces a natural cutoff in the ionizing photon production rate density.

The Astrophysical JournalVol. 1001(2)
Université Claude Bernard Lyon 1 (FR), Rutgers, The State University of New Jersey (US), University of Geneva (CH), Tufts University (US), University of Copenhagen (DK), Vassar College (US), Centre National de la Recherche Scientifique (FR), Space Telescope Science Institute (US), Stockholm University (SE), Institut d'Astrophysique de Paris (FR), Sorbonne Université (FR), University of Chicago (US), Centre de Recherche Astrophysique de Lyon (FR), Canadian Institute for Theoretical Astrophysics (CA), Université Bourgogne Franche-Comté (FR), AlbaNova (SE), Massachusetts Institute of Technology (US), The University of Texas at Austin (US)
National Science Foundation, National Aeronautics and Space Administration, Space Telescope Science Institute, National Research Foundation, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Danmarks Grundforskningsfond, University of Toronto, Vetenskapsrådet, Canadian Space Agency, Staatssekretariat für Bildung, Forschung und Innovation
Openalex Percentile: Top 17%
Galaxies: Formation, Evolution, Phenomena
DOIOpen Access PDFOpenAlex
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